The Mastery and Uses of Fire in Antiquity
The Mastery and Uses of Fire in Antiquity J.E. REHDER
McGill-Queen's University Press Montreal & Kingston· London· Ithaca
© McGill-Queen's University Press
2000
ISBN 0-7735-2067-8
Legal deposit quarter 2000 Bibliotheque nationale du Quebec Printed in Canada on acid-free paper This book has been published with the help of a grant from the Humanities and Social Sciences Federation of Canada, using funds provided by the Social Sciences and Humanities Research Council of Canada. McGill-Queen's University Press acknowledges the financial support of the Government of Canada through the Book Publishing Industry Development Program (BPIDP) for its activities. We also acknowledge the support of the Canada Council for the Arts for our publishing program.
Canadian Cataloguing in Publication Data Rehder, ].E. The mastery and uses of fire in antiquity Includes bibliographical references and index. ISBN 0-7735-2067-8 I. Pyrometallurgy - History. furnaces - History. 1. Tide.
2.
Ceramics - History. 3. Metallurgical
TN688·5·R442000
All figures were drawn by the author. This book was typeset by Typo Litho Composition Inc. in 10/12 Sabon.
To Nonnie, wife and best friend
Contents
Acknowledgments Foreword Preface
Xl
xiii
xvii
Introduction
3
1
The Nature of Heat and the Management of Its Temperature 9
2
How Furnaces Work
13
3 The Properties and Combustion of Biomass 4 Furnace Configurations for Biomass Fuel
25 38
5 Products Made in Antiquity in Biomass Fuelled Furnaces 46 6 The Manufacture and Properties of Charcoal 7 Combustion in Beds of Lump Charcoal 8 Combustion Air Supply for Charcoal
55
63 74
9 Furnace Configurations for Charcoal Fuel
84
10
The Reduction of Metals and the Functions of Slags
11
The Smelting of Copper
I2
The Smelting, Forging, and Properties of Iron
I I
3 122
101
Vlll
Contents
13 Fuel Consumption by Pyrotechnology in Antiquity 14 Fuel Su ppl y and Deforestation
153
15 Artifacts from the Operation of Furnaces
160
Appendices I
Combustion in Fuel Beds of Charcoal
2
Pressure Drop in Tuyeres and Fucl Beds and Power Required 175
3 Natural Draft in Fuel Beds
167
180
4 A Furnace to Reliably Make a Bloom of Iron Glossary References
195 199
189
145
Tables and Figures
TABLES I
Pro-Forma Heat Balance
2
Proximate Analyses, Carbon Contents, and Heat Contents of Selected Biomass Fuels on Oven-Dry Basis 29
20
3 Biomass Fuel Consumption by Furnace Product
152
4 Effects of Air Supply Rate on Temperature and on Its Extent in a Coke Fuel Bed 17 I FIGURES I
Effect of Furnace Size on Space Velocity Necessary to Maintain Temperature 16
2
Effects of Space Velocity and Fuel Reactivity on the Height of the High Temperature Zone 18
3 Heat Loss Rate as a Function of Wall Thickness and Inner Face Temperature 23 4 Effects of Moisture Content and Excess Air on Flame Temperature of Biomass Fuel 28 5 Reverberatory or Air Furnace
43
6 Gas Composition and Temperature in a Charcoal Fuel Bed 65
x Tables and Figures 7 Approximate Plumes from Tuyeres in Fuel Beds
68
8 Temperature Distribution in a Fuel Bed with Single Tuyere 86 9 Iron Ore Reduction in a Small Bowl Furnace
87
IO
Carbon Content of Iron Blooms versus FeO Content of Associated Slag 126
I I
General Arrangement of Experimental Bloomery furnace I91
Acknowledgments
It was on the gentle but persistent encouragement of Professor Emeritus Ursula M. Franklin that the writing of this book was started. I have taken advantage of her practised teacher's eye to try to keep the writing style reasonably simple and interesting, while being informative. The subject matter has many facets, and it is all too easy to drift into a byway that can confuse the issue. Her continued interest and support in both the subject matter and its transmission have been invaluable. Dr Martha Goodway of the Smithsonian Institution and Professor Bruce Trigger of McGill University read an early draft and made useful comments. The support of Professor Alex MacLean and the late Professor Alan Miller of the Department of Metallurgy and Materials Science in the University of Toronto is appreciated, especially in making available space and the services of a technician for the construction and operation of a bloomery furnace to make several blooms of iron. In particular I appreciate the excellent work done by my editor, Maureen Garvie, in smoothing out the wrinkles and style changes in the manuscript, occasioned by its having been written over a period of several years, and the ease of working with coordinating editor Joan McGilvray.
Foreword
It is a great privilege for me to write this foreword to the book by my friend and colleague J.E. Rehder. The book is an important and quite unique work, and I would like to illustrate its great potential and at the same time give the reader an indication of the philosophy behind the work. As a contribution to the reconstruction of the past, Rehder's book is essentially a companion, a knowledgeable friend to have by one's side while studying ancient objects and thinking about the accomplishments of those who made them. This companion, like its author, is both a scholar and a practitioner, steeped in the complexities of the field but waiting to be asked, trying not to overwhelm the inquirer with unessential details - yet at the same time also not permitting illusions of quick comprehension or an easy glossing of the intricacies of technical processes. Like any real teacher and friend, the book makes it easy to come back to a particular question as more and deeper knowledge is needed. The appendices serve this process by providing additional information without intimidating the initial enquirer, and it is here that the basic philosophy of Rehder's approach to the study of ancient materials and processes becomes apparent. Two central considerations inform this approach - evident not only in this book but also in his many scientific papers in this field. The first relates to artifacts as primary historical evidence, and the second focuses on the requirements for scientific and technical rigour when interpreting ancient processes and products.
XIV
Foreword
First, the "reading" of ancient artifacts, based on their microstructures and compositions, provides essential sources of historical evidence. There is, however, an important proviso here: those "reading" and interpreting technical evidence must know the "grammar" of technology, i.e., the relevant laws of physics and chemistry. And in this proviso we find the second strand of Rehder's approach - to give to the archaeologist and historian technically and scientifically correct and consistent guidance. Today's engineers may express the processing variables in terms of symbols and equations, while the ancient artisans transmitted the knowledge of their hands and minds in the language of their craft and culture; yet the underlying reality is the same. It is on the bases of these premises that Rehder gives those who do not have an adequate pertinent scientific or technical background the tools to assess the basic technical processes of antiquity. The book begins with an outline of the natures of temperature and of heat, acknowledging that the combustion of biomass was essentially the sole source of manageable heat in antiquity. The reader is then quickly introduced to the modes of heat conduction and loss, and to the realization that if materials are subjected to higher than ambient temperature, problems of containment arise - and furnaces and their precursors appear. At the same time it is made clear that the combustion of biomass does not only result in the production of heat but the chemistry of burning changes the composition of the products of combustion, often resulting in complex composition and temperature profiles in the furnace. These in turn must be taken into account in the design of furnaces and the tasks to which they are to be put. Rehder looks at charcoal, the synthetic fuel used particularly for metallurgy throughout antiquity and into the early twentieth century A.D. In a series of quite unique compilations on the making of charcoal, he provides information on its technology in a concise and accessible form not available anywhere else, and the appendices guide the more technically sophisticated to some of the roots of the issue addressed. Having laid this thorough foundation, Rehder turns his attention first to the furnace designs appropriate to charcoal fuel and then the tasks to which such furnaces arc put. Here we find ourselves in the heartland of the development of the smelting and uses of metals, the important role of slags, illuminating details on the metallurgy of copper, and the complexity of the production and manipulation of iron. The reader will find these chapters powerful and totally convincing and will realize that it is the author's thorough understanding of fur-
xv Foreword nace and high temperature reactions - in terms of chemistry, physics, and spacial arrangements - that brings about this seamless understanding. The cobwebs of antiquarian jargon and all hints of magical knowledge or long-forgotten processes known to the ancients vanish in the light of the clear, consistent rationale offered here. The book's final chapters of fuel consumption and deforestation in antiquity round out the work in the same spirit of unbending integrity, so that by taking into account the growth rates of forest, some conclusions on causes of deforestation in antiquity are arrived at that are quite different from those in the current literature. Ursula M. Franklin
Preface
The text which follows considers the subject of pyrotechnology from the perspective of the unchanging rules of physics and chemistry, and my fifty years of experience in industrial pyrotechnology in operations, research, and consulting, with pu blication of more than one hundred papers on the subject. The book is an expansion of a series of papers on ancient pyrotechnology that I have been publishing occasionally in archaeological journals since I986, as a senior research associate in the Department of Metallurgy and Materials Science in the University of Toronto. It also uses data from my industrial research and development papers published some time ago and contains some of my unpublished experimental work on replicated ancient furnaces. The approach I have taken is from a furnace operator's point of view, so that the writing style is that of the natural sciences rather than that of the social sciences. This can create problems in understanding across cultures because of differences both in accustomed jargon and in habits of style, but these arc perennial difficulties, and a conscious effort has been made to avoid jargon or to explain it when unavoidable. While development of furnace practices through time is necessarily involved, this book is not intended as a history of pyrotechnology. Many of my examples of ancient pyrotechnology are taken from the archaeological literatures of the east end of the Mediterranean basin where the evidence is over the longest term, in most abundance, and most easily available. Furnaces operate on the same physical and chemical bases everywhere, and I refer to, for example, South America or Africa only for interesting or unusual practices.
xviii Preface Two existing books may seem to be on the same subject as this one, but in fact are not. One is The Beginning of the Use of Metals and Alloys, edited by R. Maddin (M.l.T. Press, I988), a collection of thirty papers from a conference in Zhengzhuo, China, in I986. Their contents deal almost entirely with evidences of early metallurgy, and only two papers describe iron smelting furnaces, each with insufficient technical detail to permit analysis of their operation. The second is the excellent book by P.T. Craddock, Early Metal Mining and Production (I995), which discusses the mining of ores and the production of a wide range of metals in useful historical detail. However, only about one-third of that book concerns matters in which there is some overlap with the material here, and its author has an understandable tendency to take the accustomed archaeological consensual approach, which is not always reliable on testable matters of natural science. Moreover, like the Maddin book it discusses only the branch of pyrotechnology that deals with the smelting of metals, while the present one includes other important products of ancient pyrotechnology such as ceramics, lime, and glass. Pottery, lime, and the two metals copper and iron were made in increasingly large quantities throughout antiquity, in time effectively on an industrial scale. The organization of this book, in addition to reflecting the nature or kind of fuel used, also is divided according to the products made. These were fired clay, lime from limestone, metals from the reduction of ores, and to a lesser extent, glass from sand. This list is short and simple, but it subdivides into many kinds of products used in quantities that increased throughout antiquity. The complexity of their production technologies increased exponentially from clay to lime to metals, and the ability finally to smelt metals from their ores took millennia to become a reasonably reliable practice. This increase in technical complexity accounts for the considerable amount of space given here to the smelting and uses of copper (for making bronze) and of iron. These were the two metals in major use in antiquity, iron having a wider range of useful mechanical properties than copper and its alloys. Moreover, the orcs of iron arc much more widely distributed geographically and on the average are richer than are those of copper. However, the metallurgy of iron is even more complex than those of copper and bronze, and the ability to smelt iron that could be hot-forged to a bar or sheet of strength and ductility superior to those of bronze required successive considerable advances in furnace technology through nearly two millennia. For these reasons, chapter I2, on the smelting and properties of iron, is the longest in the book. Other notes on organization: The first three appendices are technical in nature, intended for serious students of furnace technology, and can
XIX
Preface
be ignored by others. Also, illustration is limited to line drawings and necessary graphs. While a valid criticism might be made that there are not enough illustrations of ancient furnaces and their associated equipment, there is a basic reason for this lack. The book covers a very long time span - about IO,OOO years - during which the technology of the uses of fire was slowly developed through simple trial and error. Thousands of different furnace shapes and kinds were tried, by definition mostly unsuccessful, and remnants of these attempts fill the extensive published archaeological record. There is then the question of selection, since this book is not intended as a history of furnace architecture. Reconstructions have been made, but they necessarily involve speculation and it is one of the purposes of this book to decrease the extent of such guesswork by supplying the technology of furnace mechanisms. Thus only a moderate number of functional graphs and a few sketches have been included.
The Mastery and Uses of Fire in Antiquity
Introduction
The material fabrics of nearly all settled civilizations have by and large consisted of things that exist only because of pyrotechnology - the generation, control, and application of heat, which at sufficient temperature can alter the properties and compositions of all materials. When the materials are of the earth itself, since antiquity the resulting products have formed a large part of the material bases of human wellbeing. The list of such products is today extensive, as a little thought will suggest: steel beams, reinforced concrete floors, brick walls, glass windows, metal and plastic pipes for water, gas, and sewage, copper wires for electricity and communication, kitchen ware - as well as trains, automobiles, airplanes, and so on. Such accomplishments are the result of some ten thousand years of development of the intentional use of fire for other than warmth and food. The early archaeological evidence is slim and scattered, but it noticeably increases towards the end of the most recent ice age. The firing of clay into pottery was apparently the first major product of pyrotechnology, but early evidence of another considerable application was the extensive use of lime plaster at Cayonu Tepesi in 6500 B.C., where a house floor was found to be made of 4.5 cubic metres of lime plaster. The decomposition of limestone into quicklime requires heat at close to I,OOOoC; however, reduction of iron ore to iron metal not only requires heat but involves complex and invisible chemical changes at different temperatures, and this took another five thousand years to master. By the close of antiquity, defined here as the collapse of the Roman hegemony before the middle of the first millennium A.D., a plateau of
4
Mastery and Uses of Fire in Antiquity
development of the uses of fire had been reached. This was exemplified by the following quotation from Pliny the Elder in his Natural History of 77 A.D.: "Fire takes sand and melts it into glass. Minerals are smelted to produce copper. Fire produces iron and tempers it, purifies gold, and burns limestone to make mortar that binds blocks together in buildings." Pliny overlooked, probably from its familiarity, the firing of clay into pottery, which probably was the earliest widely used product of high temperature heat. When our modern extensive abilities in the uses of heat arc considered, arguably the growth of skill in such an important and complex technology could be considered as the longest continuous intellectual endeavour undertaken by humankind. Indeed there is justification for considering that the extent and the complexity of uses of heat is a measure of the level or quality of a civilization. This position has been noted by Mumford (I946), White (I962), and Kef (I967). Caasen and Girifalco (I986) have created a chart showing that a linear relationship exists between per capita consumption of heat energy and personal income for twenty-eight countries around the world, from a Pacific island to the United States. The practice of pyrotechnology evidently was basic to the civilizations of antiquity, and so is of proportionate importance to archaeology and to anthropology. However, as far as I know, the subject matter and its importance receive only minor attention in the teaching of archaeology, and there also seems to be no publication that discusses in useful detail how pyrotechnology was practised in antiquity, using the limited knowledge and sources of heat then available. Such information is essential to both the tracing of its development through its artifacts and to the understanding and replication of ancient practices, and it is the objective of this book to supply it on the basis of modern knowledge of the subject. Curiously, the word "pyrotechnology" is in few dictionaries, though its entomology is obvious. Apparently its earliest appearance was in Italian as the title of the famous work of Biringuccio published in 1540, Pirotechnia, which described in useful technical detail the contemporary uses of heat to alter materials. In modern times the late Theodore A. Wertime used the word in this sense in his many well-known publications on ancient pyrotechnology, as have others, and it is now common in archaeology and anthropology. The history of the development of pyrotechnology following the end of the last icc age was outlined in a collection of papers edited by Theodore and Steven Wertime (I982). Yet almost without exception, this and other modern studies of ancient pyrotechnology have dealt with the artifacts that are the products of furnaces and, to a much lesser
5 Introduction
extent, with the functioning of the furnaces themselves. Considerable attempts have also been made to replicate the operation of furnaces recovered by archaeology and anthropology, but with limited success, since little has been published in the archaeological literature on the basic principles on which such furnaces work. The appropriate conditions are today well known from modern industrial experience, and it is the purpose of this book to add to the resources of archaeology and the other disciplines that deal with past civilizations a detailed discussion of how pyrotechnology was practised throughout antiquity. This discussion makes clear what kinds of artifacts need to be collected, and in what detail, to supply the necessary data base for detailed analysis of particular furnace operations, permit such analysis, and guide the construction and operation of replica furnaces. THE STUDY OF PYROTECHNOLOGY AS PRACTISED IN ANTIQUITY
Details of the first long stage of development of pyrotechnology during the Pleistocene are largely speculative, with archaeological evidence spread very thinly through time. No written record exists until about 3000 B.C. at Sumer, but even then, throughout most of antiquity, not only was there a low levcI of general literacy but the pervasive opinion among the literate and the powerful was that matters dealing with use of the hands for other than fighting were not worth recording. Xenophon in 400 B.C. noted, "What are called the mechanical arts carry a stigma and are rightly dishonoured in our cities - it is not legal for a citizen to ply a mechanical trade." The question of the social position of craftsmen in antiquity is complex and has been discussed in detail by, for example, Burnford (I972), and its connection with technical innovation in antiquity has been treated by Finley (1985). As an example of minimal information, iron is mentioned in literatures from the cuneiform of the Hittites in the second millennium B.C. to the Latin of Plutarch in the second century A.D., but very little was written about details of any of the technical or manufacturing practices involved. Our knowledge today of what was made, by what procedures, and for what purposes therefore depends almost entirely on interpretation based on modern scientific and engineering knowledge of the artifacts recovered by archaeology during about the past century and a half. An excellent book on artifacts in general published in I964 by Hodges has been reprinted several times. However, the objectives here are limited to those artifacts that have been produced by the influence of heat, and to the methods by which this was done.
6
Mastery and Uses of Fire in Antiquity
MATERIALS
A general classification of materials necessary or convenient to civilized human life in various ways is that they are either organic or inorganic in composition. Organic materials have their origin in some kind of life, such as wood, plant fibres, foods, leather, and horn. Composed largely of carbon, oxygen, and hydrogen, they are generally combustible but start to decompose when their temperature is raised above about 250°C. Most varieties also resist the destructive effects of the environment poorly over time. Inorganic materials such as clays, stones (some, like limestone, being consolidated skeletons of previously living matter), sand, and minerals of various metals are generally harder and stronger than organic materials. When heated to elevated temperatures, they change in composition and properties to create new materials that are useful to humans. Lime mortar, for example, is used to join building stone and to make plaster walls, floors, and ceilings. Fire-hardened clay can be made into pottery, and minerals can be sources of metals for tools and weapons. Inorganic materials' resistance to environmental effects over time varies but is better on average than that of organics, so they tend to predominate as artifacts. The practice of pyrotechnology in antiquity discussed here deals almost entirely with the effects of heat on such inorganic materials. Throughout antiquity the necessary heat was generated by the combustion of organic materials, and only two fuels were used. The primary one was some form of biomass, almost entirely organic in composition: vegetable or other organic matter living or recently dead such as wood, shrubs, vine prunings, straw, and dung. On the artifactual evidence fossil biomass such as coal, oil, and natural gas was very little used. Theophrastus (370-288 B.C.) makes mention in his History of Stones of the use of lignite by metalworkers in Italy and Greece but includes no useful detail on how, what for, and to what extent it was used. Coal was used in Roman Britain but only for space heating (Birley I977), and in China a few centuries later (just outside our time frame here) for pyrotechnology (Needham I958). The other fuel was charcoal, made by heating biomass out of contact with air to above a temperature of about 250°C. It then decomposes to form charcoal, which is largely carbon, and a gas that is combustible but in antiquity was wasted to atmosphere. Charcoal burns in a very different pattern from that of biomass, and this difference is sufficiently large to require different furnace structures for the combustion of biomass or of charcoal. It also gives abilities to reach different maximum temperatures and to make different chemical changes in materials.
7
Introduction
The heat developed by the combustion of each fuel was therefore used for quite different purposes. That from biomass was typically for making pottery, quicklime, and glass; and from charcoal for smelting ores. For these reasons the discussions in this book of furnaces and the uses of heat are divided on the basis of the fuel used. DIFFERENCES BETWEEN ANCIENT AND MODERN PRACTICES
Pyrotechnology was conducted in antiquity differently from today for several reasons. First, heat was generated almost entirely by combustion of some form of biomass and to a lesser extent, charcoal. The enormous amount of heat used for today's extensive pyrotechnology is still largely from combustion but of one of the fossil fuels. A second difference involved the supply of the all-important air necessary for combustion. Throughout antiquity this was limited to bellows or blowpipes driven almost entirely by the power of human muscle; today the necessary power is generated by engines themselves driven by combustion. A third difference followed from the low population density in antiquity; lower demand for product could thus be supplied by smaller furnaces. The fourth and intractable difference was complete lack of a tradition of science throughout antiquity. All exploration of what could be done and how to improve on it was by trial, error, keen observation, and a good folk memory that became embedded in mythology. Some statements made in the course of this book about the practice of pyrotechnology in antiquity, particularly in metallurgy, will be found to differ from ideas found in the literature of archaeology until quite recently. However, these statements arc easily verifiable, and in every case are the results of either modern published industrial research and experience, which has been little noticed in the archaeological literature, or of associated specific experimental work published by myself, or both. The more important issues dealt with in these pages include demonstrations that maximum furnace temperatures attainable in antiquity with biomass fuel could be about IAOOoC, and with charcoal, a little over I,6oooc, sufficiently so that temperature limitation could be necessary; that carburization of iron while being heated directly in a forge fire does not occur; that combustion air is not preheated to a useful extent in long tuyeres, and in any case preheat is not necessary to make high carbon steel in charcoal fuelled furnaces; that bellows air supply can generate temperatures in a charcoal fuel bed about 400°C
8
Mastery and Uses of Fire in Antiquity
higher than is possible with a blowpipe, making possible the smelting of iron, and is many times more efficient in the use of human muscle power to move air; and that the practice of pyrotechnology may not have been the major factor in the deforestation that was noted in the ancient literature.
I
The Nature of Heat and the Management of Its Temperature
Heat has three singular and interesting properties that affect how it can be generated and used. The first and most important is its intensity or temperature, which measures its ability to affect materials. The second is that heat flows instantly from a higher to a lower temperature. The third is that heat cannot be confined, since there is no known material that does not conduct heat to some extent. From the instant it is generated, heat leaks everywhere and constantly, but since different materials have different conductivities, some control can be achieved over the rate of flow or loss of heat. It is because rates of flow are involved that a time factor enters all discussion of the generation and use of heat. When heat is generated by combustion of a fuel, its quantity and temperature can be easily and directly controlled by the rate of air supply. Then ease of generation and control, and incessant but controllable loss, are the key factors that make possible and then govern the generation, control, and application of heat for human benefit. Because heat escapes so readily, a container for its generation is necessary, which should obviously be made of a material with suitably high softening temperature. The material should also have low thermal conductivity to slow the rate of heat loss to surroundings, which both saves fuel and can increase the temperature of combustion. Such a container is called a furnace. The earliest evidence of furnace use is apparently early post-Ice Age, but it existed certainly by 7000 B.C. at Catal Huyuk in the Near East. Enclosure of fire was a major advance in pyrotechnology; furnace development then began to accelerate into
10
Mastery and Uses of Fire in Antiquity
a wide variety of shapes, sizes, and internal designs, governed by the kind of fuel used and the kind of change to be made in the treated material. THE MANAGEMENT OF TEMPERATURE
The chemical compositions of all materials are sensitive to heat, and the effectiveness of heat in changing their chemistry and properties increases exponentially with temperature. There are also pronounced threshold effects of temperature, in which a particular change in a material will not take place until some minimum temperature is reached, no matter what quantity of heat is supplied. Obvious examples are that water will not boil until its temperature reaches 1000e and a metal will not melt until its specific melting point is reached. The temperature of heat and the ability to control it are therefore of primary importance to pyrotechnology. CREATION AND CONTROL OF TEMPERATURE
In the combustion of a fuel, the maximum temperature of the heat generated can be calculated from the thermochemistry of the combustion reaction, and is called the adiabatic flame temperature or AFT. It varies with the composition of the particular fuel, such as biomass versus charcoal, and with the source of combustion oxygen, such as human breath through a blowpipe versus ambient air from a bellows as source of oxygen. AFT is calculated on the basis of zero heat loss, but in practice there is always some heat loss so in a real furnace the AFT of the fuel can be only approached. MAXIMUM TEMPERATURE IN A FURNACE
When using a particular fuel and source of combustion oxygen, the rate of heat loss from a given furnace is one major factor in determining the actual maximum temperature attainable in it. The other factor is the rate of heat generation, since if this is less than the rate of heat loss, furnace temperature will decrease. An analogy is trying to fill with water a bucket with a hole in it; if water leaks out faster than it is added, it can never be filled. The maximum actual temperature in a given furnace will be less than the AFT of the fuel by the same percentage that the rate of heat loss is of the rate of heat generation. For example, with a heat generation rate of 1,000 MJ (megajoules) per hour and a heat loss rate of 100 MJ per hour, the heat loss rate is IOOir,ooo or 10 per cent of the
II
Heat and Temperature Management
heat generation rate. Then with a fuel AFT of 2,000°C, maximum furnace temperature will be 2,000 - (0.10 X 2,000) = I,800 o c. HEAT LOSSES
Heat losses from furnaces are by conduction through containing walls and then by radiation and convection from the exterior to surrounding air. Heat is also lost in flue gases. In practice, then, the management of temperature consists of selecting a fuel, which defines the AFT and maximum possible temperature, and then burning it in an enclosure so that heat is generated at a rate sufficiently exceeding the rate of heat loss from the furnace used to develop a desired fraction of the AFT of the fuel. As will be shown in chapters 3 and 7, the rate of heat generation is controlled directly by the rate of supply of combustion air, and the rate of heat loss depends largely on the furnace size, wall thickness, and materials of its construction. QUANTITY OF HEAT
In today's international SI units, the standard quantity of heat is a called a joule. As this is a very small unit equal to one watt per second or to 0.239 gm calories in the old CGS system, energy units of practical size require multipliers. These are thousands of joules, or kilojoules (KJ), for example, oak wood having a potential heat energy of 18.7 KJ per kg; millions of joules or megajoules (MJ), such as a ceramic pot at a temperature of 1,000°C containing 0.84 MJ per kg of heat energy; and thousands of millions of joules or gigajoules (GJ), such as 4.0 GJ of heat per hour being generated in the firebox of a large pottery kiln. MEASUREMENT OF TEMPERATURE IN ANTIQUITY
The temperature of heat is measurable by its effects on materials, such as the expansion of mercury in a thermometer, or the electrical voltage generated at the junction of two dissimilar metals (a thermocouple), or radiation in the visible or ultraviolet spectrum which can be measured by optical instruments. Since the control of and the ability to reproduce a given temperature is essential to being able to repeat a successful heat process, a note is in order as to how temperature was measured without such instruments in antiquity. Evidently this was, and today still to some extent is, done simply by noticing the colour of the object being heated, since there is a direct and quantitative relationship between visible colour and temperature. A practised eye can (in my and others' ex-
12
Mastery and Uses of Fire in Antiquity
perience) in this way estimate a temperature and recognize its reproduction to within at most 20°C of the temperature measured by a modern pyrometer. This is close enough for most practical purposes. Colours represent specific wavelengths of visible light that are functions of temperature and are difficult to describe in words; but, for example, the lowest temperature that can be seen with the naked eye in a darkened room is about 550°C, visible as a very faint dark-red glow. A bright red is 850-9 50°C, a yellowish red is 1,050-1,1 50°C, and a white colour is above about 1,500oC. Higher temperatures are simply more intensely white and can be estimated only by use of a darkening glass. Ranges are given here because of personal differences in perception or in naming of colours. The measurement of the temperatures of gases is a complex subject, and their temperature can be estimated visually only if they contain enough finely divided particles of matter, such as ash particles or unburned carbon, that radiate energy at wavelengths visible to the human eye. Then, even with modern thermocouples and radiation pyrometers, accurate measurement is difficult due to the complexity of flow patterns at different temperatures in a combustion system, caused by poor mlxmg. As will be noted in any combustion engineering handbook, the reasonably accurate determination of the temperature of gas within a furnace, or of average temperature of flue gases, requires many measuring points and a statistical analysis of their readings. For this reason measurements made on modern replicas of ancient furnaces are largely meaningless if taken at single points. An example is given in chapter 7 of how temperature measurements of a gas flow in a tuyere, which were seriously in error due to faulty technique, led to conclusions about process that were seriously mistaken.
2
How Furnaces Work
HEAT LOSSES
As a general custom the word "furnace" implies that temperatures above about 250°C are involved, as this is the level above which most organic materials start to decompose and form a char. Enclosures of fire used for lower temperatures are mostly for food preparation, which is incipient decomposition, and are called ovens. Two common inorganic materials whose properties can be usefully altered by such moderate temperatures are gypsum (discussed in chapter 5) and flakeable stones such as flint and chert (not discussed here). A kiln is a high temperature furnace that was used through antiquity for firing ceramics and making lime from limestone. A primary objective of a furnace is to be able to provide an enclosed space in which an atmosphere of controlled temperature and composition can be created and maintained. A secondary objective is to use as little fuel per hour as possible. This depends mostly on the heat loss rate from the furnace, which must therefore be kept as low as possible. This becomes a question of shape, size, and materials of construction. Since heat starts to flow from the instant that it is created, and since all materials conduct heat at rates characteristic of the material, heat is continuously lost through the furnace walls. There are in addition two other ways in which heat is "lost" from a furnace. One is in the form of the gaseous products of combustion of the fuel, which must escape from the furnace and carry amounts of heat that depend on the conditions of combustion. The other is the heat absorbed by the objects in
14
Mastery and Uses of Fire in Antiquity
the furnace that are being heated, ranging from an ore to be smelted into metal to pottery to be hardened. This last "loss" is of course the objective in operating the furnace, but it must be taken into account in order to understand and so control the furnace's operation. The first consideration in designing a furnace is the maximum temperature to be reached, and this affects the choice of fuels. Then the furnace must be made of material that not only resists the softening effect of heat at that temperature but can slow the rate of loss of heat to surroundings. The shape of the furnace must also assist the uniformity of transfer of heat and products of combustion, from burning fuel to the objects to be heated, and the addition of unheated objects and removal of heated ones must be convenient. BATCH VERSUS CONTINUOUS OPERATION
From a functional viewpoint, furnaces can be operated in either of two ways. One is to treat material in batches, by heating it to some desired temperature, holding at temperature if necessary, cooling both furnace and contents to ambient temperature, removing the processed material, refilling the furnace with fresh material, and repeating the heating cycle. The firing of pottery in a kiln is an example. The other method of operation is to maintain part or all of the interior of the furnace at some desired temperature, passing the material to be heated continuously through the furnace. An example is a shaft furnace with combustion and high temperature near its base, and lower temperatures at higher levels, for smelting ores of metals. These two procedures produce large differences in the amount and rate of heat loss. In the batch method, the furnace structure is unavoidably heated along with its contents, and all of this heat is wasted to open air on cooling the furnace for emptying and refilling. Since the weight of the furnace structures of batch furnaces can be several times that of the contents of a single filling, the heat loss per unit weight of material heated is large, and the thermal efficiency of use of the fuel is correspondingly low. In antiquity this was between about .5 to 4 per cent, which resulted in high fuel consumption per unit weight of product. In the continuous method, the empty furnace body is preheated by combustion to "fill" the furnace walls with heat, so that when material mixed with fuel is added and passed through the hot furnace, the heat loss rate through the walls is stable and temperature is under better control. The ratio of furnace weight to that of total product becomes low, and thermal efficiency is then much increased. In antiquity this was 10 to 25 per cent, depending on the size of the furnace, and fuel consumption was accordingly lower.
15
How Furnaces Work
The one major qualification of the use of continuous processing is that the material to be heated must be in small pieces relative to the size of the furnace, in order to be able to flow with the lump fuel down the furnace interior. Preheating is usually done with miscellaneous biomass fuel since charcoal has a considerable labour content, as will become evident in chapter 6. There is of course little to be gained by preheating a batch furnace such as a pottery kiln. TEMPERATURES ATTAINABLE
The maximum actual temperature attainable in a furnace (discussed in chapter I) was demonstrated experimentally some years ago. A wellinsulated shaft furnace 410 mm inside diameter (I.D.) was used, containing only coke fuel. It was supplied with combustion air at the rate of 120 m 3 per m 2 of internal cross-section per minute, considerably above that used in antiquity. The AFT of the fuel was 2,070oC and the measured maximum temperature of the fuel bed was 2,020 o C or 97.6 per cent of the AFT (Draper et al. 1979). It mnst be noted, however, that there was nothing but fuel in the furnace, and if material had been added to be melted or smelted, the heat absorbed by it would be a "loss" and so would have decreased the maximum furnace temperature by a predictable amount. The AFT of biomass fuel is more variable than that of charcoal, as will be shown in chapters 3 and 7; but it can be as high as I,6oo°c and the temperature in a kiln can then reach I,400oC or 87 per cent of the AFT. The lower percentage of AFT than in the Draper experiment is due to the higher heat loss rate in typical kilns. EFFECTS OF FURNACE SIZE AND SHAPE
Since heat is generated by combustion in a volume of space, and heat losses to and through walls arc through enclosing surfaces, the heat loss rate increases with the ratio of surface area to volume enclosed. As any container becomes smaller or more complex in shape, the ratio of its surface area to volume increases, and so as furnace size is decreased a larger proportion of the heat generated is lost to and through walls. This results in decreased thermal efficiency, and also means that heat input rate must be higher in a smaller furnace to achieve a given internal temperature. This effect rapidly becomes seriously limiting as furnaces become quite small, as is shown in a pro-forma way in figure I for shaft furnaces. In this example wall thicknesses were taken as 10 per cent of their inside diameter, and an inner face temperature of I,200 o C was to be maintained after five hours operation. To maintain I,200°C in a fur-
16
Mastery and Uses of Fire in Antiquity
50
'"
10
O~----r-----~--~~--~----~ :zoo 400 o I, (JOt) 600 furrtA-t!t!- l D. w",
eoo
Figure I Effects of Furnace Size on Space Velocity (Heat Input Rate) Necessary to Nlaintain Temperature
nace with less than about 100 mm I.D., the necessary rate of flow of combustion air could blow the fuel out of the furnace. Throughout antiquity, this size effect was an important consideration in furnaces, which were small, particularly in early pyrotechnology, because of smaller population density and thus limited objectives in quantities of material to be heated. The general tendency in antiquity of furnaces to be rounded in shape (a round furnace or circular domed kiln will have lower heat loss rates than the same volumes enclosed by a square or cube) is evidence that the effects of size and shape were recognized. A mechanical effect of the shape of a furnace is how it directs and controls the flow of hot products of combustion through the contents of the furnace. In the case of a kiln filled with pottery, the arrangement of the ware can thus affect the uniformity of temperature reached in various parts of the kiln. In the case of a charcoal-fuelled bowl or shaft furnace, the way in which the plume of products of combustion issues from the nose of a tuyere into the fuel bed depends on the height and the lateral shape of the fuel bed and the placement of tuyeres. WALL EFFECT
An important effect of the inside diameter of a bowl or shaft furnace fuelled with charcoal is the indirect effect that it has on the optimum
17
How Furnaces Work
lump size of the fuel. When lump material lies in the form of a bed against a uniform flat or curved surface, its void fraction (inter-lump volume as a fraction of total bed volume) adjacent to the surface or wall is considerably higher than within the bed, for purely geometrical reasons. Gases thus flow more readily up the wall-bed interface and partly escape reaction with the bed. The phenomenon is known in today's chemical industry as the "wall effect." In a circular container, it increases with the ratio of diameter of lumps to diameter of container, and becomes a serious factor when the average lump size is more than 8 to 10 per cent of the container diameter; a ratio of about 8 per cent has become accepted as the practical desired limit. This means that for best operation of a charcoal fuelled furnace, average fuel lump diameter should be proportionately changed as the inside diameter of the furnace is changed. SPACE VELOCITY
Rate of supply of heat is clearly an essential matter, and in the case of the combustion of charcoal fuel, it will be shown in chapter 6 and appendix I that one cubic metre of air at ambient temperature, burning charcoal by passing vertically through a contained bed that is more than 200 to 250 mm deep, will generate typically about 2. I MJ of heat energy, varying moderately with the carbon content of the charcoal. There is therefore a direct connection between volume of combustion air and the quantity of heat generated. This is not, however, the case when the fuel bed is of biomass, because of the structure of such a bed and the composition of biomass, as will be discussed in chapter 3. A furnace containing a charcoal fuel bed has an average internal cross-sectional area measured in m\ and the rate of air supply is measured as m} per minute. This is a velocity in m per minute in the empty furnace at ambient temperature, and is called "space velocity," or less often, "specific air rate." It is also a heat supply rate in MJ per m 2 per minute, as will be noted in chapter 7. In practice, of course, the actual gas velocity in the interstices of a fuel bed is higher due to the void space between lumps and to the large expansion of the hot products of combustion. However, space velocity has become a widely adopted convention developed in the chemical industry for fluid flow in packed beds, and it has been employed for many years by myself and others as a useful measure of rate of heat energy supply in charcoal and coke-fuelled shaft furnaces, such as cupolas and blast furnaces. In a given furnace, as the space vcIocity (i.e., the rate of heat supply) is increased, the volume of fuel bed at high temperature is increased by
r8
Mastery and Uses of Fire in Antiquity
~ ~
.
U
2S'tJ
I:>
~ ~
,
,.1 CJ¥
8-
(,"
~oo
....tl ~
'" ..... <1/
15:0
ttl
. :!
/00
SO
10 ~o 30 ,sflcu:e Velocr'ty m!mt'n
q.a
Figure 2 Effects of Space Velocity and of Fuel Reactivity on Length of High Temperature Zone
vertical extension of a given temperature, from the level of maximum temperature, which in charcoal-fuelled shaft furnaces is usually a short distance above air entry. This is demonstrated by results shown in figure 2, determined experimentally in a 600 mm I.D. shaft furnace fuelled with brown coal char, by Evans et al. (1959) and reported in more detail in appendix I. The slope of the line is an inverse function of the reactivity of the fuel, and from the reactivities of a char and of charcoal given in Reeve et al. (1975), I have drawn the line for charcoal; both of these lines will change somewhat in slope with the actual reactivity of the particular fuel used. At high space velocities, larger fuel lump size also was shown to increase the volume within the furnace at high temperature. However, below a space velocity of about 45 to 50 metres per minute, this effect of fuel lump size became minor, and since all of the charcoal combustion practice in antiquity was well below these rates, this factor was not involved. An upper limit on space velocity is in practice set by when the mass flow rate of gas upward approaches the weight of the column of mixed material and fuel in the furnace, and so can lift and open it. In modern
19
How Furnaces Work
blast furnaces with a low ratio of coke to ore and therefore high stock column density, space velocities as high as 60 to 70 m per minute can be used without starting to open or fluidize the column. However, in antiquity, with charcoal fuel much lighter than coke and a ratio of fuel to ore several times higher than today, the stock column density was much lower, and space velocities above about 20 to 30 m per minute could start to open the bed. But space velocities in antiquity apparently did not exceed about 10 to IS m per minute since they were limited by only muscle power being available on bellows. Smelting furnaces as a result had accordingly only moderate rates of production of metal, because this is directly dependent on the space velocity. An important side effect of space velocity that can occur particularly ncar to and at the top of the fuel bed in shaft furnaces is that when it is sufficiently increased, smaller particle sizes of some charge materials cannot be retained and will be blown out of the furnace. This was not a problem in antiquity since at the maximum space velocities evidently used, gas velocity in interstices of fuel would then be about 30 m per minute, a gentle breeze of about two km per hour. Particles of iron ore (for example, one mm in diameter) would not be blown out and would smelt satisfactorily. In the case of furnaces such as kilns where the fuel is burned separately from the objects to be heated and there is no movement of the objects being heated, the same effects of rate of supply of combustion air and of heat energy apply, and the proportion of furnace volume at high temperature increases as heat supply rate is increased. HEAT BALANCE
I noted above that there is a quantitative balance that always exists between the generation and the loss of heat, and that this heat balance, when made explicit, can be very useful. In accounting for quantities of heat a dynamic system is involved, because heat flows continuously, measured in joules per minute. An ongoing credit deposit to the furnace is thus made, and debits consist of losses of heat in the same terms. This is a convenient place to mention the accuracy of figures used in calculation of heat generation and flow. Although some laboratory measurements of heat parameters can be very precise, in practical situations many factors can be measured with only moderate accuracy. In engineering and in heat accounting it is nearly always pointless to report or use data to more than three-figure accuracy. Final accuracy is limited to that of the least accurate component and is seldom closer, even today, than within about 10 per cent of the true value.
20
Mastery and Uses of Fire in Antiquity
Heat balances can be revealing as to what conditions matter most in the successful operation of a furnace. This can be made clearer by examination of a pro-forma heat balance such as given in table I, the numbers being purely illustrative of the effects of furnace size. Two furnaces are considered, of different inside diameter but supplied with combustion air at the same space velocity and therefore the same energy supply rate per unit of furnace volume. Table I Pro-Forma Heat Balance Furnace size
small
large
100
100
Losses to walls
35
10
Losses to top gas
10
20
Absorbed by work
30
30
Total heat lost and absorbed
75
60
25
40
Heat input rate, MJ/m 3 Heat absorbed, MJ:
Surplus heat available
The larger the amount of surplus heat available, the greater will be the proportion of the AFT of the fuel developed, and the greater will be the volume of fuel bed at high temperature. The larger furnace will therefore operate at considerably higher temperature than will the smaller furnace at the same ore-to-fuel ratio, or alternatively, less fuel will be necessary to maintain a given furnace temperature. If the rate of heat input (i.e., the space velocity) were to be increased, the heat losses to walls and top gas would increase less than proportionately, and the heat absorbed by the work would increase only moderately due to the higher furnace temperature, so there would be increased net heat available for the creation of the higher temperature. Thus a smaller furnace can reach high temperature if blown sufficiently hard (high space velocity); but this can have serious negative side-effects such as decreased residence time, rapidly increasing pressure of air supply necessary, or fluidization of the furnace contents. TEMPERATURE CONTROL
The effective controls over furnace temperature are therefore space velocity, heat loss rate through furnace walls and in top gases, and the ratio by weight of work to be processed to fuel added. It should be
21
How Furnaces Work
noted that throughout this book ratios by weight of work to be processed to fuel added are given in that order, i.e., an ore-to-charcoal ratio. In much of the archaeological literature dealing particularly with smelting furnaces, the ratio is given in reverse (i.e., charcoal: ore). The latter is an accountant's or plant manager's figure that gives information on the cost of operation; the ore: charcoal ratio is that of a furnace operator who knows that the heat supply rate is determined by the air supply rate and that the ratio of ore to fuel in a given furnace then determines the furnace operating temperature which must be controlled. MATERIALS OF CONSTRUCTION
Since nearly all materials decrease in strength as their temperature increases, the primary requirement of a material with which to build a furnace is that it possesses sufficient strength at maximum operating temperature to prevent the collapse of the structure. This is particularly true of the roof arches of kilns and of the perforated floors that are characteristic of certain kiln structures where the biomass fuel is in a firebox separated vertically from the working chamber. The inner surfaces of both wall and roof materials reach face temperatures that are somewhat lower than the interior of the furnace, because of heat loss to the exterior face by conduction. The temperature of the wall material decreases through its thickness from the interior surface at a rate that increases with its thermal conductivity, and so it is the middle and outer cooler and stronger layers of the wall that carry the compressive stresses of wall or arched roof. However, internal perforated floors are immersed in high temperature gases on both sides, so their interior temperatures can nearly reach gas temperature. They therefore must be made relatively thick and have poor thermal conductivity to decrease centre temperature in order to last through a full kiln cycle, and supporting pillars must often be used. A kiln is a permanent structure, and with continued use the roof in particular will gradually become thinner from being fluxed by fuel ash carried in the flame. Applying insulation to the exterior of the roof to decrease heat loss rate will accentuate this action by increasing the inner face temperature and so will decrease the life of the kiln. Another cause of deterioration in structure is the cracking that occurs from expansion and contraction of material on repeated heating and cooling, so the kiln structure should be examined after every firing and patched or rebuilt as necessary. The area of most severe wear is the arch or slot connecting the firebox with the work chamber and the area of roof near it, since this is where the gas temperature can be highest. Kilns in
22
Mastery and Uses of Fire in Antiquity
general present more serious refractory material problems than do bowl and shaft furnaces, primarily because of high stresses in roofs and in perforated floors. The comment has been made by Freestone (1987) and quoted by Craddock and Meeks (1987) that no refractories were known in antiquity, in the western world at least, that could have withstood temperatures of 1,400oC, which limited furnace temperatures. This is mistaken, since sandstone is a not uncommon rock and its high silica content can withstand appreciably higher temperatures. It was certainly used in antiquity and to this day is used routinely in modern shaft furnaces with inner wall temperatures of about 1,600 o c. Also higher alumina clays such as kaolin make useful mortars for such temperatures. The difficulty concerns what is meant by the word "withstand" since, for example, the ability to resist fluxes or a rise in internal temperature is related to the time involved. As many furnace operations in antiquity lasted only hours or a part of a day, the thinned or distorted refractory could be replaced before re-use of the furnace. In bowl and particularly shaft furnaces using charcoal as fuel, as will be shown in chapter 7, temperatures over 1,600 o c can occur in a short region about 50 to ISO mm high just above air entry. The inner face of the local wall is then at somewhat lower temperature for the same reason described above for kiln walls; but in this narrow region, erosion of lining material is indeed more rapid than anywhere else in the furnace. However, with good clay and/or pieces of sandstone in this region, several hours of operation can be obtained before serious thinning occurs, and the area is easily repaired for the next use. Above this region, temperatures decrease rapidly with height, and only occasional patching is necessary, wear being mostly due to abrasion by the descending burden. The question of mechanical strength of the wall material in a shaft furnace is much less important than in kiln construction, since vertical stresses are largely due to the weight of the wall. Hoop stress from the weight of the burden is usually resisted by tapering the wall thickness to be greater at the base, particularly in taller furnaces. Walls arc usually tapered to be thicker at the base, and circumferential binding with vines as used in some African furnaces can help control cracking if the wall is thick enough or the smelting time is short enough so that the surface temperature just above tuyere level does not destroy the vine. Above temperatures of about 1,000oC, materials with good strength become scarcer. The most common material that has good strength at high temperature is, as noted, sandstone, which can retain useful compressive strength to about 1,500oC; but other igneous rock materials such as diorite and serpentine can carry loads up to about 1,200°C or so.
23
How Furnaces Work 2..S
...." /s
\
cJ
111TJ~r
Fo.ct:
~ ~~~!.
~·C I,OOO'C
oo·c
o+-----.-----.-----.-----,-____~ o ISO WAil
rio Ie lc.'TIess - m=
Figure 3 Heat Loss Rate As a Function of Wall Thickness and of Inner Face Temperature
The softening temperatures of different kinds of fired clays are quite variable and unpredictable because of varying composition. Of the reasonably common clays, kaolin has the highest softening temperature, followed by the clays associated with coal measures. Common glacial clays can be very plastic but usually have moderate softening points. Selection in antiquity was necessarily by trial and error, and in some localities the materials available for kiln construction were not good enough for the temperatures necessary for firing a local pottery clay. This is evidenced in the archaeological record by badly sagged or completely collapsed kilns. The importance of a low heat loss has been discussed, and different refractory materials conduct heat at widely different rates. Solid materials such as a fired clay brick have thermal conductivities which are measured in watts per cm 2 per degree c, in the order of 0.8 to 1.0, while air at ambient temperature has a conductivity of 0.013. Thus porous materials such as pumice or ashes have low thermal conductivities which decrease as their density decreases. As the refractory wall of a furnace is made thicker, the rate of heat loss through it becomes smaller (see figure 3). This has three effects. One is that less fuel is then necessary to reach a desired internal temperature; another is that more heat is stored in the refractory and so wasted; the third effect is that the internal face temperature increases
24
Mastery and Uses of Fire in Antiquity
unless the rate of heat supply is decreased. The first effect is advantageous, but the third effect can give trouble by softening wall material, and sprung arches for this reason should not be made too thick nor should exterior insulation be applied to save fuel. The effects on shaft furnace operation of heat loss rate through walls can be shown by comparison of the results of iron smelting runs published by Tylecote et al. (I97I) and unpublished similar runs which I made in the late I980s. The two furnaces were approximately the same size, 300 and 240 mm LD. respectively, as were the air supply rates at 300 and 280 Iitres per minute. However, the Tylecote furnace was heavily insulated, being built of a three-part wall of firebrick, sand, and insulating brick, of total thickness 330 mm, while the wall of my own furnace was simply 32 mm of castable refractory in a thin steel shell. The heat loss rates through the two walls, calculated from data in Trinks (I95I), were 2.9 and 8.9 KW respectively. Primarily because of the large difference in heat loss rates, my furnace consumed twice as much charcoal per kg of bloom made, and the rate of bloom formation per hour was a little less than half that of the Tylecote furnace. While the latter was abnormally well insulated, this demonstrates the considerable advantage of a well-insulated (and preheated) furnace and is also instructive in assessing fuel consumption in different furnaces. CONDITIONS IN ANTIQUITY
The above discussion on furnace operation and construction has been in terms of modern knowledge of the properties of materials and of heat generation and flow. This was necessary to demonstrate the effects of various conditions, but the knowledge in turn can be applied to the study of remnants of ancient furnaces, to approach an understanding of how they were built and for what purpose, and also to recognize features that were in fact unnecessary. As furnaces in antiquity, particularly early ones, were quite small, this meant large heat losses per unit of output, and the necessity of relatively large rates of heat input. The latter depends quantitatively on the rate of combustion air input, which in fact was not fully understood until in the nineteenth century A.D.; and the importance of leaks in bellows and at connections was not appreciated. Similarly, in natural draft kilns, the importance of height of the top of the smoke-hole above fuel chamber floor on the rate of combustion air flow was not fully recognized.
3 The Properties and
Combustion of Biomass
Biomass has become the all-inclusive name for living and recently dead vegetable matter which occurs in large quantity and in enormous variety of types and species spread over the face of a large part of the earth. Among its qualities is that it can be burned readily, and it was essentially the only fuel used throughout antiquity. The more common varieties of biomass that have been and still are widely used as fuel include trees both as trunks and as branches, shrubs, grasses, straw, and chaff from food grains, trimmings from orchard and vineyard pruning, other waste vegetable and organic products, and at one remove, animal dung. Seaweed and other aquatic plants such as water hyacinth can make excellent fucI if sufficiently dried, but the difficulties of harvesting and drying make them practical only where land-grown biomass is not available. Large quantities of biomass exist in fossilized condition as coal, lignite, peat, oil, and natural gas, but these seem to have been little used in antiquity. COMBUSTION OF BIOMASS
Three features of biomass are held in common, and are of basic importance to how it burns and to the properties of the products of combustion. One is the presence of a variable but often considerable amount of water in the structure of the material as grown. Another is the complex geometry of pieces of biomass when it is cut to the sizes that can be assembled as a mass or bed for burning, which creates a high fraction of
26
Mastery and Uses of Fire in Antiquity
void space in the bed. The third feature is that the combustion of biomass is characterized by its decomposition into a complex mixture of gases that is called volatile matter (VM), and a residue of solid material called charcoal, that is high in fixed carbon content (FC). The VM burns largely above the fuel bed and the FC burns within it, their products of combustion combining to produce a luminous high temperature flame. WATER CONTENT
The water content of biomass as harvested can be as high as 80 per cent, and for woody types can be 60 per cent or more depending on the species. Wood is difficult to ignite above about 60 per cent water content. Water also decreases the potential heat content of the biomass, which decreases the AFT of combustion. Biomass is therefore air dried as far as practicable before combustion. The importance of wood fuel being as dryas possible was recognized in an Assyrian chemical text, reported in Oppenheimer et a1. (1970). This stated in part, "The wood which thou shalt burn - shalt be of styrax, thick decorticated billets which have not lain exposed in bundles, but have been kept in leather coverings, cut in the month of Ab. " The time necessary for drying biomass can be highly variable, depending on the difference between the harvested and the desired water contents, the density of the material, the average section thickness of component pieces, and the local climate in terms of sunshine, wind, and air humidity. It can require many months. For example, maple wood cut to a certain size range and air dried for three months contained 27.0 per cent water, and after two years drying still contained 18.7 per cent water. Pine wood of similar size contained 44.7 per cent water after three months drying, and 10.5 per cent after two years (Mallock and Baltzer 1935). Softwoods tend to have higher water contents as harvested, but to then air dry more rapidly than do hardwoods, probably related to the difference in their densities. It is unusual to find air-dried wood with less than about 20 per cent water content in temperate climates unless it is in thin pieces and has been dried for a year or more. However, wastes that are naturally thin, such as straw and chaff, often contain only 10 to 15 per cent water. EXCESS AIR
The high proportion of empty spaces or voids in a fuel bed of biomass facilitates the entrance of combustion air and passage of products of combustion, but the void fraction is usually sufficiently large that part of the air entering the bed passes completely through it without react-
27
Biomass Combustion
ing with fuel. Since biomass is usually burned on the ground or on the floor of a firebox, an appreciable amount of the air supply also passes over the fuel bed. These air flows combine to become part of the products of combustion and are called "excess air." This is measurable by the free oxygen content of the total products of combustion, and represents air flow above the amount necessary for perfect combustion of the fuel. The amount can be considerable, being seldom less than 25 per cent and more typically 50 to as much as ISO per cent of that necessary, depending on the kind of biomass and the condition of the fuel bed. The two disadvantages of excess air are that by dilution it decreases the AFT of the products of combustion, and that they are then quite oxidizing in chemistry. EFFECTS OF WATER CONTENT AND OF EXCESS AIR
The effects of water content and of excess air on the AFT of woody biomass have been studied by Tillman (1987, 211) for combustion by air at ambient temperature. These are shown graphically in figure 4 for Douglas fir wood, from which it is seen that a I per cent increase in water content decreases AFT by I 1°C, and a I per cent increase in excess air decreases AFT by goc. Thus when Douglas fir at 25 per cent water content, attainable by extended air drying, is burned with 25 per cent excess air, an AFT of I,700oC is attainable, but at 60 per cent excess air its AFT would be decreased to I,420 o C. Tillman showed that the AFT depends as well on the species of wood being burned. While excess air and water content can have serious effects on the temperature and composition of the products of combustion, they only moderately increase the amount of air necessary to supply to the fuel bed. For example, if the air necessary for perfect combustion of a particular oven-dry wood is 6.2 m 3 per kg, then when its water content is 25 per cent and the excess air to burn it is 50 per cent - which would be similar to conditions in a kiln in antiquity - it can be shown that the air necessary would be 7.0 m 3 per kg. CHEMICAL ANALYSIS OF BIOMASS
A standard method of analysis of biomass materials, both living and fossil, used today is called "proximate analysis," and its results are reported on an oven-dry basis. In this method the sample is first dried and then weighed and heated to 950°C in a weighed platinum crucible with a cover to exclude air. It is held at temperature for seven minutes,
28
Mastery and Uses of Fire in Antiquity :
1,800
~I
~
1,600
1.4-00
/, ()OO 0
/0
50
Figure 4 Effects of Moisture Content and Excess Air on Flame Temperature of Biomass Fuel
which drives off very nearly all of the VM, and the loss in weight is taken as the VM content of the sample. The residual material in the crucible consists of mineral ash and charcoal, and when the crucible is reheated in a current of air to oxidize all of the charcoal, the further loss in weight is called the "fixed carbon" (FC) content. The final residue is of course the ash content, since the sum of VM, FC, and ash percentages equals 100. VOLATILE MATTER
Biomass materials are composed of cellulose, the hemicelluloses, lignin, and "extractives," and as harvested always contain water. When such material is heated out of contact ruith air, first water is evaporated, and then at temperatures starting at about 225°C, pyrolysis of the celluloses and then of the lignins begins. Pyrolysis is the destructive distillation of carbonaceous materials in the absence of air and results in the formation of four kinds of product: gas of mixed organic composition, a material called" pyroligneous liquid," tar, and charcoal. The pyroligneous liquid contains water formed from the hydrogen and oxygen in the chemical structure of the biomass, as well as a wide variety of organic chemicals. All of these products except the residual charcoal are gases at and above the temperatures of their formation and contain half or
29
Biomass Combustion
more of the heat content of the biomass being burned. They also burn with a luminous flame because of the presence of microscopic particles of unburned carbon from particularly the 5 to 15 per cent of tar. Table 2 gives the proximate analyses and heat contents of a selection of biomass materials, taken from Tillman (1978). Included in this table are the elemental carbon contents of some of the materials, which are different from FC because they are determined by a different, purely chemical procedure. Also included are the "higher heating values" (HHV) to be discussed. In the relevant literature on the properties of wood there are of course very many more analyses of different kinds of biomass, and it is characteristic of the subject that often moderately different figures are published for what is named as the same item. This is because of the enormous variety of relationships between soil, climate, and genetics, so that samples taken from an oak tree in one location may have appreciably different chemical analysis, heat content, and ash content from one of a different species grown a few kilometres away. Table 2 Proximate Analyses, Carbon Contents, and Heat Contents of Selected Biomass Fuels on Oven-Dry Basis
Ash o/r)
C
HHV
ex)
MJIKg
12.6
0.1
50.6
87.0
12.7
0.3
Red alder wood
87.1
12.5
0.4
49.6
19.3
Black oak wood
85.6
13.0
1.4
49.0
18.7
Western hemlock bark
73.9
24.3
0.8
Cotton gin trash
75.4
15.4
9.2
42.8
15.6
Grape pomace
74.4
21.4
4.2
54.9
21.8
Olive pits
80.0
16.9
3.1
49.1
19.4
Rice hulls
63.6
15.8
20.6
38.3
14.9
Walnut shells
81.2
17.4
1.4
19.5
Straw
67.0
25.0
6.0
13.5
VM
FC
%
~}
])ouglas fir wood
ll7.3
Western hemlock wood
20.4 19.9
22.0
Burning biomass in a simple heap on the ground is wasteful, since most of the heat in the flame is developed above the fuel bed and lost to the atmosphere. However, if it is burned in an enclosure so that the flame can be collected and directed over and among objects to be
30
Mastery and Uses of Fire in Antiquity
heated in a separate chamber, much of the heat in the VM can be recovered. If the fuel mass is on a grate in the enclosure so that air is drawn through the mass, there is much better mixture of air and fuel, although typically about 25 per cent excess air still gets through. The use of biomass as fuel thus largely determines the shape and size particularly of the firebox but also of the furnace in which it used. HEAT CONTENT
Heat content, also called "heating value" and "heat of combustion," is determined by burning a sample of material in a specified way in pure oxygen in a device called a bomb calorimeter. This measures the total quantity of heat developed, and the value is called the "higher heating value" (HHV) when it includes the heat in the steam that results from the hydrogen that is in all biomass; it is the value usually given in handbooks and is the practice in this book. The" lower heating value" (LHV) results from ignoring the heat of condensation of the steam, and averages about 94 per cent of the HHV. It is a useful empirical fact that the heat contents of a wide variety of biomass including most fossil biomass are directly proportional to their elemental carbon contents (not their FC contents) on an oven-dry basis. This was shown by Tillman et al. (1981, 110) to be given with a correlation factor of 0.96, as: HHV
=0.475
xc - 2.38 where HHV = MJ/kg. C = per cent carbon
Biomass has a fairly narrow range of carbon contents, wood and woody materials being mostly in the range 48 to 50 per cent, and straw and food plant residues being in a wider range of about 40 to 50 per cent. This means that on an oven-dry basis, wood has a heat content of 20-4 to 21.4 M]/kg, and plant residues contain about 19.0 to 21.4 MJ Ikg. These values do not quite agree with those of the specific materials given in table 2, but this is typical of the small disparities in published figures mentioned above. For comparison, the heat content of an average bituminous coal, which is fossil biomass, is 30.0 MJ/kg. Dry wood substance has a considerable range of densities, from 1,080 kg per m 3 for ironwood to 120 kg per m 3 for balsawood, and since the heat contents of woods are measured per unit weight, the heat content per unit volume of dry wood substance in a firebox varies considerably with the species of wood. In addition, biomass is usually
3I
Biomass Combustion
burned as a heap or bed of individual pieces that are of an irregular stick geometry, such as cordwood, branches, shrubs, and pruning debris. This causes the high and variable void fraction noted above, which also gives a low and variable heat content per unit volume of fuel bed. For example, brushwood of bulk density 400 kg per m 3 with a water content of 46 per cent and a void fraction of 0.70 would have a heat content of about 1.4 GJ per m 3 , while hardwood of bulk density 700 kg per m 3 and a 30 per cent water content, cut into short lengths and piled with 0.50 void fraction, would have a heat content of about 5.2 GJ per m}. Such an increase of 280 per cent can have a considerable effect on the relationship between size of firebox and intensity of the heat generated. ASH CONTENT
Biomass contains a variable but on the average quite low content of mineral matter or ash that remains after complete combustion. This has been taken up from the soil and water in which the biomass was grown, on a selective basis that varies with species, and so has a widely variable chemical composition. Wood ash is usually very low in sulphur and silica contents and high in alkali content, especially lime, and to a lesser extent, potash, but particular trees and shrubs produce ashes of odd composition that can be used as raw materials for other purposes. For example, the ash of certain shrubs in Mesopotamia contains as much as 30 per cent of sodium oxide, which was valuable as a necessary component in glass making (Oppenheimer et al. 1970). The ash of temperate climate elm and ash woods contains 62 to 77 per cent of calcium oxide, which is quicklime; bamboo and some varieties of straw ash have very high silica content. Ash contents and their chemical compositions have been discussed in some detail by Kurth (1944), who provides a table containing the compositions of the ashes from twenty-three species of wood. This shows an average ash content of 1.24 per cent of the dry wood, composed of an average of 44.3 per cent CaO, 19.8 per cent K 2 0, 5.1 per cent Na 2 0, 7.4 per cent MgO, 1.8 per cent Fe 20}, 5.2 per cent Si0 2 , 10.2 per cent P20" 3.0 per cent SO" and 1.8 per cent Cl. From an archaeological viewpoint the interest in ash has two aspects. One is that chemical analysis of an ash layer on the floor of a firebox could make possible the identification of the species of biomass used. The other aspect of note is that the quantity of ash can indicate how much heat had been generated and fuel consumed. If the heat content of air-dry wood is about 15.0 MJ/kg and the average ash content is 1.24 per cent, each
32
Mastery and Uses of Fire in Antiquity
kg of dry ash represents the generation of 1.21 GJ of heat from 80.5 kg of air-dry wood burned. This could be very useful in analysing the operation of the kiln. FLAME DEVELOPMENT
"Flame" is the word used here for the products of combustion of biomass that contain the heat from both the VM and the charcoal. It is usually visible because of microscopic bits of unburned carbon, and is then an excellent emitter of radiant heat. The development of the flame is a function not only of the amount of VM in the fuel but also of the thoroughness of mixing of excess or secondary combustion air, and the velocity of the gases, i.e., the rate of combustion. High VM content, poor mixing of secondary air, and high rate of combustion tend to give a long flame, which in a furnace with too short a distance between firebox and flue will still be burning as it leaves the furnace. This a serious waste of heat, resulting in lower maximum furnace temperature and possible damage to flues. On the other hand a flame that is too short for the furnace will create high temperature near the firebox and considerably lower temperature further into the furnace. Biomass of various kinds contains between about 50 and 90 per cent VM by weight, and the length of the flame developed, other conditions remaining the same, will vary accordingly. Flame length often can be thus adjusted to suit the volume and shape of combustion space by choice of fuel. Charcoal contains only from about 5 to 15 per cent VM, which is driven off in the upper part of a bed of charcoal fuel, and which can produce only a small and invisible flame. Charcoal by itself is an unsatisfactory fuel for kilns for both this reason and the serious loss of the potential heat in biomass, as will be noted in chapter 6. These conditions are easily visible to a kiln operator, and matching of furnace proportions and rate of firing to the VM of the fuel available is not difficult to do on the basis of experienced observation of results. In antiquity, furnaces were relatively small because of limited need for productivity, and in most cases there must have been considerable loss of heat as flame coming from a flue. COMBUSTION AIR SUPPLY
In practice biomass fuels have been nearly always burned with air supplied by natural draft, which is discussed in some detail in appendix 3. Briefly, because of the large void fraction of such fuel beds, resistance to gas flow is low; since hot products of combustion are much lighter
33
Biomass Combustion
than ambient air, they rise and draw heavier fresh air in at the base of the fuel bed, an action that can be seen in any bonfire. The negative pressure or suction due to the hot gas is quite small, but it increases in proportion with increase in the vertical height between the lowest level at which air enters a fuel bed, and the highest level at which products of combustion are released to atmosphere, and less directly with the average temperature within the furnace. A principal advantage of enclosing a fuel bed is the much better control of natural draft possible, and the possibility of increasing its strength by increasing the height between air entry and flue. The low resistance to gas flow of biomass fuel beds, particularly when burned in a firebox attached to a kiln, allows quite moderate levels of suction to draw in air at high volumetric rates, and so to create high rates of combustion and of heat generation as well as higher percentage of the AFT of the fuel used. Furnace temperatures of IAoooe can thus be achieved with suitable well-dried wood, with the considerable advantages of no need to construct and maintain bellows, requiring no physical effort whatever to supply combustion air, and furnace temperature being controllable both as to rate of increase and maximum attained. A desirable precaution in the use of natural draft is that the walls of the furnace should be as impervious as possible, since any leakage of air inward through the walls will short-circuit the internal negative pressure or suction due to the flue. The walls can easily be sealed with a clay wash on the outside, and this can be a diagnostic marker on a remnant of a furnace wall that it was operated by natural draft. Rate of temperature increase and its maximum are determined by rate of combustion, but for biomass fuels in a firebox this requires control of both rate of fuel replenishment and rate of air flow. As an example, consider a kiln with a particular size of firebox opening, rate of heat loss, and height and size of flue opening. When the firebox has been freshly replenished, there is some moderate room above the fuel for air to pass over the bed, to become part of the excess air passing through it. AFT will be inversely proportionate to this excess. Then, as fuel is consumed, the space above it increases, total excess air increases, AFT decreases, and rate of temperature increase in the kiln slows. For these reasons there will be a level of partly consumed fuel in the firebox corresponding to a rate of heat generation and AFT, and the kiln operator, knowing his kiln, will regulate his rate of fuel addition according to his objectives. However, there will remain a cyclical effect on kiln temperature, which is dampened by the thermal inertia of the kiln and its contents.
34
Mastery and Uses of Fire in Antiquity
A sheet damper or shutter over the opening to the firebox can control air flow rate independently, but must be periodically removed to permit addition of fuel and then replaced, making a repeated change in conditions of combustion. On the other hand, a shutter over the flue opening will also decrease air inflow rate to the firebox, which can then be left open. There is very little evidence on how combustion air supply was controlled on ancient kilns, since few of them have apparently survived complete enough to show whether a shutter was used over the firebox entry or over the flue. There is, however, pictorial evidence on several pieces of Attic black and red figured pottery of mid-first millennium B.C., showing men opening or closing covers over flues of kilns (Noble 1988), and this may have been a general practice in antiquity, a flue or hole in the top part of the kiln being considered necessary simply to "let the smoke out." It was not until in middle of the seventeenth century A.D. that a treatise was published (Glauber [1646] 1942) that explained what was happening in natural draft and pointed out the quantitative effect of chimney height on increasing air flow rate. The likely mode of operation in antiquity was to regulate the rate of heat generation and the AFT by the rate of addition of fuel, the maximum rate of heat generation and the AFT being predetermined by the area of firebox opening and the height and area of the flue, as part of the individual kiln design. A cap or cover over the flue would be used primarily when a reducing atmosphere was wanted in the kiln, which would be only for particular products such as Attic ware. There is no reliable general relationship between the volume of combustion air drawn in to a kiln and the quantity of heat generated, as there can be for fuel beds of charcoal. This is because the amount of air necessary for combustion of one kg of biomass varies with its particular water content and excess air, and the latter varies as the space above the fuel bed varies as the firebox becomes low in fuel and is replenished. However, rough estimates are given in appendix 3. In antiquity biomass fuel was burned in kiln fireboxes usually on a flat surface, with combustion air drawn over and through the mass or pile of fuel. Modern experience is that woody biomass burned in such manner will generate heat at the rate of about 50 to 100 kilowatts per m 2 of covered firebox floor (Vimal and Bhatt (989). This translates to burning air-dry wood at the rate of about 30 to 40 kg per m 2 per hour. If the fuel is supported on a grate so that all air must go through the fuel bed, the rate of heat generation per m 2 can increase by several times; but I am unaware to what extent or when grates under fuel beds of biomass were used in antiquity.
35
Biomass Combustion
REDUCING ATMOSPHERE
A reducing atmosphere is one in which the ratio of its carbon monoxide to its carbon dioxide contents is high, and as a result oxygen can be removed from a heated oxide of a metal, thus reducing it to metal. Clay often contains appreciable quantities of iron, and when it is fired in a biomass-fuelled kiln with its normal oxidizing atmosphere, the iron is oxidized to hematite (Fe 2 0,), which is bright red and colours the fired clay proportionately to the" amount of iron present. H the kiln atmosphere is made to be less oxidizing by decreasing its contents of air and carbon dioxide and increasing that of carbon monoxide, the iron will be oxidized to only wustite (FeO) or to magnetite (Fe 3 0 4 ), both of which are black; so a range of colours from red through browns to black will be produced depending on the degree of oxidation or reduction. Also glazes consist of mixtures of oxides of metals, and the colour of some glazes depends on their state of oxidation; so for aesthetic reasons a reducing atmosphere is sometimes desired in a kiln. This can be done by restricting the combustion air supply so that not only is there no excess air but there is not enough air for complete combustion. This decreases carbon dioxide and increases carbon monoxide contents but also decreases the temperature of combustion. Partially closing an air inlet or a flue will do this, as will the addition of a mass of fresh green fuel, the evaporation of its water content absorbing heat. H these measures are applied too vigorously or for too long, the temperature of the combustion and so of the fuel can decrease sufficiently so that the kiln temperature no longer increases but falls. "RUNAWAY" POSSIBILITY
Although the relationship between volume of combustion air and the quantity of heat developed when burning biomass is not a uniform one, certainly a higher rate of supply of combustion air in general produces heat at a greater rate, and so in a kiln of particular construction and heat loss rate a higher air supply rate can generate furnace temperatures that are a higher proportion of the AFT of the fuel. However, as furnace temperature increases, the power of the natural draft then increases, which further increases air flow rate, so there is then a runaway condition. As much as 90 per cent of the AFT of the fuel could be thus developed in a well-proportioned kiln, and with an AFT of, for example, I,600 o c, such as could be possessed by a well-dried hardwood, the furnace temperature could soften or mclt some of the materials of
36
Mastery and Uses of Fire in Antiquity
which a kiln is constructed and possibly cause the kiln roof to collapse - a disaster. There are examples of such over-heated kilns in the archaeologicalliterature, and these are evidence of the high temperatures that can be developed in simple biomass-fuelled kilns. In practice the maximum air rate was often limited by chance or ignorance, such as by too small a firebox or its opening, and/or too short a height from air entry to smoke-hole. DUNG, PEAT, AND STRAW AS FUELS
Dung is biomass that has been processed by a digestive system, and the most common varieties are from herded herbivorous animals such as cattle, sheep, pigs, and llamas. As produced, the chemical composition is similar to that of the feedstock but may contain more or less water, VM, and ash, depending on the species of animal and associated feedstock involved; the fibre structure is less organized than that of the feedstock. Drying is important to increase the heat content and the AFT of dung when it is burned, and on a completely dry basis its heat content varies with the species of animal but not widely, from about 17 to 18 MJ/kg (Winterhalter et al. 1974). This is about 10 per cent less than the heat contents of grasses and low resin content woods. Air-dried dung is an important fuel for warmth and the preparation of food in tropical and sub-tropical countries even today, particularly where fresh or fossil biomass is scarce or expensive. As a fuel for generating high temperature heat, dung dried to similar water content could satisfactorily replace or supplement wood in natural draft kilns and reverberatory furnaces, since it not only can produce a flame but is already in a particulate form that makes a fuel bed of moderate void space. This would decrease excess air and so give higher flame temperature (chapter 4). However, as a metallurgical fuel for bowl or shaft furnaces, dried dung would act like wood and be converted to charcoal in the upper part of the fuel bed, which is then burned as it sinks to air entry level, as described in chapter 7. Peat, which is vegetable matter that has started to decompose out of contact with air and which in time can become lignite and eventually coal, has high moisture content as found. It often has little directional structure and contains VM, and so can be similar in properties, heat content, and combustion characteristics to dung. Archaeological evidence of its use for metallurgy is slim, though it would be a good kiln fuel if thoroughly dried. Straw and chaff are common agricultural residues and burn readily. Their advantage is a usually low moisture content and easy drying; disadvantages are a quite high ash content, heating value about 17 per
37
Biomass Combustion
cent below that of wood, and in particular, small size and very low bulk density. The latter limits the rate of heat generation possible since as combustion air rate is increased, its higher velocity can entrain larger amounts of fuel that can escape in the flue gases before being fully burned. But in practice these fuels can be successfully used, and even today straw is burned in kilns for firing clay building brick.
4 Furnace Configurations for Biomass Fuel
A fuel bed of burning biomass generates heat both within the bed and as a flame outside the bed. Thus to transfer heat to one or more objects, they can be immersed in the fuel bed before its ignition, or contained in an enclosure to be heated by the flame from biomass burned in a separate, but connected, firebox. Each has its advantages, but the latter arrangement can be under much better control and is more efficient in the use of fuel. It became dominant in the Near East apparently by about the sixth millennium B.C. OBJECTS MIXED WITH FUEL
The simplest procedure to heat objects is to mix them with the fuel as a heap on the ground before ignition as a bonfire. Heat is transferred to the objects largely by radiation and conduction, but there are large losses of heat as flame and radiation to surroundings, so fuel consumption is high. Also since access of combustion air is uneven due to variable wind and to bed structure, there is uneven temperature distribution, particularly in hollow objects such as clay pots; this can result in their cracking due to uneven thermal expansion. If the heap is covered with turf and potsherds before fuel ignition, heat loss is considerably decreased, and access of air, and resulting temperature, can be not only more even but controllable by openings in the base of the cover. This arrangement is still in some scattered use and was the progenitor of the separation of the generation and application of heat.
39
Biomass Fuelled Furnaces
WORK SEPARATE FROM FUEL
An important and considerable step in the ability to control temperature, increase its maximum level, and decrease fuel consumption was taken when the combustion of fuel was separated from the work to be heated. Since objects placed in a work chamber by themselves have much lower resistance to the passage of gases than when the spaces between them are filled with fuel, much higher rates of gas and therefore heat flow can be created by natural draft. This considerably increases the maximum furnace temperature attainable. An important further result is the ability to develop and more efficiently use the hot luminous flame from the combustion of the VM in the biomass fuel. There are many ways of joining one or more fireboxes to a work chamber, with three basic objectives. These are obtaining as complete combustion of fuel as possible, providing adequate length of combustion space to fully develop the combustion of the VM flame, and giving good distribution of hot products of combustion in the chamber holding the work. One widely used variation from a simple work chamber with one or more attached fireboxes at the same level was used at least as early as the middle of the six millennium B.C. at Yarim Tepe (Merpert and Munchaev 1973), and is still in use today. This was to place the work chamber over the firebox with a perforated floor between. A smokehole or flue in the top of the work chamber created natural draft, which drew combustion air through openings in the lower wall of the firebox and then hot products of combustion through the perforated floor of the work chamber to heat the work in it. This construction is thermally efficient and can increase the uniformity of distribution of temperature. To this end there was considerable variation through time in the geometry of the supports for the floor of the work chamber, to give necessary support while interfering as little as possible with uniform distribution of air. A commonly used variation was to build a permanent combustion chamber with a flat perforated roof on which ware to be heated was placed, and over which was built a domed cover with vent on top, which was demolished again after firing was completed. Another variation was to place the firebox at a side or end of the work chamber but at a lower level, and to make several horizontal loosely covered channels across the floor of the work chamber, connecting the exit from the firebox to a common flue up the end wall of the kiln. The floor thus became a radiant distributor of the flow of hot products of combustion, as a conductive and radiant heater. In later antiquity there was also considerable use in kilns of walls that had verti-
40
Mastery and Uses of Fire in Antiquity
cal clay pipes set into them to carry the products of combustion, thus separating the gases from the objects to be heated. This was extended to the use of ceramic plates with knobs on them as spacers, fastened to the wall to create a continuous open space for products of combustion (Brodribb 1978). Kilns inherently use large quantities of fuel because of their low thermal efficiency, and variations in geometry of kilns to decrease fuel consumption andlor increase uniformity of product were endlessly explored by trial and error. Illustrations of some of the very wide variety actually used can be seen in, for example, Majidzadeh (1975), Barnard and Tamotsu (1975), and Alizadeb (1985), to mention only three of many collections. Classification of kilns by geometry is difficult because of the great number of small and large variations in the archaeological record, but a system based on how the hot gases from the firebox are distributed to and among the work would seem to directly use a basic factor. Temperatures Attainable The important details of kiln design and operation that lead to increased maximum temperature are as follows: sufficiently high ratio of volumes (or floor areas) of firebox to those of work chamber, and of areas of flue opening and total air entry; choice of kind of biomass for flame length suited to the length of path of combustion in the work chamber; greatest possible dryness of the biomass; and adequate internal height from the floor of the firebox to the outer edge of the smoke outlet or flue opening, which directly controls the draft pressure available. A kiln will reach an equilibrium maximum temperature that depends on these factors, and to the extent that they can be determined on an excavated kiln, we can begin to retrieve its operation. In kilns of moderate size such as 10 to 15m2 work floor area, a firebox floor area of about one third this volume can be sufficient to develop kiln temperatures of more than 1,200°C. In a kiln of 16 m 2 work floor area excavated at Ayia Triada from fourteenth century B.C. in Crete, chemical analysis of droplets of wall refractory (Levi and Laviosa 1986) showed it to be very similar to that of a slag that had its free-flowing temperature measured experimentally as 1,250°C (Gale et al. 1985; also sec chapter 10). Since the temperature of the kiln atmosphere must have been higher than this because of heat loss through the wall, its temperature would have been at least 1,300oC. There is also the evidence of ancient collapsed kilns mentioned on page 36. However, if a kiln is small, of less than about one m 2 floor
41
Biomass Fuelled Furnaces
area, its heat loss rate becomes sufficiently high that the firebox area may need to approach that of the work chamber to achieve high temperature.
Temperature Distribution in the Work Chamber Combustion of the VM and the development of flame have been discussed in chapter 3, but the distribution of temperature in a kiln is also important. When a kiln is filled with pottery and firing is started, the combustion air is at first drawn in slowly because of the low temperature of the interior of the kiln. But it will increase as kiln temperature rises (see appendix 3), and control of air rate, i.e., combustion rate, is usually necessary to restrict the rate of increase of temperature to avoid cracking of some ware by uneven thermal expansion. Then as kiln temperature increases further to where the ware is less likely to crack, the VM flame can be developed further through the work chamber, by increasing the air supply rate or changing to a biomass fuel that gives a longer flame. The hot products of combustion decrease in temperature as they flow because of the heat being absorbed by the ware and the kiln walls, so throughout most of the firing cycle the temperature of the ware nearest the firebox will be well above that of ware near the flue. The latter will reach desired temperature as the ware and the kiln structure as a whole "fill up" with heat and temperature; but the ware ncar the firebox will then have been at temperature for some time and can become over-fired if care is not taken in the rate of temperature lllcrease. This problem of achieving reasonably even temperature distribution in the ware throughout a kiln can never be solved completely, but the variation can be decreased by judicious placement of objects, placing less sensitive or more massive pieces near the firebox, and the erection of permanent partitions within the kiln to deflect and guide the flow of hot gases, leading to modern terminology of "up-draft" and "downdraft" kilns. The use of clay pipes along kiln walls to carry the hot products of combustion to the flue(s) noted above would much improve uniformity of temperature in the kiln. Although every change in the direction of flow of a gas increases its pressure drop and therefore decreases its rate of flow with a given draft pressure, in practice there is often a self-compensating feature. This is that while the need for guidance of the hot gas flow increases with the size of the work chamber, the increase in size usually includes an increase in the height of the roof, so that the vent or flue in the top cre-
42
Mastery and Uses of Fire in Antiquity
ates stronger draft; heat loss rate is also lower as kiln size increases. However, in smaller kilns it can be difficult to adequately control gas flow path without decreasing net draft and therefore air flow rate, and so decreasing maximum temperature.
Smelting of Ores and Melting Metal in a Kiln It is now well established in the archaeological literature that in Neolithic times pottery kilns fuelled with biomass were able to develop temperatures in the order of I,200°C, sufficient to both smelt copper ores and to remelt copper and bronze. It was noted in chapter 3 that the atmosphere in a biomass-fuelled kiln is very oxidizing and cannot directly smelt an oxide ore to its metal, but as will be discussed in chapter 10, if a copper oxide ore, a flux, and a source of carbon are pulverized, mixed, and placed in a crucible, the mixture is then unaffected by the atmosphere in the kiln and when temperatures of 1,100 to I,200 o C are reached, will form molten copper under a layer of molten slag.
Reverberatory or "Air" Furnace A rectangular plan of kiln with the firebox at one end, a shallow hearth in the middle, and the flue at the other end was eventually developed into an effective and simple furnace for smelting and melting metals, and for melting glass. As shown in figure 5, the sidewalls are low in height, the low arched roof becoming very hot on its interior and radiating heat downward onto the hearth - which is why this shape of kiln or furnace is called a reverberatory furnace. The low roof also decreases the cross-sectional area of the furnace interior, which elongates the flame from the firebox to increase longitudinal heat distribution. The length of the hearth depends on the length of flame of the biomass used as fuel. In size, such furnaces ranged from as little as one metre for glass-melting in England in the seventeenth century to six or seven metres in China in the sixteenth century. The hearth of the furnace is formed as a shallow elongated dish in a bed of rammed silica or beach sand, which is retained by the side walls and a low brick wall at each end. The top of the firewall next to the firebox should be only a small distance above the level of the molten contents of the hearth, to permit the flame to sweep the contents as soon as possible. The top of the bridgewall at the flue end is usually a little higher than the firewall since its height determines the cross-sectional open area above it, which should be less than that above the firewall to help retain heat within the furnace.
43
Biomass Fuelled Furnaces
Fue!
"nl
Illy
'ca.I" Ifemovabk>
~/.y p/"9\..L~"==r~""",, .:~~;~;:[(j?:y~~;:
I ,/',0:" , I
C"'f'".~;tr ~o()l:>kJ
Figure 5 Reverberatory or "Air" Furnace
Material to be melted can be added through a temporary opening in the roof or, before lighting the fire, by removing two or three rows of roof bricks and then replacing them. An opening that can be shut tight (a leak will short-circuit the draft) is usually placed in one sidewall just above liquid level, to permit periodic observation of the contents and to manipulate them as necessary with a bar. A small peep-hole near the base of the stack or flue just above bridgewallievel permits viewing the state of combustion and the evenness of its lateral spread. The temperature of the flame decreases as it flows through the furnace, transfering heat to the material on the hearth and to furnace walls, but it must be high enough at the flue end to continue to transfer heat to melted material on the hearth and not cool it. Flue gas temperature is therefore high, and since the strength of draft increases with temperature, a strong draft can be created by a short flue or stack. The rate of heat generation in the firebox is a direct function of the rate of combustion air supply, and this can be controlled by a movable damper over the firebox feed-hole. Another arrangement that gives smoother operation, since it permits regular and frequent additions of fuel, is to leave the firebox entry partly open most of the time, and to regulate the rate of air flow by a damper across an opening made at the base of the stack, as shown in figure 5 (top). This opening short-circuits the draft suction, and so regulates the rate of gas flow through the furnace. The peep-hole nearby permits frequent observation and adjustment. As with kilns in general,
44
Mastery and Uses of Fire in Antiquity
the ratio of firebox area to hearth area is important. The balancing of their proportions and the ratios of the areas of openings have been discussed for modern practices, which are not greatly different, by Rehder (1953). Temperatures of 1,400oC in the metal, which are ample for melting bronze and cast iron, were regularly attained in Europe in the nineteenth century with wood fuel and natural draft. ATMOSPHERE COMPOSITION CONTROL
The effects of the furnace atmosphere can be avoided by enclosing the ware in a separate container of low porosity. This can be as simple as a covered clay pot or "saggar," which can both keep atmosphere out and provide support for delicate pieces that might deform under their own weight at high temperature. A more extensive step is to construct a double-walled or "muffle" furnace, the inner chamber being in effect a very large inverted pot. In both saggars and muffles, the atmosphere within can be made reducing even at high temperature by enclosing a small amount of any form of dry biomass. The kilns mentioned above with closed channels in floors and up walls were muffle furnaces. FUEL ECONOMY
The hardness and strength of fired clay increases, and its porosity decreases, as firing temperature is increased; and clearly temperatures adequately high to make low porosity or waterproof pottery from many clays have been possible from very early times with biomass fuel and natural draft. However, examining structures of pottery from most of the Near East suggests that much of it was not fired above about 900°C. Also, it was a common practice to coat the insides of amphorae for wine with pitch, so there must have been porosity needing sealing. This has supported speculation that high firing temperatures were somehow not possible at the time, but since it is clear from the discussion above that they were attainable, the reason for lower temperatures was more likely to have been the cost and/or availability of fuel. AN EGYPTIAN KILN OF ROMAN DESIGN
Another instructive example comes from Roman Egypt, in a papyrus dated A.D. 243 (Cockle 1979). The treatise concerns the lease of a fully equipped factory for making pottery, with a capacity of fifty thousand jars per year for the wine trade. Operation was through the winter and spring of the year and finished by June. The fuel used was
45
Biomass Fuelled Furnaces
apparently straw and chaff. The jars held nineteen litres each and were lined with pitch, forty-seven kilograms of pitch being required for each thousand jars. Assuming the operation of the factory to have been for about seven months of the year or thirty weeks, and that wastage was IO per cent, I,830 jars per week were fired. If the weight of a jar is estimated as nine kg, and the firing temperature was 700°C, giving a heat content of the clay of 1.04 MJ/kg, then the heat necessary to fire one jar was 9.4 MJ. The potential heat content of straw and chaff is about 13.5 MJI kg, and if it is burned at a thermal efficiency of 2.0 per cent (the kiln is moderately large but combustion of such lightweight fuel would be incomplete), the quantity of fuel required per jar would be thirty-five kg. One week's production of 1,830 jars would require sixty-four tonnes of fuel, which at an estimated bulk density of I50 kg/m3 would occupy a volume of 430 m 3 • For the thirty-week season nearly I3,000 m 3 of fuel would be required; and the logistics of gathering and transporting this volume of fuel during the season would have been considerable.
5 Products Made in Antiquity in Biomass Fuelled Furnaces
The categories of products in antiquity were simple - ceramics, lime, glass, and the smelting and melting of metals - but each of these was divided into many kinds of products. Fired clay, for example, was used for pottery and other containers, building bricks both plain and glazed, roofing tiles, mosaic tesserae, and small statuary. CLAY PRODUCTS
Through antiquity fired clay was a widely used product of high temperature heat. Our discussion of furnaces so far has taken as examples mostly kilns that have been used for firing pottery, a practice that has continued into the present day. The hardness and strength of fired clay pottery increases, and its porosity decreases, as firing temperature is increased. However, as noted in chapter 4, there is considerable evidence that the majority of pottery used in antiquity was not fired to temperatures much over 900°C, possibly as practical compromises between serviceability, the properties of a local clay, and economy of fuel and of kiln time. Yet stoneware requiring temperatures of 1,200 to 1,300oC was made in some quantity. PLASTER OF PARIS
Two materials, plaster and lime, are made from thermally decomposed stones that when pulverized and mixed with water can make a smooth, adherent mass that hardens with time. The material we call Plaster of
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Products of Biomass Fuelled Furnaces
Paris is made very simply by heating the common mineral gypsum (a hydrated calcium sulphate) to between 128 and 163°e, which drives off half its water of crystallization. If this material is re-hydrated by moistening with water, it will harden into a plaster. However, if in the original heating the temperature of the gypsum has exceeded 163 °e, all of the combined water has been lost and with it the ability of the material to be re-hydrated. If a temperature of 128°e has not been reached, no water of crystallization is lost and no re-hydration is possible. The temperatures involved are those of a food-cooking oven, and must be carefully controlled to within this narrow range. Plaster from gypsum was and still is widely used for making a smooth coating on a wall or ceiling and can easily be moulded into decorative shapes. When hardened, it is finely porous and has moderate hardness but will not withstand elevated temperature. Its porosity when hardened makes it a useful mould material into which a clay slip can be poured, which then forms a leathery skin of a thickness that increases with time, as water is absorbed from the slip by the plaster. When the thickness of skin is what is desired, the remaining slip can be poured out and the leather-hard clay object taken out to be dried and fired. LIME
Lime, which is calcium oxide and is also called "quick-lime," is made by thermally decomposing limestone, which is calcium carbonate of various degrees of purity. When lime is hydrated to calcium hydroxide by the addition of water, it can be made into a smooth plaster that hardens, first by evaporation of water and then by absorption of carbon dioxide from the atmosphere, to re-create calcium carbonate. Since this is the composition of the limestone from which the lime was made, hydrated lime can make a plaster that is much stronger than Plaster of Paris. It can be used as a binder of sand to make a strong mortar. When mixed with crushed pozzolan, a variety of lightweight volcanic silicoaluminate, hydrated lime reacts with it to form a strong concrete that will set under water; this process was extensively used by the Romans. Considerable quantities of lime were used in the Near East from an early date. Excavations at Asikli Huyuk have shown a lime plaster floor in a dwelling dated to 7000 B.e. that contained 1,800 kg of lime. At Jericho a millennium later, many houses had lime plaster floors and walls that would have required the calcining of an average 450 kg of limestone per house (Gourdain and Kingery 1975). A minimum temperature of about 9000e is necessary to decompose calcium carbonate to calcium oxide and carbon dioxide, and the rate of decomposition increases with higher temperature. If during the firing
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Mastery and Uses of Fire in Antiquity
of the limestone its temperature rises much over about 1,100°C, the lime made increases in density and becomes more difficult to hydrate, so good temperature control is necessary in the kiln. Since the atmosphere in the kiln has negligible effect on the decomposition, biomass fuel is satisfactory. Limestone is preferably broken into small lumps before heating, as decomposition occurs at the rate that heat penetrates from the surface of a lump; small lumps thus decompose more rapidly. These can be heated loose on the floor of a kiln or in a crucible, or can fill a specially shaped kiln, as will be described below. As heat penetrates a lump of limestone, the progress of its decomposition inward from the surface of the lump alters the appearance of the stone. Completeness of reaction can be followed by periodically extracting and examining a broken lump. The decomposition of calcium carbonate requires a considerable amount of heat. It takes 0.96 MJ per kg to increase its temperature to 1,OOOoC, and then 1.75 MJ per kg is absorbed by the decomposition reaction. However, limestone is seldom pure calcium carbonate, averaging possibly 85 per cent. In antiquity an average of probably not more than So per cent of the calcium carbonate was decomposed, some of it remaining in the centres of large lumps. The heat of decomposition would therefore be 1.75 x 0.S5 x o.So = 1.19 MJ per kg of limestone, which added to the heat necessary to raise it to decomposition temperature (its enthalpy) would be 1.19 + 0.96 = 2.15 MJ per kg of limestone. Lime (calcium oxide) is 56 per cent calcium carbonate, so the lime made would be 0.85 x 0.80 x 0.56 = 0.38 kg per kg of limestone, and the heat necessary per kg of lime made would be 2.15Io.3S = 5.7 MJ. This is 42 per cent more than the heat necessary to heat and reduce iron oxide to one kg of iron as a bloom, and more than eight times as much heat as is necessary to fire one kg of pottery to Soooc. Clay and limestone were both fired in biomass-fuelled kilns which have low thermal efficiencies, so lime-making would have been a major consumer of fuel through antiquity. Limestone in small lumps can be decomposed slowly in a windblown wood fire, or in an ordinary pottery kiln heated to 9001,OOOoC, but it can be more efficiently done in a vertical kiln, which is geometrically a shaft furnace. In a rare example of recording details of technical matters in antiquity, Cato in early Roman times (165 B.C.) described how to build such a kiln to make lime: Build the lime-kiln ten feet across, twenty feet from top to bottom, sloping the sides in to a width of three feet at the top. If you burn with only one door, make a pit inside large enough to hold the ashes, so that it will not be necessary to clear
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Products of Biomass Fuelled Furnaces
them out. Be careful in the construction of the kiln; see that the grate covers the entire bottom of the kiln. If you burn with two doors there will be no need of a pit; when it becomes necessary to take om the ashes, clear through one door while the fire is in the other. Be careful to keep the fire burning constantly, and do not let it die down at night or at any other time. Charge the kiln only with good stone, as white and uniform as possible. In building the kiln, let the throat run straight down. When you have dug deep enough, make a bed for the kiln so as to give it the greatest possible depth and the least exposure to the wind. If you lack a spot for building a kiln of sufficient depth, run up the top with brick, or face the top on the outside with field stone set in mortar. When it is fired, if the Harne comes out at any point but the circular top, stop the orifice with mortar. Keep the wind, and especially the south wind, from reaching the door. The calcining of the stones at the top will show that the whole has calcined; also, the calcined stones at the bottom will settle, and the flame will be less smoky when it comes out.
Such a kiln is a shaft in the sense that it is considerably taller than wide, though this one was distinctly conical in vertical interior profile. The twenty-foot (6.I metre) height would give a strong natural draft, and the narrowing of the shaft towards the top was probably intended to give increased uniformity of heating by decreasing the weight of stone to be heated per vertical metre. Over-burning of the lime near the grate would be unavoidable. The kiln may be estimated to have contained about 36,000 kg of lump limestone, and assuming it to be of the quality estimated above for heat content necessary, the lime made at each firing would be I3,700 kg or I3.7 tannes. Modern practice would be to place holes for air access around the base of such a shaft, and fill it with a mixture of fuel and lumps of limestone in a ratio of about ten to one by weight. After ignition at the base, the rate of combustion of fuel is controlled by shutters over the air-holes. Combustion of fuel allows the mixture to sink, and when fully burned lime appears at an air-hole, a layer of it is extracted with rakes, and fresh fuel and limestone are added at the top. In this way a continuous operation is achieved with less fuel consumption and more evenly burned lime. Since at the time of Cato's writing the Romans had a widespread empire and good communications, and evidently were not aware of the advantages of continuous operation, it seems unlikely to have been used elsewhere in antiquity. GLASS-MAKING
Glass-making appeared in the Near East after copper smelting was well developed and could have been a development from experience
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Mastery and Uses of Fire in Antiquity
with metallurgical slags. These usually contain considerable silica derived from the gangue of the ore, and this contributes the quality called "glassiness," described below. Slags are usually opaque because of their content of other oxides such as those of iron, manganese, and other metals, but on occasion can be quite transparent, especially in thin section. There seems to be no archaeological record of steps by which clear glass could have been derived from semi-transparent slag, and the first all-glass vessels were not made until the end of the sixteenth or the early fifteenth century B. C. (Oppenheimer et al. 1970). However, faience had been known for a long time, made from clean silica sand mixed with a little soda or potash, which acted as a binder of grains of sand when heated to moderate temperature. If the mass was heated to a higher temperature, a plastic mass of clear or coloured glass would form. The addition of small amounts of oxides of iron, manganese, copper, and cobalt can create a wide range of colours in glass, with tin oxide producing a white opacity. As long as the silica in a completely fused mixture is more than about 45 per cent, the solidified and cooled melt has the rigidity of a crystal with the random atomic structure of a liquid, so that it is amorphous in structure. This may be taken as a definition of the "glassy" state. Silica is the most common glassifier, though there are others such as boron and phosphorus oxides. The naturally occurring obsidian is a glass that is solidified lava of suitable composition and cooling rate. As a corollary to its disordered molecular state, glass has no distinct melting point, and when its temperature is increased, it simply decreases in viscosity from an extremely high value at room temperature to a pourable liquid at sufficiently elevated temperature. Modern convention is that the "working point" of a glass, where it is manipulable somewhat like warm toffee, is at a viscosity of 1,000 pascal-seconds (Pa.s), and the "melting point," where it can be poured out of a container like a heavy oil, is at 10 Pa.s. Viscosity is the basic property in the handling and forming of glass, and the temperature at which a particular viscosity is reached can vary by 500°C or more with change in composition of the glass. Typical practical working points or temperatures would be in the order of 1,000 to I,200°C. The viscosity-temperature curves for glasses can be quite steep, and this creates a major problem in melting and blending the raw materials to make a clear glass. Until high temperatures are reached, internal mixing is very slow, and inhomogenieties not only can show at the surface of the finished glass but can distort light passing through it. Excessive temperature is not wanted since the container of the materials
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Products of Biomass Fuelled Furnaces
is itself composed of metal oxides and can dissolve and spoil the glass or let glass leak out. The problem was solved in most cases throughout antiquity by melting the raw materials at least twice, the first melt being cooled or quenched in water and then pulverized, thoroughly mixed, and remelted (Oppenheimer loco sit.). A second difficulty in melting glass is that the mixture of granular solids contains about 50 per cent void space, decreasing its thermal conductivity by about half, so that it is slow to absorb heat by conduction from a hot crucible wall. However, the rough surface of a granular mix is a good receptor of radiant heat, so use of shallow saucer-shaped crucibles is more effective. Such saucers were indeed used in ancient Mesopotamia (Oppenheimer loco cit.) but so were more normally shaped crucibles. The kinds of furnaces used for glass-making seem to be remarkably poorly reported both in written literature and in archeological work, attention tending to be on the glass produced. My impression is that the furnace was usually a round or sometimes rectangular biomass-fuelled kiln with a low roof, in which radiation from the VM flame was the important factor. There were various ways of forming the hot plastic mass into useful objects, such as simple manipulation against gravity of a viscous" gob" taken from a crucible on the end of an iron rod; moulding and pressing a gob in a shaped mould; casting into moulds like metal; and blowing with an iron pipe. All were used in antiquity, although the making of blown glass was late. A kiln would be the versatile furnace for all of these methods, including heating crucibles and moulds, but it is puzzling that apparently little use was made of crucibles in beds of charcoal with bellows air supply. Various arrangements of the interior of a kiln were used for glassmaking. Crucibles were of different shapes to increase the rate and uniformity of heating. Apparently a common arrangement was to place crucibles around the interior walls of a circular or rectangular kiln. This was described in a later time period by Theophilus in 1100 A.D. and by Biringuccio in 1540 A.D. It is little different from a pottery kiln design with a low roof. The contents of each crucible had to be accessible through its own hole in the wall of the kiln for the removal of glass for forming when it was in satisfactory condition. Removal was done at intervals to form objects, so the crucible had to be kept up to temperature and the glass at suitable viscosity for considerable periods of time. Another property of glass that made a modified domed pottery kiln an excellent furnace for its manufacture is that unless it is slowly cooled from the temperature at which it can be shaped, thermal stresses build up within the material as its viscosity increases to high levels.
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Mastery and Uses of Fire in Antiquity
These stresses can be strong enough to shatter the object before it is cool enough to handle, or to make the cooled object fly into pieces from the smallest external stress. The necessary slow cooling is called "annealing," and covers a range of temperature from a level of about 700 to 400°C (depending on the glass composition) to room temperature. This essential annealing can be done in a compartment or portion of the upper part of the kiln where the products of combustion on their way to the flue are too cool to soften glass appreciably but are hot enough to anneal it. Since the remnants of most of the kilns used in antiquity are no longer complete enough to determine their full original structures (particularly their height and therefore available draft and attainable temperature), it may be difficult to distinguish between pottery kilns and glassmaking furnaces, and so it is difficult to follow their independent developments. However, one of the objectives in this book is to point out the technical boundaries within which developments necessarily took place, and not their chronology. Glass is today melted directly on a hearth in a reverberatory furnace as was described above for melting metals, but to my knowledge this was not done in antiquity. Useful data on ancient glasshouse operations are scarce, but records are available from sixteenth-century England when glass was still being made with wood as fuel (Kenyon 1967). These furnaces were apparently operated continuously for a week, making 2,000 kg of glass, which represents a daily output of about 128 litres, the equivalent of a 23 cm cube of glass. This required 19 cords of wood per week, which at 1,600 kg per cord of air-dry wood is 15 kg of wood per kg of glass. If the heat content of molten glass is taken as 1. 5 MJ Ikg, and of air-dry wood as 14.0 MJ/kg, the thermal efficiency of use was 0.7 per cent. The volume of glass made per day indicates a small furnace with inherently high heat loss rate and low thermal efficiency, and so may have relevance to ancient practices. It would certainly result in glass being a relatively expensive material. MELTING AND SMELTING METAL WITH BIOMASS FUEL
Chapter 4 noted that the melting and smelting of metal can readily be done in a crucible in a pottery kiln, but that the elongated, low-roofed kiln called a reverberatory furnace is much better suited when quantities of metal larger than 40 or 50 kg are required. A description of how such a furnace can be built was also given to show its simplicity. When a considerable quantity of metal is necessary such as for the casting of a series of ingots or statuary or ritual vessels, it could be
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Products of Biomass Fuelled Furnaces
made in several or many crucibles placed on the flat hearth of a kiln for melting. But their removal when filled with molten metal takes time, temperature loss in the metal would then be considerable, and composition of the poured object variable. Melting it all as one mass gives the important advantages of uniform composition and temperature of the whole mass of metal as poured into the mould. So in general, for pouring large objects in bronze or cast iron, the reverberatory furnace is much simpler, safer, of lower cost, and under better control. The furnace would be built specifically for the large casting to be made and placed close enough to the mould and at a suitable level for the molten metal to be transferred by a short trough or "launder" directly to the entrance to the mould. The material to be melted could be scrap metal, or the mixtures of pulverized ore and carbon to make directly reduced metal as described in chapter 10. The building of the furnace would likely be at less cost than the preparation of the mould; there would be no difficulty in delivering molten metal to the mould at nearly its maximum temperature in the furnace; and more than one large casting could be made by preparing a new mould. A furnace with a hearth, for example, one metre wide and three metres long, containing a 200 mm depth of molten bronze, would contain about 5,000 kg that could be poured at one time. It seems very likely that sections of impressive monuments such as the Colossos of Rhodos, built in 280 B.C. and reportedly 32 metres tall (Sarton 1959), would have been cast from such a furnace. I am of the opinion that air furnaces were used in antiquity from an earlier time and more extensively than is generally believed, because of their simplicity, lack of need for blowing equipment, and easy adaptation from kiln practice. However, archeological evidence is sparse, partly because archaeologists have been unaware of the possibilities, and partly because after two or more millennia the oblong ridges of burned earth or clay that could be the remains of such furnaces can very easily be mistaken for those of pottery kilns. Melting in such a furnace has a valuable metallurgical effect on the composition of cast iron in particular. The hot gases above the metal are quite oxidizing because of the presence of oxygen in excess air, and the metal surface is large relative to its volume. Cast iron contains 2 to 4 or more per cent of carbon and as will be noted in chapter 12, becomes higher in strength as its carbon content decreases. Therefore when cast iron from a shaft furnace (which is usually high in carbon content), and pieces of scrap iron to be recovered are remelted together in an air furnace to accumulate a large quantity of molten iron, the strength of the iron can be considerably increased because of the removal of carbon by oxidation. Carried far enough, this process can make steel, and then would be called" fining."
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Mastery and Uses of Fire in Antiquity
Similarly, in remelting copper or bronze that have too much iron in them from poor smelting practice (see chapter II), the iron is readily oxidized preferentially, to be absorbed as an iron silicate slag by a silica flux. Some of the ancient furnace remains shown in the Chinese archaeological literature and termed "blast furnaces" seem much more likely be the remains of reverberatory furnaces. TEMPERATURES IN BIOMASS FUELLED FURNACES IN ANTIQUITY
I showed above that the combustion of biomass in simple furnaces can easily produce the temperatures necessary for much of the pyrotechnology practised in antiquity, and that not infrequently temperature had to be prevented from rising too high for the purpose intended. It is interesting to note in passing that while the maximum temperature necessary for most of the purposes noted is in the order of I,200 o C, the maximum temperatures that occur in nature (with the exception of lightning) are in volcanoes and are about this temperature. These natural temperatures are not as high as generally thought, many careful measurements having shown that the temperature of molten lava varies from about 800°C to a maximum of I,225°C, the latter being basalts (Williams and McBirney 1979), which can be similar in chemical composition to many smelting slags.
6 The Manufacture and Properties of Charcoal
When biomass is burned with ample access of air, a series of reactions and decompositions takes place that were described in chapter 3. If, however, combustion is stopped before it is complete, as for example by rain or by excluding air by a cover of earth, some charcoal usually remains. Charcoal has very different physical and chemical properties from those of the biomass from which it is made, and for reasons to be given in chapter 7, it was universally used in antiquity for the smelting of ores of metals. It was therefore necessary to be able to manufacture charcoal from biomass with some control of process, and the method developed by an early but unknown date was to burn an enclosed heap of biomass slowly with a very limited supply of air. The heat generated pyrolized (decomposed) some of the unburned biomass to charcoal, and the air supply was limited to burn as little as possible of the charcoal made. MANUFACTURE
When biomass is heated out of contact with air, the products are a solid residue (charcoal) and gaseous volatile matter (VM) in proportions that are sensitive to temperature. As temperature is increased, charcoal starts to form above about 22SoC and the fixed carbon (FC) content of the charcoal increases approximately in proportion to the increase in temperature. For example, when wood is heated to soooc, the charcoal may contain 80 to 8 S per cent FC, and at 900°C, 90 to 9 S per cent FC, the charcoal becoming stronger, harder, and less chemically reactive as
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Mastery and Uses of Fire in Antiquity
the final temperature of its formation increases. The yield of charcoal as a percentage of the weight of the original air-dry biomass also decreases at higher temperatures due to loss of VM and is moderate even in modern externally heated kilns. Depending on the species of biomass, its water content, and final heating temperature, this would be a maximum of 35 to (at most) 50 per cent. However, throughout antiquity yield was much lower. Charcoal was made in antiquity by the heap method, or its inversion, the pit method. In the heap method, wood is piled vertically into a low cone or mound, with an opening left at the centre as a flue. The mound is covered completely by turf except at the flue, and openings are left in the cover around its base to admit combustion air. The wood is ignited by a brand thrown down the flue, and the rate of combustion is controlled to a low level by adjusting covers over the air entry holes. When the heap is considered on the basis of experience to be completely converted to charcoal, a time of one to two weeks, all air entries are closed and the whole left to cool for several days. If the mound is opened while still too warm, the charcoal will burst into flame because of its high chemical reactivity. The method is simple, and many detailed descriptions and illustrations have been published that show the variety of ways in which it can be conducted (e.g., Overman 1854 and Theophrastus 280 B.C.). Yields are inherently low since part of the biomass is burned to generate the necessary heat, some of the charcoal is unavoidably burned, and not all of the biomass in a heap may be sufficiently well carbonized for use in metallurgy and must be discarded. Yield throughout antiquity would vary depending on skill and raw material, with 10 to 15 per cent possibly being an average. The species of biomass used to make charcoal has a strong effect on both the strength of the charcoal and on its chemical reactivity. For example, a hardwood such as birch will make a lump charcoal of good strength and hardness, while a softwood such as pine under the same conditions will make lump charcoal of lower but still useful strength and hardness, but of higher reactivity. Some species of biomass make charcoal so soft that it is not coherent and appears as a powder. Recorded conversations about the smelting of bloomery iron in Africa (Cline 1937) show that for each practitioner trees of a particular local variety were the only ones that his experience showed to make good charcoal. Harder and denser charcoal is the preferred product of a heap, since it results in charcoal of larger average lump size after the abrasions of digging it out of the heap, transporting it to the site of use, and measuring it into a furnace. Size is an important factor in charcoal's efficiency as a smelting fuel for reasons discussed in chapter 2 and again in chapter 7.
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Charcoal Manufacture and Properties
Modern experienced manufacturers of charcoal (Overman r854, Constantine 1975) have noted that small-sized wood, less than about 70 mm diameter, makes harder, denser, and less reactive charcoal than will wood split from the trunk of the same tree. The reason lies in the difference in the coarseness of the "ray" structure of the wood, which means that younger wood, and branches of a tree, make better charcoal than does wood from its trunk. This advantage of smaller wood for charcoal making has three implications. One is that it seems likely that throughout antiquity most of the charcoal made was from smaller size and therefore younger wood. This is of some importance to modern c- 14 dating of charcoal. The date measured is that of the cessation of growth of the wood, and often some years are added to the measured date to allow for growth of the wood. However some of these additions have been unreasonably large, by assumption that the charcoal was made from a century-old tree. Addition of ten or twenty years reflecting charcoaling of limbs and coppices would be a more practical average. Another is that large trees would be felled only when the trunk was necessary for some structural use, or more often as clearance for farming. The felling of a large-diameter tree in antiquity was a major undertaking because of the brittleness of early stone or flint axes, and the relatively low hardness of the bronze axes available later. The third implication is that the preferred use of smaller wood meant that when a large tree was felled for making lumber or ship masts, the branches would be preserved and used for charcoal making, resulting in excellent use of the tree as a whole and in that sense decreasing the rate of deforestation. PHYSICAL PROPERTIES
In heap charcoal making, temperature and time at temperature will differ from the bottom to the top of the heap, partly because of the skill applied in control of the combustion, and partly because temperature is highest at the air entries at the base of the heap. There will thus be a range of properties of the charcoal in different parts of the heap, and selection is necessarily made while taking out the finished charcoal. As noted above, the hardness of charcoal is significant because of its effect on final size distribution and on its reactivity. There are clear visual clues to charcoal quality. A sixteenth-century A.D. writer, Biringuccio ([r540] 1942), notes for example, that if charcoal is "rightly baked it is thick and strong, and when struck with another piece is as resonant as glass." A nineteenth-century comment (Overman r854) was that the variation in a freshly opened carbonized pile can be recognized by the relative appearance and hardness of the
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Mastery and Uses of Fire in Antiquity
charcoal, and that on digging out the pile, the charcoal that rings when struck and breaks with a shiny, conchoidal fracture is the best. These criteria were as easily recognized eight or more millennia ago as today. Judging from several nineteenth-century reports on charcoal making when some knowledge of modern chemistry had become available, it would appear that the desirable minimum FC content of smelting charcoal was about 80 per cent. The negative effects of low Fe content are discussed in appendix 1. The average quality of charcoal made in antiquity was probably very good, because of the direct and reliable connections between the visible appearance and the hardness of charcoal and its effectiveness as a smelting fuel. The operator of a smelting furnace undoubtedly examined the charcoal closely before accepting it for use, and if it was unsatisfactory, it would have relegated to some minor use such as cooking or warming fires. The charcoaller would have had to adjust his practices accordingly. The description by Theophrastus in the third century B.C. of how to make and to recognize good charcoal matches those of the nineteeth century A.D. Each lump of charcoal contains considerable very fine porosity as manufactured, resulting from the cell structure of wood, so it has large internal surface. It can therefore absorb water rapidly, so much so that it is always kept under cover to keep it as dryas possible. Absorption of water increases the weight of a piece of chacoal but does not change its volume since it is in its pores; so a basketful of lump charcoal contains about the same weight of FC wet or dry. Wet charcoal absorbs heat by evaporation of its water in the upper part of a burning fuel bed, which can actually be an advantage since it can decrease consumption of charcoal, as will be explained in chapter 7. Charcoal as a mass or fuel bed has an inter-lump void fraction usually in the range of 0.35 to 0.50 depending on the shapes and the size distribution of the lumps; these in turn depend to a considerable extent on the species and physical size of the wood used. The bulk density is a function of the specific gravity of the charcoal itself, which includes its internal porosity, its water content, and the void fraction of the lumps as a packed bed. This was a fortunate circumstance throughout antiquity since there is no archaeological evidence of use of weight to measure charcoal or ore into a furnace, volume being invariably used. In modern practice, bulk density is customarily taken to he that of dry, or nearly so, charcoal. The bulk density of hardwood charcoal is typically about 280 kg per m 3 and of softwood charcoal about 175 kg per m 3 • If the heat of combustion of charcoal is taken as 28 MJ/kg, then the volumetric heat content of a fuel bed of hardwood charcoal may be taken as 7.84 GJ per
59
Charcoal Manufacture and Properties
m 3, and of softwood charcoal as 4.90 GJ per m 3. It will be noticed that these heat contents per unit volume average about twice those of fuel beds composed of wood or woody materials given in chapter 3, and they are less variable. A higher intensity of combustion can therefore be achieved with a charcoal fuel bed, and higher temperatures can be developed as a greater percentage of a higher AFT of the fuel. The bulk density of charcoal is important also because of a hidden effect on the operation of ancient smelting furnaces. A matter of first importance in burdening or charging a smelting furnace is the ratio by weight of carbon in the charcoal to that of the ore. However, as noted above, both charcoal and ore were measured into furnaces by volume, as some number of baskets or barrows. Variability in the bulk density of charcoal meant that the weight of carbon in a basketful, and so the ratio of charcoal to ore, could vary without the change being visible. This created an undetectable source of uncertainty in the temperature of smelting and is another reason why individual smiths adhered to using charcoal made in a particular kind of heap, from wood from particular trees. This was in order to achieve, without realizing why, some uniformity of bulk density of charcoal to obtain better repeatability of smelting results. The crushing strength of a piece of hardwood charcoal is considerably lower than that of coke made from coal, while that of softwood charcoal is lower than that of hardwood. In a furnace smelting an ore (which has several times the bulk density of charcoal), the average bulk density of a complete charge of ore plus charcoal determines the pressure on the charcoal in the hearth. However, with the high ratios of charcoal to ore used throughout antiquity, the average bulk density of the furnace contents remained well below the strength of the charcoal to support the burden, even in furnaces several metres tall. This point needs emphasizing, since well into the second millennium A.D. blast furnace operators believed that the use of charcoal fuel limited the height that furnaces could be built. This view was mistaken, as experience with large and very tall furnaces has shown. Charcoal continued to be used for smelting both iron and copper in the West until early in the eighteenth century A.D., then was slowly replaced by coke for reasons of cost and availability, not strength. However, in North America, charcoal was low in cost and ample in supply, and by late in the nineteenth century, charcoal-fuelled furnaces twenty-four metres high with hearth diameters of 2.5 metres were still in production (Sweetser 1908). During the same period the ratio of ore to charcoal was increased for economy, which increased the bulk density of the burden to 400 - 500 kg 1m 3 • At no time was there difficulty due to the fuel, unless it was admittedly badly made.
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Mastery and Uses of Fire in Antiquity
CHEMICAL PROPERTIES
One of the most important properties of charcoal is its chemical reactivity, which is among the highest of all forms of carbon. Usually a compromise was made between reactivity and strength and size of charcoal without realizing it, since of course the concept of reactivity was not known and mixtures of softwood and hardwood charcoal were often intentionally used for convenience or economy. When high chemical reactivity is combined with the high fuel surface area that is due to desirable lump size being relatively small, as was noted in chapter 2, the total reactive power is high. The result is that in fuel beds of charcoal more than about 10 to IS fuel lump diameters thick, all of the oxygen in the combustion air is consumed in passing through. Thus not only is excess air as described for biomass fuel in chapter 3 then not possible but the products of combustion above this level are then a mixture of nitrogen and carbon monoxide that is strongly reducing in character. This makes charcoal by far the most effective fuel for smelting oxide orcs of metals, considerably more so, in fact, than the less reactive coke made from coal. The VM content of oven-dry charcoal is the inverse of its FC content since ash content is usually low and is a measure of the success of its removal during manufacture, being typically in the order of 5 to IS per cent. When charcoal is charged to a furnace and sinks with the burden as it is consumed at a tuyere, it descends through rising temperatures, and most of the contained VM is driven off in the upper part of the furnace. Although VM is a strongly reducing gas, it is removed in a temperature range too low to be very effective in reduction of orcs. The heat content of charcoal kept reasonably dry and burned completely to CO 2 will be in the range of 27.0 to 30.0 MJ/kg (Tillman et al. I98r) and the AFT will be in the range of r,820 to 2,OOOoC depending on the FC content and reactivity of the charcoal (Rehder 1990). However, it must be noted that while the chemical and physical properties of charcoal are the reasons why it is so effective as a fuel for smelting ores, as a heat source alone it is grossly wasteful. On the assumption that in antiquity the yield of charcoal is taken as 15 per cent or 0.15 kg per kg of air-dry wood, the heat content of this wood at 25 per cent moisture can be taken as 15.0 MJ per kg; charcoal used for smelting is burned nearly completely to carbon monoxide (as will be described in chapter 7 and appendix I), with a heat production of 8.3 MJ per kg for charcoal containing 80 per cent FC. The heat produced by the charcoal made from one kg of air-dry wood is therefore 0.15 x 8.3 = 1.24 MJ, which is a loss of 13.8 MJ or 92 per cent of the
61
Charcoal Manufacture and Properties
heat content of the original wood. Some heat is recovered from the carbon monoxide through partial oxidation by ore which decreases the loss somewhat, but this is balanced by the fact that wood suitable for charcoal-making is only about 60 per cent of a tree, and most of a tree can be burned directly as biomass, so the waste from a whole tree is larger. Charcoal has an ash content derived from that of the initial wood and varies with species from about 0.25 to as much as 5.0 per cent. However, common temperate climate species such as pine, oak, beech, and ash have moderate ash contents in the order of 0.3 5 per cent. If the wood used for charcoal-making contains, for example, 87 per cent VM, 12.6 per cent FC, and 0.35 per cent ash on a dry basis, then the ash content of the FC in the resulting charcoal would be 0.35/12.6 = 2.8 per cent. But in the heap practice used in antiquity, due to chemical reactions during carbonization, the recovery of charcoal is variable so the ash content of the charcoal would be on the average about half this or in the order of I. 5 per cent. Actual analyses vary with the time-temperature cycle in charcoaling but are about this size. Because, as has been shown, lump size distribution, bulk density, FC content, and reactivity of charcoal all can have major effects on the operation of smelting furnaces, it is important not only to preserve the lump charcoal found in the archaeology of smelting contexts but, wherever possible, to measure these properties. TRANSPORT OF FUELS
Materials were transported extensively by ship in antiquity (Casson 1959), but in the long list of items given by Casson, wood and charcoal are not included and so were apparently infrequent cargoes. A principal reason may have been the low bulk density, particularly of charcoal, and so a relatively low value per unit of volume: the cost of maintaining and manning a ship must be recovered in terms of the value of the volume of its hold per unit of time. Biomass fuel has airdry bulk density of from about 400 kg per cubic metre for cord-wood, to 200 or less for bundles of brush, and charcoal has typically about 200 kg per cubic metre. Since (as will be shown) the efficiency of use of the energy in the fuel was low, the bulk density of delivered fuel in terms of heat content was in the order of only 10 kg per cubic metre, which made fuel, either as biomass or as charcoal, expensive to transport very far. As late as the eighteenth century in England the maximum economic distance for transport overland of charcoal to a blast furnace was about five miles. Above this, it was cheaper to move the furnace.
62
Mastery and Uses of Fire in Antiquity
COKE
Certain kinds of coal can have most of their VM removed to create a high FC residue called coke using exactly the same methods as for making charcoal from biomass. The earliest evidence of such usc, however, is not until about I,OOO A.D. in China, and several hundred years later in the West, both outside the time frame of this book.
7 Combustion in Beds of Lump Charcoal
The combustion of fuel beds of the various forms of lump carbon low in VM, such as charcoal, coke, and anthracite coal, has been thoroughly studied both in the laboratory and industrially. Basically only two simple chemical reactions are involved, which occur in sequence and at quite different rates. The result is that in the same fuel bed, oxidizing gas at high temperature and reducing gas at lower temperature can be found at different levels. These can be changed as the lump size and air supply rate are changed, giving remarkable versatility in the use of lump fuels, particularly for the smelting, melting, and refining of metals. These fuels differ in their chemical reactivities, which affect their rates of reaction and so of heat and gas distribution; and as noted in chapter 6, charcoal has particularly high reactivity. But since coke and anthracite coal have little evidence of use in antiquity, discussion of their combustion is irrelevant here, except for a note in appendix I concerning problems that can arise in trying to reproduce ancient forging practices using coke as fuel. THE PHYSICS AND CHEMISTRY OF COMBUSTION OF CHARCOAL
The combustion of charcoal (for more quantitative detail, see appendix I) is essentially the combustion of carbon in two stages, the first reaction being between carbon and the oxygen of air. This takes place very rapidly to produce carbon dioxide and a large quantity of heat that
64 Mastery and Uses of Fire in Antiquity increases the gas temperature. On further passage of the resulting very hot mixture of carbon dioxide and nitrogen through the fuel bed, the carbon dioxide is reduced to carbon monoxide in a second reaction. This also consumes charcoal but proceeds much more slowly, and absorbs a moderate quantity of heat that decreases the temperature of the gas. The result, if the fuel bed is deep enough and air is admitted at near its base, is the complete gasification of the original carbon to a mixture of carbon monoxide and the nitrogen of the initial air. There can be no "excess" air due to the lower void space and high reactivity. Since the chemical reactions are sequential in time and space, the chemical nature of the products of combustion in a bed of charcoal fuel change from very hot and oxidizing close to air entry to very reducing and somewhat cooler at some distance from it. With charcoal as fuel, the maximum carbon dioxide content of the gas developed in the initial reaction with charcoal is in practice 10 to 12 per cent, depending on the source of the charcoal - considerably less than the 20.9 per cent theoretically possible. At the air entry rates used typically in antiquity, maximum gas temperature and carbon dioxide content are reached within milliseconds at a distance of two to three fuel lump diameters from the point of air entry. With usual average lump sizes, this is typically about 30 to 60 mm from air entry. However, the complete reduction of this carbon dioxide to carbon monoxide as it passes further through the fuel bed occurs at a much slower reaction rate, and so requires an appreciably further distance. This can be from as little as six to as much as fifteen fuel lump diameters from air entry, again depending on the source of the charcoal. The increased gas temperature in the initial stage of combustion depends on the amount of carbon dioxide formed, and at the carbon dioxide contents of about 10 to 12 per cent typical of charcoal, this creates an AFT of 1,880 to 2,000oC, with r,940oc as a practical average. It is usually possible to find with a thermocouple a spot near a tuyere nose in the bed of an operating charcoal furnace that will show a temperature as high as 1,800°c. This value is moderately lower than the AFT of the fuel due to heat losses, as discussed in chapter I and further in appendix 1. The heat absorbed by the ensuing reduction of the carbon dioxide to carbon monoxide decreases gas temperature to about 1,000°C, below which the reduction of carbon dioxide effectively ceases for thermodynamic reasons. Figure 6 shows experimental results from combustion of a charcoal fuel bed (Hiles and Mott 1944) that reflect the effects described above; they are directly applicable to furnace conditions in antiquity because the space velocity of 13m per minute that was used happened to be in the same order of size as that used in antiquity. The charcoal had an
65
Combustion in Lump Charcoal Beds CO COt. CO/C02
%
40
30
n7TIf. C 2,000
't.o
/,000
/0 COil
o
O~~~--~~-r~--r-~~~ () ~ 4/0
b
Fu..t.! Lt.<7l\P P;4.m,r'e,.~
8
t)./u,v!'
t1'lC"Y
Figure 6 Gas Composition and Temperature in a Charcoal Fuel Bed
average lump diameter of 25 mm and was burned on a bed 300 mm deep on a grate, in a furnace 340 mm inside diameter with an open top. The orientation of the graph in figure 6 is with air entry from the left, and the spatial distributions of gas temperature and of temperature are well illustrated. The maximum temperature reached (at the location of maximum carbon dioxide content) was only I,420 o C, about 73 per cent of the AFT of the fuel, for reasons given in appendix 1. QUANTITY OF AIR REQUIRED
Combustion in fuel beds of charcoal is usually complete to carbon monoxide and nitrogen gases, beyond distances of more than about 10 to 20 fuel lump diameters away from air entry, and with space velocities below about 30 m per minute. As noted in appendix I, the combustion air then required is 3.9 m l per kg of 80 per cent FC charcoal consumed, a figure that agrees well with measurements in charcoal blast furnaces operating in the late nineteenth century A.D. It should be noted that when oxide ores are being smelted, some carbon dioxide will always appear in the top gases even in tall furnaces. This comes from the reaction of the carbon monoxide in the gases with the oxygen of the ore in the upper part of the furnace, and the carbon dioxide then produced is at too Iowa temperature to be re-reduced by
66
Mastery and Uses of Fire in Antiquity
the surrounding charcoal in the time available before its exit from the top of the bed. EFFECTS OF CHARCOAL LUMP SIZE
Particular note must be taken of the effects of change in the average lump diameter of the fuel on the course of combustion. These are independent of the wall effects described in chapter 2. Larger average size decreases the surface area of fuel per unit of bed volume, which is the surface at which the carbon of the charcoal reacts with air. The very fast initial reaction rate at air entry minimizes any effect of surface area, but the ensuing reaction to carbon monoxide is at a much slower rate and the effect of a change in surface area of the carbon is then much more pronounced. A decrease in surface area due to larger lump size means that at a given air inlet velocity less carbon dioxide is reduced at a given distance from air inlet. Then the level above air entry in a shaft furnace at which carbon dioxide is completely reduced to carbon monoxide is much higher. An extreme example of this effect was unwittingly displayed in the operation of a reproduction of a natural draft furnace in Yarde, Denmark, intended to reduce iron ore to iron (Tylecote 1969). The charcoal fuel was carefully cut into large blocks 100 by 100 by 125 mm in order to give high void space and so low resistance to gas flow. These were charged to the 240 mm I. D. furnace with roasted bog iron ore containing 64 per cent iron, at a weight ratio of 2 of ore to I of charcoal. A space velocity of about 8.4 m per minute was achieved which generates about 18.0 MJ of heat per minute per m\ which should have been more than enough to smelt iron. After about eight hours of operation, the furnace was cooled and dismantled, and very little reduced iron was found in a considerable amount of solidified slag. The carbon dioxide content of the furnace gas at 520 mm above tuyere level had been 9 per cent after seven hours of operation, and the maximum temperature reached just above tuyere level was I,290oC. Since little iron was reduced, the carbon dioxide measured was largely from incomplete reduction of what was produced by initial combustion at air entry level. The ratio of the surface area of one of the charcoal blocks to that of a 25 mm cube, which would be the normal size for the furnace, was I to 4. TUYERES, AND GAS DISTRIBUTION IN THE FUEL BED
In the case of combustion air entry through a grate beneath the fuel bed, as in figure 6, gas flow is well distributed across the cross-section
67
Combustion in Lump Charcoal Beds
of the bed, and conditions across any plane above the grate will in principle be uniform, except for near the wall as noted in chapter 2. This arrangement of air entry to the fuel bed was necessary for experimental purposes, but it is not practical when charcoal-fuelled bowl and shaft furnaces arc used for smelting. The product then made is usually a molten slag and often a molten metal, and until quite recently there was no material of which a grate could be made that could withstand such erosion for more than a short time. In general, depending on the size of the furnace, one or more ceramic pipes or tuyeres are used for air entry, arranged so that their exit ends or "noses" are not far from the bottom of the fuel bed. A single tuyere may penetrate the bed from the top, angled in toward the centre from one side of the top; but usually tuyere(s) enter the fuel bed through the furnace wall near the base of the bed, horizontally or at a small angle downward. Only a short refractory pipe is necessary, with just enough protrusion into the fuel bed to be clear of the "wall effect" mentioned in chapter 2. It was shown experimentally some time ago (Shires I960) that if a tuyere is angled downward slightly, about 15 degrees being optimum, the maximum horizontal distance of gas penetration into the bed is at a maximum; all tuyeres arc so angled today. Interestingly, tuyeres in many shaft furnaces in antiquity were similarly angled downward, an apparent example of the result of experience and close observation of furnace operation. The consequences of air injected through a tuyere into a bed of lump fuel are simple in principle but varied and useful in practice. An expanding plume of products of combustion is formed that (reading figure 6 from left to right) is seen to quickly reach high temperature and a very oxidizing composition, and then decrease moderately in temperature while becoming very reducing in composition. The high initial temperature reached causes a large expansion, at an almost explosive rate, of the initial products of combustion. The increase in volume is in the order of six to seven times, and at a typical air entry velocity from the nose of a tuyere of about 15 to 25 metres per second, this occurs in about five to three milliseconds. The subsequent much slower rate of reduction of carbon dioxide to carbon monoxide decreases gas temperature so that its volume will decrease again, but the decrease is lessened by the fact that one volume of carbon dioxide creates two volumes of carbon monoxide. The gas stream carries the momentum of the incoming air and so penetrates among the fuel lumps, but the dispersive effect of the fuel lumps on gas flow broadens the plume, and the geometric constraints of the bottom and walls of the fuel bed container turn the plume upward towards the top of the bed. The effect is simulated in figure 7,
68
Mastery and Uses of Fire in Antiquity
Figure 7 Approximate Plumes from Tuyeres ill Fuel Beds
which shows four effects: that of a deep fuel bed and restricting walls as in a shaft; a difference in air entry velocity; the use of two opposing tuyeres; and a shallower bed and a more distant far wall as in a hearth. In a shaft or bowl furnace with a single tuyere, an arrangement commonly used through antiquity, the gas composition and temperature across a horizontal cross-section of the fuel bed will evidently vary greatly above tuyere level. This effect will decrease with height above tuyere level due to the diffusing effect of the fuel bed, but the pattern of temperature and chemistry will start distinctly to slant away from the tuyere nose. The use of two opposing tuyeres for the same total air flow will make a marked increase in horizontal uniformity of gas composition and temperature; in general, more tuyeres give more uniform gas distribution. However, due to the physics of air flow through tuyeres, there are important other effects from using more than one tuyere. From equations (I) and (4) developed in appendix 2, it can be shown that for a given rate of air flow to the fuel bed, if one tuyere is replaced by two of the same inside diameter, the power necessary to maintain the same total flow rate becomes only one-quarter of that when one tuyere is used. This is large enough to become evident to the operator of the bellows. Alternatively, if the work effort necessary for one tuyere is maintained for two tuyeres, the air flow rate to the fuel bed would be increased by 60 per cent and the furnace would run faster and hotter.
69
Combustion in Lump Charcoal Beds
Similarly, if one tuyere is replaced by four of the same size, the air rate would be increased by 250 per cent. These considerations became very important from the beginning of the first millennium A.D. when water wheels began to be used to operate bellows. However, throughout antiquity as few tuyeres as possible were probably used, both because of the difficulty of connecting multiple tuycres to a single source without introducing considerable leakage at joints and the lack of measure of effective power on bellows with only human muscle involved. TUYERE DIAMETER AND AIR EXIT VELOCITY
A tuyere produces other important effects in a fuel bed. It is clear from the discussion so far that if the velocity of the air stream coming from a tuyere is increased, the products of combustion will be propelled further into the fuel bed due to higher momentum, and the cone of prodlIcts of combustion will be elongated. In addition, the higher velocity decreases time available for the relatively slow rate of reduction of carbon dioxide to monoxide. This extends the length of the plume of high carbon monoxide content gas and therefore the vertical distribution of reducing gas in the furnace. Tuyeres of quite small inside diameter have thus been habitually used to increase air exit velocity. It is remarkable how narrow the range of tuyere diameters has been through antiquity and also the Middle Ages in bellows-blown furnaces. Most of those I have noticed have been in the range of about 20 to 35 mm 1. D., giving tuyere velocities of about 15 to 25m per second for the space velocities that were typically used. This order of velocity seems to be that necessary to give adequate penetration of an average charcoal fuel bed in the relatively small diameters of furnace commonly used. Very high tuyere velocity can create a horizontal tunnel or "raceway" in a lump fuel bed just off the nose of a tuyere which increases in length with further increase in velocity and momentum. It apparently has no particular chemical or combustion advantage with charcoal as fuel, although it does with coke which has considerably lower reactivity than charcoal. Appearance of a raceway is a factor of fuel density and lump size, and the momentum of entering air. Modern experience with charcoal fuel beds is that raceway formation starts somewhat above a tuyere velocity of about 50 m per second, which is well above that commonly used in antiquity. There is an appreciable price to pay for increased tuyere velocity, since a tuyere acts as a nozzle, and the pressure required to force a gas
70
Mastery and Uses of Fire in Antiquity
through a nozzle increases as the inverse fourth power of its inside diameter. For example, while a given air flow rate through a 25 mm tuyere will give twice the velocity as through a 35 mm tuyere, it will require four times the pressure in the supply. If the decrease in tuyere I.D. is due to slag adhering at the nose and more pressure is not applied from the bellows, the rate of air flow will decrease and furnace operation will be slowed; so keeping tuyeres clean is important. In charcoal fuelled furnaces the pressure drop through the tuyere(s) constitutes usually more than 90 per cent of the total of that through tuyeres and fuel bed. These issues are discussed in more detail in appendix 2. Another factor in control of the shape of the gas plume from a tuyere in a fuel bed is the shape of the cross-section of the tuyere opening or nose. This was and is today in most cases circular or nearly so, and the initial shape of the issuing air stream and resulting plume will be the same. However if the opening is rectangular or D-shaped with the longer dimension horizontal, the stream of air and then the cone of products of combustion will be spread further horizontally than vertically. The zones of high temperature and then of maximum reducing power will be similarly spread, which would serve to fill the square or rectangular moderately shallow bed in a bowl or hearth furnace, with active working volume. This was demonstrably useful in the smelting hearth that eventually became known in the eighteenth and nineteenth centuries as the widely used "Catalan forge." LONG TUYERES
Tuyeres that are nearly horizontal do not normally penetrate the fuel bed by more than 10 to 20 mm beyond the inner furnace wall, since temperature is lower close to the wall, and deeper penetration increases the rate at which the nose of a tuyere is fluxed or melted away. However, deep penetration of a tuyere into the fuel bed can serve to deliver combustion air nearer the centre of the bed of a larger furnace, which can avoid the necessity for high tuyere velocity and associated high air pressure and harder work on the bellows. Long tuyeres can be a necessity if the air supply is to be by natural draft, as will be discussed below. The longest tuyere penetration into a charcoal fuel bed seems to have been in African ironmaking furnaces used in an apparently limited geographical area, where a single tuyere 1,000 to 1,500 mm long was used, penetrating the fuel bed vertically from the top, reaching down at the start of operation, to within a short distance of the hearth (e.g., Cline 1937, Sassoon 1964). This is not practical for large furnaces because of the difficulty of supporting the then necessarily long and heavy pipe. The conditions of combustion and the temperature distribution
71
Combustion in Lump Charcoal Beds
are complex because the air from the tuyere must immediately reverse direction to flow upwards, and the pulsating supply from a bellows makes further changes in air and temperature distributions. The tuyere diameters are quite large since the exit velocity of air need be only a few metres per second. During operation, the nose of the tuyere recedes upwards by being fluxed away, at a rate of typically 25 to 50 mm per hour. The result when iron ore is being smelted is that the reduced iron is very heterogeneous in carbon content and in physical structure (David et a1. 1989), requiring much effort in selection and in welding small pieces into a bloom of usable size, and with low final recovery of iron from the ore. Long tuyeres penetrating from the side of the furnace were also used in Africa, when natural draft instead of bellows was used for air supply (Cline 1937, Bellamy 1904). For natural draft to be effective, it is essential to keep the pressure drop in the tuyeres as low as possible (as is explained in appendix 3), and multiple or large tuyeres were used to give sufficiently large total tuyere area to decrease the resistance to gas flow. As a result tuyere velocity was low, and long tuyeres were necessary to deliver air closer to the centre of the fuel bed. The effect of tuyeres penetrating deeply from the wall into the bed is that they concentrate combustion and the generation of heat to within the circle of their noses. This has been demonstrated experimentally in modern times in a shaft furnace - an iron-melting cupola using coke fuel - which was shown by the use of horizontally movable watercooled tuyeres to perform in accordance with the area enclosed by the noses of its tuyeres, nearly independently of the area of the fuel bed as a whole. A disadvantage of the arrangement is that when the heat is thus concentrated at the centre, the periphery of the fuel bed which can constitute a large proportion of the furnace area and volume is less active. The furnace is in effect made smaller in working volume. Another effect from the use of deeply penetrating tuyeres is that as tuyeres are fluxed away, not only is heat concentration decreased but there is an increasing amount of slag made from the tuyere material, which absorbs heat in being melted and increases metal loss to the slag. Particularly detailed data acquired by Bellamy (1904) on a natural draft furnace with long tuyeres then still in usc in Africa can be used to compare such furnace operation with that of a bellows-blown single tuyere shaft furnace. This is done in some detail in chapter 12, showing that their performances were very similar. The apparently relatively limited use of the technique in Africa could well be because the same metallurgical performance can be obtained from a smaller diameter furnace with bellows and short tuyeres, avoiding the cost and trouble of making long tuyeres and a larger furnace
72
Mastery and Uses of Fire in Antiquity
and giving more controllable operation. The true functions of deeply penetrating tuyeres, i.e., decreased pressure drop and concentration of heat, seem to have been quite misunderstood by modern investigators of African iron smelting furnaces. AIR PREHEAT
As combustion air is increased in temperature, so does the AFT of the fuel involved, and today air preheat temperatures of more than 1,200°C are universally used in blast furnaces, primarily to decrease fuel consumption. Every conceivable method has been tried over the last ISO years to do the preheating; very expensive ceramic "stoves" fired by waste gas from the blast furnace arc now commonly used. In 1978 and 1979 the considerable claims were made and subsequently published in several journals that the long tuyeres used in some primitive African iron smelting furnaces sufficiently preheated the air to be considered to be using hot blast as an important early innovation in iron smelting practice; furthermore, this was necessary to the formation of high carbon content blooms (Avery and Schmidt 1979 et seq.). However, the field measurements of air temperature on which the claims were made were seriously in error and the claims were quite unjustified in more than one way, as discussed in criticisms by Rehder (1987a) and Eggert (1987). BELLOWS EFFECTS
The whole question of combustion air supply in antiquity will be discussed in chapter 8, but one factor due to the intermittent nature of bellows operation and, as will be noted below, also in blowpipe operation, seriously complicates gas and temperature distributions in the fuel bed. Called here the" bellows effect" for lack of a better term, it is important particularly with the relatively small bellows used in antiquity. From a bellows the stream of air pulses in volume through each stroke to a variable degree that depends on the type of bellows and how it is worked. The velocity of air exiting from the tuyere changes accordingly during each stroke from a maximum to zero or even a short reverse if valveless bellows are used. The extent of penetration of air into the fuel bed must similarly change, and with it the shape and size of the plume of products of combustion. At any given point in the fuel bcd, therefore, the composition of the gas, its carbon dioxide-monoxide ratio, and its temperature change through each stroke of the bellows. This condition or series of events is inseparable from bellows air supply. It becomes less widely cyclic as
73
Combustion in Lump Charcoal Beds
bellows size increases and as its operation becomes more uniform such as with mechanical bellows drive, but the latter was not to, my knowledge, used in antiquity. Another style of tuyere was based on an aspirator effect, which accentuates the effects described above. The tuyere or pipe that penetrates the furnace wall into the fuel bed enlarges at the exterior end to an open cone; a pipe coming from the bellows is supported so that its end is centred inside the cone but at a distance from touching the cone. When air is expelled from the bellows, its velocity entering the cone acts as an injector that draws ambient air into the cone, increasing the total air into the furnace. However, extra effort is then necessary on the bellows to supply the high velocity injection, which could just as well be exerted on a directly connected larger bellows. The bellows need contain no valve, but intake of air for each stroke then must come through the annular space between cone and nozzle, which creates a negative pressure or suction on the cone that can reverse the flow in the tuyere itself. This accentuates pulsation, and combustion reactions are further disturbed for no evident advantage. Whatever the detailed chemical and metallurgical effects of this condition, ores were in fact successfully smelted by the average positive air flow that was forced through the fuel bed. However, it should be clear to any student of gas flow in packed beds that attempts to measure gas compositions and temperature with modern sensors in a reconstructed bellows-blown furnace are doomed to large uncertainty in accuracy and therefore in usefulness. The accurate sampling of gas streams in shaft furnaces is notoriously difficult even in modern furnaces with well-distributed air supply. The important point is how irregular the furnace working conditions were which the smiths in antiquity had to deal with. Early in the twentieth century in the large blast furnace industry in the United States, the idea arose that a pulsating blast would have operating advantages, and expensive equipment was installed on several furnaces to create pulsations in their blast. No advantage whatever could be detected, and the equipment was scrapped.
8 Combustion Air Supply for Charcoal
By now it must be clear that the rate of air (i.e., oxygen) supply to a fuel bed is the primary control over the rate of generation of heat and the performance of furnaces. In antiquity, combustion air supply for charcoal fuel could be from three sources: as human breath through a blowpipe, as ambient air from a bellows, or as ambient air drawn in by natural draft. All were used at some time or in some location, and each source of oxygen produces its own pattern of limitations and possibilities. Each will be discussed separately in the order given, but first the mechanics of the flow of gases in pipes and other conduits must be reviewed. Air and the gaseous products of combustion have specific gravities that differ by only a few per cent, and for practical purposes the mechanics of their flows in furnaces may be taken as the same. The following discussion pertains only to the combustion of fuel beds of charcoal in bowl and shaft furnaces and is taken from standard handbooks on combustion. MOVEMENT OF AIR
The weight of air necessary for combustion is considerably greater than that of the weight of the fuel to be burned. As is shown in appendix I, one kg of 80 per cent Fe charcoal burned as a bed in a smelting furnace requires for its combustion close to four m 3 of air, which weighs nearly five kg. There are therefore considerable weights of air to be drawn or pushed through entry nozzles such as tuyeres and through the fuel bed.
75
Combustion Air Supply for Charcoal
This requires considerable amounts of power; and as a result, the amount of mechanical power available to supply combustion air has from antiquity to the present day determined the maximum rate of heat generation possible by combustion. In antiquity the only practicable source of power for blowing or pushing air was human effort, and this was a sharply limiting factor in the production rates of smelting furnaces. The exception was the use of natural draft for kilns and reverberatory furnaces as well as for suitable designs of shaft furnace. The movement of a gas through a pipe, an orifice such as a tuyere, or a fuel bed encounters resistance from friction with internal surfaces. The pressure necessary to overcome such resistance can be easily calculated for simple shapes such as ducts, pipes, and orifices but less easily so for beds of fuel and for irregular volumes and changes of shape. The details of the calculation of pressure drops of air in pipes, tuyeres, and furnaces, and of the power necessary to overcome them, are given in appendix 2. HUMAN BREATH AS AIR SUPPLY
We all know that blowing on a fire will brighten it by increasing its temperature, and it is well known that furnaces can be operated using human breath through blowpipes. However, it may be less well known that the temperature that can be reached in a fuel bed by use of a blowpipe is limited, since the chemical composition of human breath gives a lower AFT in its combustion of charcoal. The rate of heat generation by one person is also limited by the physiology of sustainable flow rate of human breath. Having explored these points in some detail elsewhere (Rehder 1994), I summarize them here. Since human breath is the waste gas from an oxidation reaction in the human body, it contains only about two-thirds as much oxygen as ambient air. A few per cent each of carbon dioxide and of water vapour arc also present, which absorb heat in combustion reactions, so the net result is that less heat is generated by one m 3 of breath than by one m 3 of ambient air. This in turn results in both a lower AFT with a given fuel and a higher rate of supply of breath necessary to generate heat at a given rate. The composition of human breath is also variable through each respiration as well as with the level of exertion of the body and with personal physiology. A typical average composition of human breath when moderately heavy effort is being made is given by Comroe (I974) as I3.7 per cent oxygen, 5.0 per cent carbon dioxide, 6.0 per cent water vapour, and 75.3 per cent nitrogen. Extensive physiological data has shown that a sustainable rate of output of exhaled air, midway between
76
Mastery and Uses of Fire in Antiquity
resting and strenuous exertion of an adult male human being, is about 70 to 80 litres per minute (Altman et al. 1958). The rate of heat generation is then about 0.075 x 1.3 = about 0.10 MJ per minute. The average output rate of females is appreciably less, so in the remainder of this discussion of use of human breath for combustion, a male is assumed to be the operator. From this data it can be calculated that as a practical average, the heat generation rate in a fuel bed of charcoal will be about 1. 3 MJ per m 3 of breath, and the AFT will be in the range of about 1,300 to I,500oC. In a well-insulated and preheated furnace, this can result in a maximum temperature in the fuel bed of about 1,100 to possibly I,300°C, depending on breath composition and rate of supply. This range includes temperatures sufficient to melt and smelt copper and bronze, with the advantage that their iron content from its reduction from slag would be moderate. At the lower end of the range, poor slag fluidity can result in smelted copper appearing as prills in a viscous mass of slag rather than as a molten pool. A collection of pre-Columbian copper smelting furnaces at Batane Grande in Peru was excavated with careful attention to detail by Shimada et al. (1982), and from their published furnace measurements and using data in appendix I below, it can be estimated that one of the 250 mm I.D. furnaces would have a heat supply requirement of 0.39 MJ per minute. The number of blowpipes necessary can then be shown to be about three, and three to four evidently had been used. Considering the variation in human physiology which affects blowing rate, this is good agreement. Also, much of the copper produced was in fact as prills in slag. In my experience the maximum furnace temperatures noted above could be marginally sufficient to smelt iron ores in a bowl or shaft furnace. Iron would be reduced in the upper part of the furnace to particles of low carbon iron, but the temperature would be only marginally sufficient for them to weld together into a bloom, and carburization would be slow. The result in the hearth would be at best small particles of low carbon iron or small beads of cast iron, both difficult to separate and then forge into a usable bloom. I know of no archaeological or experimental evidence that iron ore has been smelted to a useful product by human breath. We may note parenthetically that the maximum furnace temperature and heat production rate attainable using human breath for combustion could well be the origin of the old or traditional idea that temperature in a charcoal fire in antiquity could not exceed about I,200 oC which would be approximately true for a mouth-blown charcoal fuel bed only.
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Combustion Air Supply for Charcoal
Another interesting fact in the excavation report from Batane Grande was that all of the many ceramic blowpipe nozzles found were quite small, 8.0 mm LD. with only minor variation. When the tuyere exit velocity is calculated for blowpipe operation, it is found to be about 25m per second. In my observation the tuyeres used for bellows operation in antiquity were commonly in the range of 20 to about 30 mm 1.D., which with the air rates used gave a velocity of about 15 to 25 m per second. As noted above in chapter 7, this velocity seems to be that necessary to give sufficient penetration of air or breath into a fuel bed of charcoal. As a consequence the 1.D. of tuyeres or nozzles found in excavating any furnace can be diagnostic as to whether bellows or blowpipes were used, and so whether or not it was possible for iron to have been smelted. As will be shown below and in appendix 3, tuyeres for natural draft furnaces must have a large total area and can be 50 mm or more in inside diameter. BELLOWS AS AIR SUPPLY
When ambient air passes through a charcoal fuel bed, the AFT developed and the quantity of heat generated per m 3 of air are, as noted in appendix I, respectively about 1,940oC and 2.0 MJ per m 3 . These are both higher than with use of human breath, and this has two important consequences. One is that the furnace temperature can then reach 1,600 oc, which is ample to smelt iron ores. The other less obvious but equally important result is that the physical effort of one person on bellows can generate heat at a much higher rate than if a blowpipe is used. A 75 kg person can generate mechanical energy on a continuous basis at a rate of about 120 watts (Reay 1977). If this is applied to operating a bellows which, due to friction and leaks, has a mechanical efficiency of the conservative figure of IS per cent, the effective power to move air is 120 X o. I 5 = 18 watts. Appendix 2, equation 4, shows that the power necessary to move air against a resistance depends on both the size of the resistance and the rate of air flow. If the total resistance of piping, one or more tuyeres, and the fuel bed in a shaft furnace is taken as typically in the order of 300 pascals, then the 18 watts will produce an air flow rate of 3.6 m 3 per minute. This results in a heat generation rate of 4.0 x 3.6 = 7.2 MJ per minute. The heat generation rate of a man using a blowpipe was shown above to be about 0.10 MJ per minute, so the increase in rate possible when the same man operates a bellows is 7.2/0.10 = 72 times. The increase will vary with the efficiency of the bellows and the pressure drop in tuyeres and fuel bed, but it is so large that once discovered, the use of bellows would be revolutionary in terms of the manpower necessary
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Mastery and Uses of Fire in Antiquity
for smelting or, conversely, the size of furnace that one man could service. In addition there is the important point that the increased AFT also created the possibility of smelting iron ores. Curiously, the use of bellows was evidently unknown in the preColumbian New World, attested by early Spanish comments and by extensive modern archaeology. This could be the major reason why iron ores apparently were never smelted there. We might also infer that since any furnace for smelting copper that was larger than about 500 mm I.D. would not have room around its circumference for the more than sixteen blowpipers necessary, such larger furnaces very probably used natural draft for air supply. While the use of bellows is apparently a necessary condition for the smelting of iron, such use is not a proof that iron was in fact smelted wherever bellows were used. It is well known that iron are can be smelted to a forgeable bloom or to molten cast iron in a biomassfuelled natural draft kiln, as is described in chapter 10. However, at the time of writing the extent to which this process was used in antiquity is not known, so caution is necessary. It seems likely that the first bellows would have been applied to copper smelting furnaces, to save manpower and/or make possible the use of larger and more productive furnaces. A change-over from blowpipes to bellows is illustrated in Egypt in tomb wall rcliefs that show only blowpipes before the arrival of the Hyksos invaders from the Near East about 1670 B.C., and bellows after that time. This has been noted by Wainwright (1944) and Zwicker (1969), and after the rise of the New Kingdom in Egypt about 1570 B.C., more iron appears in the archaeological record. BELLOWS MECHANISMS
The basic mechanism of a device such as a bellows to blow air against a resistance such as a tuyere and a fuel bed involves simply drawing air into a cavity, trapping it with one or more valves, and then expelling it by a force that decreases the volume of the cavity. Numerous mechanical arrangements to do this were developed around the world, the variations being in how to make a collapsible volume and where to place valves. There were three widely used solutions, the first consisting of simple leather bags made often from whole skins, the second ceramic or wooden pots with loose, flexible leather covers, with valves operated by a finger or toe. The third took the form of wooden leaves hinged at one end, with sides and opposite end closed by flexible leather by automatic valves. Piston bellows apparently originated in China, but after the time span of this book.
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Combustion Air Supply for Charcoal
All of these devices were probably leaky in antiquity, both because the quantitative importance of air was not directly appreciated, and because of the difficulty of making tight joints between bellows and tuyeres. With clay pot bellows reconstructed to match primitive bellows, for example, the efficiency of conversion of mechanical effort into a volume of air moving against a resistance can be as low as 15 per cent (Friede 1977). Leaks at the connection between the outlet of a bellows of any type and the outer end of one or more tuyeres are important since they conceal an otherwise visible relationship between rate of operating a bellows and rate of progress of a smelt. Air is intangible, and most archeological evidence and remnant primitive practices show lack of attention to leaks in the bellows-tuyere connection. Considering these many variables in leakage, it would appear that in antiquity something of the order of only 15 per cent or less of the work effort on bellows would appear as air moving out of the nose of a tuyere into the fuel bed. However, this could still be sufficient to operate a furnace with one person on the bellows. The air-moving equipments in antiquity were on such an uncertain efficiency basis that in most cases, when retrieved, they are not useful guides to air flow rates that could be used as bases for analysing associated furnace performance. Analysis of such furnaces and practices must usually depend on tuyere sizes and charcoal lump sizes and burning rates, where available. CONSEQUENCES OF THE USE OF BELLOWS
We can only speculate on the form of the first bellows tried, but its first usc through a pipe or tuyere into a fuel bed would immediately and markedly increase its brightness (i.e., temperature) over that with a blowpipe. This was readily visible and important, but other consequences had to be encountered and then dealt with by trial and error. The positive effects were that much less physical effort was necessary to maintain a fuel bed at a desired temperature, considerably higher fuel bed temperatures were possible, and a single worker could maintain temperature in a size of furnace that would have required many blowpipes and workers. This opened the way to building larger and more productive furnaces, and the higher temperatures possible not only made good slag fluidity easier to achieve but eventually allowed the smelting of iron ores. However, too high temperature could introduce more iron into copper in copper smelting, and make iron of too high carbon content in iron smelting; so an important side effect of bellows use was that it necessitated the limitation of temperature to a level acceptable for a given purpose.
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Mastery and Uses of Fire in Antiquity
LIMITED AIR SUPPLY RATE NECESSARY IN ANTIQUITY
The bloomery process for smelting iron (discussed in some detail in chapter 12) could make a bloom of iron whose size was controlled by the rate of supply of combustion air and the length of time the furnace was operated. Size of bloom made was also independently limited by the mechanical power necessary to forge it, which increased exponentially with its size. Only human muscle was available for forging blooms, and the archeological evidence shows that even into Roman times when iron was being smelted in considerable quantity, typical bloom size was in the order of IO to I5 kg (Tylecote I976). To make larger objects, bars from two or more blooms had to be forge-welded together to make one of the desired size. Such objects had to be of simple shape, since not enough power was available to forge the larger piece as a whole. The relatively small bloom size meant that there was in practice little need to build shaft iron-smelting furnaces of more than about I,OOO mm I.D. This in turn limited the air flow rate necessary per furnace, so that it could be supplied by one or two bellows operators. The later Romans well knew the increased power available from water wheels, but I know of no evidence that they used them to operate bellows for smelting or to operate a forge hammer. When large amounts of iron were needed to outfit an army, they were made by using many relatively small furnaces. NATURAL DRAFT
Combustion air can be drawn through a fuel bed of charcoal by natural draft, which is a negative pressure caused by the density of hot gas within the fuel bed being lower than that of ambient external air. The strength of the negative pressure developed increases with the average temperature of the fuel bed and with its height, so that in a correctly proportioned furnace natural draft can produce rates of flow that develop temperatures sufficient to smelt copper and iron ores. The lack of need for bellows and the manual effort to operate them simplifies furnace operation, and it would seem likely that such furnaces were used particularly in early antiquity. The archeological evidence that this was so is small, and the possibility of larger scale use will likely remain inconclusive. The remains of a natural draft furnace are at most a remnant ring of wall with spaces or holes in the circumference, which can easily be missed or mistaken for a small pottery kiln. The presence of smelting slag would be the only positive indication of a smelting furnace.
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Combustion Air Supply for Charcoal
Natural draft operates satisfactorily whether the ambient air is still or moving. If a wind is blowing, the added pressure effects tend to be self-cancelling, so that the net increase in air flow rate is quite moderate. However, if the tuyeres are rearranged to be all facing the wind, an appreciable increase in air flow rate can be had but at the price of the rate becoming more dependent on wind velocity. In this case the furnace is more correctly called a "wind furnace." A summary of the effects of natural draft when using charcoal fuel in shaft furnaces will be given first; quantitative relationships for both natural draft and wind furnaces are given in appendix 3. Modern reproductions of natural draft charcoal fuelled furnaces have been shown to reach a temperature of 1,400 o C (Tylecote and Merkel 1987), and maximum temperatures approaching 1,600°c can undoubtedly be attained with sufficient tuyere area and furnace height (appendix 3). Use of natural draft furnaces in Africa for smelting iron is well known, although it accounted apparently for a minor proportion of the total iron made, with the remainder being from bellows furnaces. In the pre-Columbian New World where bellows were unknown and blowpipes were used to smelt copper, remains of furnaces and associated smcIting slag have been found that seem to be much too large (e.g., 1.0 to 1.5 m LD.) to have been operated by blowpipes. While their antiquity is not known, further investigation may show that they were operated by natural draft. In Europe, iron was smelted in natural draft furnaces of considerable size, for example, in the Jura of Europe of unknown early date (Gowland 1899). The negative pressure developed by natural draft can be quite small, and since the resistance in tuyeres is usually much greater than that in the fuel bed, in order to keep tuyere flow resistance sufficiently low for adequate flow rate, total tuyere area must be increased. The minimum total cross-sectional area of tuyere necessary to produce air flow increases with the space velocity required and decreases with the furnace height. The quite large tuyere areas then necessary result in low air velocity through the tuyere, which decreases penetration of the fuel bed. In the smelting of copper orcs in which the metal is produced in the molten state, this does not create difficulty in smaller diameter furnaces; but for smelting iron ores where the objective is a solid state bloom of iron, low tuyere velocity can result in the production of many small nuggets of iron rather than a larger one-piece bloom, particularly in furnaces of large inside diameter. An example of possible misidentification of a natural draft furnace is given by Tylecote (1975), in which Nok culture iron smelting furnaces at Taruga in Nigeria, C-14 dated to 300-400 B.C., were investigated. The remains of several furnaces, ranging from 400 to 1,000 mm LD.,
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Mastery and Uses of Fire in Antiquity
provided no evidence of original height. Short pieces of tuyeres 30 to 50 mm IoD were found, but there was no evidence of bellows or their placement. The furnaces were considered to have been bellows blown since the tuyeres found were believed to be too short for natural draft, so a reconstruction drawing was given showing tuyeres and bellows. However, according to the criteria given for natural draft in appendix 3, the furnaces could in fact have been operated quite satisfactorily on natural draft with tuyeres of such diameters. The larger furnaces would have had inactive centres because of low tuyere velocity, but small masses or nuggets of iron would form at tuyere noses around the inner walls. Many small artifacts of iron were indeed found, so the archeological evidence is distinctly in favour of the furnaces having been operated with natural draft. WIND FURNACES
Anyone who has noticed the difference that a wind makes to the rate of combustion and the temperature of a bonfire would think that wind would also increase the rate of operation of a natural draft shaft furnace. There are indeed effects of wind, and they are complex, but they have been studied in some detail by architects to determine the distribution of their pressures on buildings. For example, a wind blowing past a shaft furnace with tuyere openings around the lower circumference exerts a positive pressure on the openings on the near side of the furnace, a negative pressure on those at both sides of the furnace, and a turbulent negative pressure on those at the back side of the furnace. Also the wind blowing across the open top of the furnace creates a vertical negative pressure on the furnace column by aspiration. When these are quantified for a vertical round shaft furnace, there is a small net pressure to move gases through the furnace, in addition to that caused by the internal hot column effect. This additional pressure increases with the square of the wind velocity but does not become appreciable until wind velocities exceed about 20 km per hour. When these effects are considered further as in appendix 3, it can be shown that a modification of the shape of the furnace to square or rectangular cross-section in plan, and a relocation of total air entry to the side of the furnace facing the wind, can make sufficient change to the design and operation of the furnace to produce molten copper and iron blooms. It is then truly a wind furnace. Such furnaces are entirely dependent on an appreciable and steady wind velocity, but they have been demonstrably effective iron smelting furnaces in Sri Lanka, as shown by a recent detailed report Uuleff 1996), and for
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Combustion Air Supply for Charcoal
copper in Israel (personal communication). However, because their rate of operation is a factor of the square of the wind velocity, and geographical locations where wind velocity is both appreciable and steady are not common, their use in antiquity may have been correspondingly limited.
9 Furnace Configurations for Charcoal Fuel
This chapter discusses in some detail the quantitative functioning of bowl and shaft charcoal-fuelled furnaces to clarify their operation, since there has been little published on the subject in the archaeological press. It is in effect an elaboration of chapter 2. If a heap of charcoal is made on the ground and ignited, and a tuyere is thrust horizontally part way into it near its base and air is supplied by a bellows, high temperature will be developed a few centimetres from the tuyere nose. However, the pattern of combustion and temperature will be spread laterally more widely than that described in chapter 7, because of the lack of walls. This narrows concentration of heat and temperature towards the centre, but while the hot gas from the tuyere nose passes vertically through less charcoal than in a shaft furnace, the top of the pile is closer to the tuyere nose, and enough carbon monoxide is present to reduce oxide ores. Since the oxides of the common metals lead, copper, and iron start being reduced at temperatures as low as about 300°C, small, suitably placed pieces of are can be reduced to metal. The melting point of lead is low enough then to be molten, but copper and iron reduced in this manner will be porous, solid-state small lumps unless they can be manipulated to be in front of the tuyere nose for sintering together. The amounts of metal arc then still small, and the charcoal consumption is high, but the practice was used in antiquity. PART I: BOWL AND HEARTH FURNACES
Several of the furnaces described in this section can be found illustrated and discussed in Biringuccio's famous Pirotechnia of 1540. As with a
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Charcoal Fuelled Furnaces
bonfire of biomass, when charcoal is enclosed within refractory walls, heat loss is markedly decreased, and flow patterns of combustion gases are restricted laterally and directed upwards. Construction can be of many kinds, from a depression or hole in the ground, to three vertical slabs of stone that form an open-fronted square, or four slabs that form a complete enclosure. The enclosed area is typically less than about 0.2 to 0.3 cubic metres, and a single tuyere or blowpipe can be sufficient, inserted into the fuel bed a short distance above its base from the open side or angled in from a top edge of a complete enclosure. The necessary condition, as was noted in chapter 7, is that the nose is near the base of the fuel bed but far enough above it for molten slag and metal to collect without reaching the tuyere nose. Such bowl or hearth furnaces can be used for smelting oxide ores of metals, for melting, or for simple heating for forging or annealing of metal. The principal disadvantage in their use for smelting is that, particularly in the smaller versions, there is not enough distance from the nose of the tuyere to where the plume reaches the surface of the fuel bed for all of the carbon dioxide initially developed ncar the nose to be reduced to carbon monoxide before it escapes from the bed. The reduction power of such a bed therefore can be limited, even though a considerable maximum temperature is developed in a small volume not far from the tuyere nose. This is one of the main reasons why some modern reconstructions have not made successful smelts. Other reasons are insufficiently high space velocity and the high rate of heat loss from any small furnace. The principal way of dealing with such conditions in the bed is to charge charcoal and ore at the side of the furnace opposite the tuyere where the reducing conditions are strongest and then, as charcoal is consumed at the tuyere, to add more charcoal and ore opposite to the tuyere. The metal reduced there is moved with a bar towards the tuyere nose for slag formation, and in the case of iron smelting, agglomeration to a bloom. This is the procedure in the smelting hearths described below. Experimenters have made many test runs in small bowl furnaces in recent decades, and the temperature distribution was shown particularly well by Hetherington (1980). This is reproduced, slightly modified, in figure 8. A single tuyere 20 mm I.D. was used in a furnace 500 mm I.D. and 300 mm deep, with charcoal lump size averaging about 25 mm, and with no ore added. Air flow rate was 150 litres per minute, and tuyere velocity was 8.0 metres per second. It will be noticed that an appreciable volume of the fuel bed is above a temperature of 1,300oC and that the location of probable maximum temperature is about three fuel lump diameters from the tuyere nose. These conditions agree with those outlined in chapter 6 and given in more detail in appendices I and 2. If oxide ore were to be added with the fuel,
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Mastery and Uses of Fire in Antiquity
r--
_---! _---,8 oo·c
So em
_ _----.900 I,oot-
_ - - ; 1/1 00 _ - - - - - . /,J.qIJ
_----r /,140
1,,#-00
{
.......
~-~--
----
/. ''0"
Positi•." of
ttJ..y
Figure 8 Temperature Distribution in Fuel Bed with Single Tuyere
temperatures would be decreased moderately because of heat absorbed by the ore and by its chemical reactions with carbon monoxide. Under such conditions a temperature in the order of I,200°C, enough to make a fluid slag, can exist in a useful volume of the bed, and successful small scale smelting can be done. This was demonstrated experimentally some decades ago (Wynne and Tylecote 1958) in the smelting of iron ore in a bowl or pot 230 mm LD. Many arrangements of charges of ore and charcoal and placements of the tuyere were tried, with iron being most successfully made when the tuyere velocity was 10.5 metres per second (which is relatively low but higher than that of Hetherington). Charcoal size was about 6 mm, and ore and charcoal were charged only against the back wall of the furnace opposite the tuyere. A section of an interrupted and solidified heat is shown in figure 9, and it is evident that the ore was reduced in the high carbon monoxide content part of the stream of products of combustion ncar the back wall. The reduced iron then became welded together to form a bloom without melting, at higher temperature closer to the tuyere. Slag from the gangue of the ore fluxed by contained iron oxide melted and flowed by gravity down and away from the growing bloom of iron. The investigators noted that "blast pressure [i.e., tuyere velocity] for best results was very critical; soft blast did not reach the ore while hard blasts were still rich in carbon dioxide when they reached the ore." This is an excellent demonstration of the nature of the plume of gas from a tuyere nose described above. It was also found that crushing of
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Charcoal Fuelled Furnaces
- - ; ! 3 em.
Figure 9 Iron Ore Reduction in a Small Bowl Furnace
the charcoal to minus 6 mm size was essential for good reduction of carbon dioxide to carbon monoxide in the short distance involved, and therefore maximum iron reduction. A SMELTING HEARTH
The smelting bowl or hearth called a "Catalan forge" was, with small variations in geometry, widely used in Europe until a century ago. Its simple shape and single tuyere originated in the earliest smelting furnaces, and the practice of manipulation of the fuel bed and ore was added at some unknown later date. Detailed descriptions of operation about a century ago have been given (e.g., Percy r864, 278; Eglestone r 879, 5 I 5). The latter is summarized here because, though shorter, it clearly typifies the reactions involved, which cannot have changed from earlier times. It is instructive in many ways, particularly in the comments made by the observer. The hearth was nearly square, about 700 by 750 mm in plan and about 800 mm deep. A single tuyere was used with a rectangular nose 45 by 22 mm, laid on its flat side and extending into the hearth about 60 mm, the nose about 300 mm above the bottom of the hearth. Much importance was attached to the angle of the tuyere being about 14 degrees above the horizontal; "too flat" giving a "greater yield of lower quality iron," too steep being "economical in fuel but giving poor yield" of iron from the ore. Air flow rate was apparently 2-4 cubic metres per minute, giving a tuyere velocity of 44 metres per second, desirable for deeper horizontal penetration. The ore used was calcined and crushed to minus 25 mm size, but the charcoal size was not given. The growth and condition of the bloom of iron was followed closely by an iron bar probe, and the hands-on nature of the operation is clearly indicated by a few extracts from the description:
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Mastery and Uses of Fire in Antiquity
The flame, when the operation is being properly conducted, will be bluish or reddish. When it changes to brilliant white, tipped with yellow, it shows that the temperature is too high. The fire must then be chilled at once either by charging ore, the quantity depending on the brilliance of the flame; or sometimes where the charcoal has burned low, by taking off some of the fuel .. , There is always danger whenever the temperature becomes too high, even for a very short time, that some of the iron will become steely [higher in dissolved carbon content], and the loupe will become less homogeneous. To avoid it the hearth must be constantly sounded, and the heat of the fire regulated by the indications of the sounding-bar, which must be thrust into different parts of the face of the loupe, and moved around its sides to ascertain its shape, the condition of the rim, and how it lies in the hearth, and plunged down into the scoria [slagJ to ascertain both its quality and quantity. The bloomsman is thus guided in his work by the hardness of the loupe, its shape, and the way it lies in the hearth, the color and fluidity of the cinder [slag], and the color of the flame ... The proper temperature is indicated by a pasty condition of the loupe, and an easily flowing cinder ... When the loupe is sounded with the bar, if the temperature is right, the tool will sink but a short distance into the face of the loupe, and when withdrawn but little of the iron will stick, and there will consequently be only a short thin thimble of iron with a round knob at the end called a button attached to the end of the bar. If the temperature is too high the loupe will be soft, the tool will sink deep, and a long thick thimble and button will be drawn out, in which case are must be charged at once ... If the furnace is too cold the bar will not enter the loupe, but will strike against it with a dull thud; no attachment will take place. In this case charcoal must be added, and the furnace left to itself until it becomes hot enough to work.
After about two hours of operation and periodic tapping of slag, a bloom weighing about I50 kg was pried up from just under the tuyere and removed. The whole operation was then repeated. The slag contained about 25 per cent silica and 55 to 60 per cent ferrous oxide. The carbon content of the bloom was quite uneven, averaging about 0.20 per cent, lower carbon content being made possible by decreasing the production rate. The observer noted particularly how easily molten cast iron was made if the ratio of ore to charcoal was allowed to become too low. While the above description was published in the late nineteenth century, far from antiquity, its value is that it clearly shows the hands-on details of operation and the control of iron smelting in a hearth entirely on the basis of experience and visual and tactile clues, using no knowledge of chemistry - simulating a practice in antiquity. However, a I50 kilogram bloom could not have been produced in antiquity be-
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Charcoal Fuelled Furnaces
cause there was no source of mechanical power large enough to supply the necessary combustion air rate, nor to forge such a large bloom if made. ANNEALING AND FORGING OF METALS
The simplest uses of bowl or hearth furnaces are as generators of localized heat and high temperature. A bowl furnace with only charcoal in it and a single, nearly horizontal tuyere whose nose is near its bottom becomes a forge fire, in which bars or objects of metal such as copper, bronze, and iron can be heated. The maximum temperature reached by the object can be controlled, since it depends both on the air supply rate and on how close the object inserted through the fuel bed is to the maximum temperature zone just in front of the tuyere nose. Annealing or softening of copper and bronze occurs above a temperature of about 350°C, which is quite moderate and shows no colour in a fuel bed. Such temperatures can be reached with little oxidation simply by placing the object on top of a fuel bed of charcoal held in a bowl or crucible, and supplied with a small amount of air by natural draft through one or two small holes in the crucible wall near its base. With an anvil and hammer within reach, copper or bronze can then be rapidly extended by alternate cold working and annealing. Since copper can absorb oxygen from heated air and become less ductile, in this case the strongly reducing products of combustion leaving the top of the bed arc protective against such oxidation. In the production of iron it is essential to forge the bloom taken from the smelting furnace, to squeeze out slag, and to consolidate the porosity in the bloom into a sound bar. This requires a temperature of about 1,100 to I,300oC, for three reasons: to make the iron sufficiently plastic to be easily deformed by hammer blows, to liquefy contained slag so that it can be extruded from the mass, and to reach welding temperature to eliminate internal porosity. Such temperatures can be found not far from a tuyere nose, but the local atmosphere is then very oxidizing and an iron oxide scale unavoidably forms on the surface. (This will be discussed further in chapter 12.) CRUCIBLE FURNACES
It is also possible to melt pieces of copper and bronze contained in a shallow bowl by covering them with a layer of charcoal and supplying combustion air from the top. One or more tuyeres are used, depending on the diameter of the bowl, held at a steep angle or nearly vertical above it, with their noses just above or touching the fuel. Primary
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Mastery and Uses of Fire in Antiquity
combustion occurs close to the top of the fuel bed and a layer of maximum temperature is created 60 mm or so below its surface. This layer has high radiant heating power and melts the objects below it, which then flow together to form a pool. The familiar plano-convex" bun"shaped ingots demonstrably can be made this way. OPTIMUM CRUCIBLE MELTING
The thermally most efficient way of crucible melting, giving the best working access to the crucible, and with its air supply provided by natural draft, is a version of the vertical circular pottery kiln in which the fuel as biomass or as charcoal is above the perforated floor rather than under it. The floor has perforations small enough to prevent fuel from falling through and is positioned only high enough for free passage of combustion air from openings in the furnace wall beneath the floor. The crucible sits on a refractory stool about 50 to 80 mm high in the centre of the floor, to hold the crucible in the position of maximum temperature above air entry through the floor, each hole acting like a tuyere. In some cases the crucible simply has its bottom made thicker to act as a stool. The distance between the crucible outer wall and the inner furnace wall should be at least that of half the crucible outside diameter at its widest. With the crucible in place, this annular space is filled with fuel, the crucible is filled with whatever is to be melted, its cover is put on, and more fuel is then added to cover the top of the crucible by 100 mm or so. The furnace wall should extend vertically at least another 100 mm to increase the strength of the natural draft created when the fuel is burning. Maximum temperature will be attained with charcoal fuel and a thick furnace wall, but biomass fuel cut to small sizes can reach temperatures satisfactory for many purposes. When charcoal is used as fuel it is preferably in moderately large-sized lumps to decrease loss by reduction of carbon monoxide in the upper part of the fuel bed, since what is desired is maximum crucible wall temperature, not a strongly reducing atmosphere. When the fuel is ignited, air flow is created by natural draft and its rate, and therefore the maximum temperature reached in the crucible can be regulated by a damper over the air entries below the perforated floor. Such a furnace is simple to build, temperatures approaching I,400°C can be reached with biomass fuel, there is easy access to the crucible, fuel economy is good, bellows are not required, and the perforated floor has good service life because its average temperature is moderated by the incoming combustion air.
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Charcoal Fuelled Furnaces
As will be discussed in some detail in chapter 10, smelting of oxide orcs can be done by filling a crucible with a thorough mixture of finely pulverized ore, flux, and charcoal, tamping it down, and heating it to temperatures of about 1,100 to 1,300°C. The earliest archeological record of a crucible furnace is, as noted in chapter 4, apparently at Abu Matar, dating from 3300 to 3000 B.C. (Tylecote 1976, 17), but the details are sketchy. The reconstruction shown by Tylecote has omitted the post almost certainly present under the crucible, probably because it would have been difficult to identify as an artifact. The crucible would not have been inserted and removed through the wall but placed on the stool and taken from the furnace from its open top, since the fuel bed would be made only a small amount higher than the cover of the crucible. If a crucible with its bottom thickened to 60 to 80 mm, i.e., a self-contained stool, were to be found, in the absence of evidence of tuyeres or bellows, it would suggest that a natural draft crucible furnace had been used. Archaeological record also exists of crucible smelting in the third millennium B.C. at Goltepe (Yener and Vandiver 1993). THE SMALLEST FURNACE
As furnaces become smaller in size or working volume, it was noted in chapter 2, their efficiency of use of fuel becomes rapidly poorer and increasingly high rates of heat input are required to obtain some useful fraction of the AFT of the fuel. With very small charcoal-fuelled bowl furnaces, the heat rate input necessary, which requires a proportionately high rate of air supply, can become high enough that the air supply levitates the fuel bed and can blow it out of the furnace. However, if a depression 10 to 20 mm in diameter and 5 or 10 mm deep is carved into the top horizontal face of a lump of charcoal, an effective very small furnace can be made. If the charcoal wall of the depression is ignited and then kept supplied with air by a blow-pipe directed at its centre, moderately high temperatures can be developed, higher if the blowpipe is supplied with a small bellows. Charcoal is a very poor conductor of heat so the charcoal block can often be handheld. The blow-pipe tip must be small in diameter, one millimetre or so, in order to localize the area of highest temperature. Human breath would be a usual source of air which with charcoal fuel could develop an AFT in the order of 1,200 o C, but a small bellows supplying normal air could create higher temperature. Since the rate at which air (i.e., heat) can be supplied is large compared to the small volume of the cavity, and there is no loose charcoal to be moved by gas velocity, a high
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proportion of the AFT can be developed in a volume of a few cubic millimetres, and fuel economy is of course of no concern. The primary limitation is the size of object that can be heated, and use of such a "furnace" is typically for jewellery. A WOODEN FURNACE
A few years ago the published discussion of strange furnaces of the second millennium B.C. at Anfun in northern Nigeria (Grebenart 1985) led me to explore the possibility of a furnace made entirely of wood. It turned out that an effective furnace with an operating life of about a day can be made from a hollowed-out short length of tree trunk or a stump. When filled with lump charcoal and combustion air directed from the top through a tuyere, temperatures over I,4oooC can be reached. (The furnace and its operation are described elsewhere in detail lRehder 1987aJ.) When such a furnace is discarded after a day's use, it can be broken up to provide part of the charcoal for the next day's work -leaving little trace for an archeologist. PART 2: SHAFT FURNACES
A shaft furnace is a flat-bottomed bowl with walls that have been extended upwards to increase the height of the top of the fuel bed above tuyere level. Then as fuel is converted to gas by combustion at tuyere level, downward movement of fuel by gravity creates a counter-current flow of hot products of combustion upwards and of lumps of fuel downward. Any objects mixed with fresh fuel added at the top of the shaft accompany it and pass through increasing temperatures and gas compositions that are at first reducing and then oxidizing as tuyere level is approached. The advantages gained over a bowl or hearth are considerable. These are increased thermal efficiency from the longer heat exchange between rising products of combustion and descending burden (furnace contents); more time for an increased amount of carbon monoxide to reduce ores of metals added with the charcoal; and continuous operation, which decreases heat loss per unit of material processed. These all decrease charcoal consumption per unit weight of product. There is in addition an automatic mixing mechanism, described later in this chapter, which eliminates the labour necessary with a bowl or hearth to move materials during processing. Technically, a shaft furnace is one in which the top of the fuel bed is appreciably higher than the level of complete conversion of carbon dioxide to carbon monoxide. This level is independent of furnace diam-
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eter at constant space velocity and (as noted in chapter 7) is typically about IO to 20 fuel lump diameters above tuyere level. It also depends on the charcoal used and increases with space velocity. As the bed height is increased above the level of complete conversion of carbon dioxide to monoxide, the residence time of ore in the furnace increases and also more heat can be recovered in the descending material being heated. For example, a furnace with 20 mm average fuel lump diameter should have a bed height above tuyere level of a minimum of about 500 mm. MELTING OF MATERIALS
If the material added to the furnace with the charcoal is not an ore but is pieces of reduced metal or inorganic materials such as slag or many kinds of stone, these are preheated but have little reaction with furnace gases while still solid. They melt at the level where they reach their melting or fluidity points and immediately trickle down between and ahead of fuel lumps, to collect as a molten pool in the hearth of the furnace, from which they can be tapped out as necessary. Since the heat absorbed is much less than for smelting, much less charcoal is necessary per unit weight melted to reach a given temperature of molten metal- typically about one-tenth as much. This procedure seems not to have had much attention in the archaeological literature, but it must have been considerably used throughout antiquity. It should be noted that the heat contents per kilogram of igneous rocks and slags are from 50 to 75 per cent higher than those of metals, and in the smelting of metals from their ores the amount of slag per unit weight of reduced metal can vary from three to as much as twenty times that of the metal, depending on the particular metal and its ore. The result in practice is that much of the heat necessary for smelting is simply to melt and fluidize the slag involved, unless the ore is very rich or has been efficiently concentrated. RESIDENCE TIME
The continuous downward flow of materials during operation is at a rate that is determined by the rate of combustion of the charcoal, which in turn is determined by the rate of air supply in a reasonably deep fuel bed. The length of time that it takes material added to the top of the fuel column to reach tuyere level is called the "residence time," and this is obviously an important factor in the amount of heat and degree of temperature that will be absorbed by any work material added with the fuel, as well as the time during which furnace gases can react with work material.
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When furnaces are relatively small and heat loss rates are proportionately high, more fuel is necessarily burned to reach some objective maximum temperature; when the fuel is charcoal, which has a bulk density considerably lower than that of ores or metals, the ratio by volume of charcoal to ore becomes high. These were the conditions in shaft furnaces throughout antiquity, and furnace contents typically contained 90 per cent or more by volume of charcoal. Typically close to 4.0 cubic metres of air is required to burn one kilogram of charcoal as a fuel bed (as was noted in chapter 7), and the residence time in minutes is then given by the rate of consumption of charcoal in m 3 per minute, divided by the furnace volume in m 3. It can be shown in more detail that the residence time in minutes is given by the product of 4.0 m} of air per minute, times the bulk density of the charcoal in kg per m 3 , and the fuel bed height in m, divided by the space velocity in m per minute. For example in a fuel bed 1.0 metre high above tuyere level in a straight-sided shaft 0.30 m I.D., with charcoal of bulk density 220 kg per m 3 and with a space velocity of 5.0 m per minute, it will take I76 minutes for material added to the surface of the bed to reach the level of the tuyeres. Since the bulk density of charcoal can vary considerably, it directly controls the residence time in the furnace; this is a principal reason why early smelters were careful of the wood used for making their charcoal, and generally preferred hardwood charcoal which is of higher and more uniform bulk density and so gives longer and more predictable residence time in the furnace. The basic determinants of residence time are bulk density of the fuel, height of the fuel bed, and space velocity; these are in principle controllable, but there are natural constraints. Bulk density of charcoal varies but in most cases will be made from a particular species of local tree which the smith is familiar with and which will make charcoal of a fairly narrow range of density. The space velocity necessary to reach a given maximum temperature in a particular furnace is largely determined by its size and wall thickness (i.e., heat loss rate) as noted in chapter 3; in general in antiquity this appears to have been limited to a range in the order of about 4.5 to possibly 7 or 8 metres per minute. Fuel bed height can be increased by taller furnace walls, but the limit here, aside from structural stability, is increased ratio of wall heat loss to top gas heat loss. The gas pressure drop increases as does leakage, leading to harder work on the bellows and more air leakage. As a result it seems that residence time might have ranged from about IOO to 180 minutes per metre height of fuel bed. There is obviously a balance to be made between the residence time in a furnace and the reducibility of an ore to be smelted. A major factor in the rate at which an oxide mineral in an ore can be reduced is its sur-
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face area in m 2 per kg, since reduction of oxides of metals in carbon-fuelled furnaces has been adequately demonstrated to be a surface reaction with carbon monoxide gas. Small lump size of ore and ample porosity either naturally or created by roasting make large differences in reduction rates. Ores of metals are natural products that as charged to a furnace can vary considerably in terms of these factors, and their suitable preparation to increase surface area per unit volume can change a poor smelting result to an excellent one. In antiquity this was often done by roasting the ore on an open wood fire, which both increased its porosity and made it easier to crush to smaller size. BOSHES
Still another, more complicated way of increasing the reducing power of a furnace of given height is to construct the furnace with a bulge or "bosh" in the walls of the shaft between its top and the tuyere level. At the level of a bulge the internal cross-section is larger, and so the rate of descent of materials downward and of gases upward is decreased in proportion at constant air rate; i.e., residence time locally and in total is increased. To my knowledge such wall distortions were little used in antiquity but became common in late medieval times in Europe and were of questionable value. FURNACE DIAMETER
In general shaft furnaces in antiquity were circular in plan or approximately so, probably because this is the shape that gives the smallest heat loss rate and therefore lower fuel consumption. However, aside from this factor, the cross-sectional shape can be oval or square or rectangular, and long rectangles have been used in the Tatara furnaces in Japan, in recent modern large copper smelting furnaces, and in the wind furnaces of Sri Lanka (described in appendix 3). At a given space velocity, the production rate of a particular smelting furnace will increase with its cross-sectional area, provided that its space velocity is maintained. The tuyere velocity usually also must be increased to obtain deeper penetration into the bed to avoid having a partly inactive centre. This occurs naturally if the number and size of tuyeres is unchanged for the larger furnace, but the resulting higher velocity requires increased pressure in the air supply as shown in appendix 2, requiring harder work on the bellows. These questions of the relationships between production rate, furnace diameter, number of tuyeres necessary, pressure drops in gas flows, and the power required to move the air through the system will
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not be pursued further here, since for any given case they can be sorted out by reference to appendix 2. They are mentioned to note their importance and the variety of interdependent choices inherent in a basically simple system, and to suggest why throughout antiquity some particular choices were repeatedly made. As noted above, in practice charcoal burns and is added at the rate completely determined by the air supply rate; the proportion of ore added per unit of fuel added by weight is adjusted so that the desired temperature profile for satisfactory smelting is created and maintained. A lower ratio of ore to fuel allows furnace temperature to increase, while too high a ratio can result in furnace temperature being too low to melt and fluidize slag, eventually resulting in a "frozen" furnace. These are also the reasons why a smaller furnace must be fed with a lower ratio of work to fuel than a larger one, the basic heat losses in the smaller furnace being higher. SUPERHEAT OF MOLTEN PRODUCTS
Considering in more detail the flow of materials down through a shaft furnace, we see that a separate and important phenomenon occurs from the moment when any solid material added with the fuel has absorbed enough heat to raise its temperature to its fusion point and becomes molten. If the material is metal, it immediately becomes a fluid of very low viscosity that by gravity trickles as droplets rapidly down through the bed past the tuyere, to collect as a molten pool in the hearth of the furnace. The droplets fall through a regime of increasing high temperature as they approach tuyere level and so arc increased in temperature to above their melting point, a condition called "superheat." The rate of heat transfer to the descending droplets as they approach tuyere level is high, because it is largely by radiation and conduction from lumps of fuel at very high temperature, and the surface area of the droplets per unit of weight is large. However, the time of fall through the high temperature zone is short, in the order of a few seconds, so the efficiency of heat transfer is low, about 4 to 5 per cent (Rehder 1977). For this reason it requires a significant decrease in the ore to fuel ratio to appreciably increase the temperature (i.e., to superheat) of a molten furnace product. THE FURNACE HEARTH
The hearth of the furnace has been referred to several times, and while the term basically means the floor of the bowl or shaft, it is commonly used to indicate the volume below tuyere level. It has been amply dem-
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onstrated experimentally in modern times that there is little circulation of gas in this volume and that the atmosphere is essentially carbon monoxide and nitrogen. There is also little consumption of fuel except when smelting iron ore to molten cast iron. In this case carbon in the charcoal in the hearth can reduce iron oxide in the molten slag, and also dissolve in the reduced iron. Except for these reactions, no heat is generated, and some heat is lost from hearth walls and floor; so most of the heat in the hearth, and thus its temperature, depend entirely on the heat carried into it by molten material descending from above. It is clear, therefore, that descending molten material on reaching the hearth must be well above its melting or fluidity temperature in order for it to not only maintain hearth temperature but remain molten in the hearth, to be tapped out at a temperature well above its solidification temperature. The volume of the hearth must be able to contain all of the slag and molten metal that come down from above tuyere levcI, and so the depth from tuyeres to furnace bottom must be proportioned to how frequently the molten products are to be removed. The smallest height necessary is when the furnace is operated only as a melter, with an open tap-hole so that slag and metal run out together continuously as produced, to be collected in a ladle or a refractory basin so that they can separate by gravity. More commonly the tap-hole is opened only at intervals, sometimes of many hours, requiring a deep well. The metal appears first, followed by slag, so that they can be separated by temporary closure of the tap-hole when slag appears. In some instances where the volume of slag is large compared to the volume of metal, a separate tap-hole at a higher level is used to remove slag only, so that the metal tap-hole need be opened less often. The weight of slag made per kilogram of metal during smelting increases of course as the metal content or the grade of ore decreases, and since slag has a density typically in the order of one-third that of copper or iron, considerable volumes of slag can be made when lower grade ores are used. In such cases the volume in the hearth can be sufficient to rise to and above tuyere nose level, thereby plugging the tuyere(s) completely or partly and increasing the back-pressure on the bellows. This is easily avoided by tapping slag out through the separate slag-hole, whenever it can be seen through a tuyere, when a bubbling sound is heard, or more effort on bellows is required. There has been much unnecessary comment, in bloomery iron-smelting practice in particular, on the question of tapping or non-tapping slag practice. It usually depends simply on the grade of ore used, plus the number of hours the furnace is in operation to make slag, since it is essential to uniform continued operation that tuyeres be not even partially closed.
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It was noted above that the contents of a charcoal-fuelled shaft furnace are typically 90 or more per cent charcoal by volume, and so the bulk density of the burden as a whole is not far above that of the charcoal itself. This is about one-tenth that of molten slag; thus when a pool of slag and/or metal is retained in the hearth, the burden floats on the slag only slightly immersed, the molten metal being under the slag. Another arrangement for taking molten metal and slag from a furnace is to continue the hearth outside the furnace as a refractory basin, with the front wall of the furnace reaching not quite to the extended hearth floor. The narrow horizontal slot thus formed acts as a separator of slag, keeping most of it inside the furnace since the levels are the same inside as out. In antiquity such arrangements were used in the smelting of copper and lead only because iron was always produced as a solid state bloom with molten cast iron as occasional unwanted byproduct (chapter 12). Modern studies have shown (as does common sense) that the friction of granular materials moving downward against vertical container walls tends to restrain the flow rate close to the wall. The material in the centre of the container then flows more rapidly, resulting in continuous mixing or folding of the wall material towards the centre. This mixing is surprisingly effective, and was demonstrated in a coke-fuelled 1,200 mm I.D. shaft furnace that was melting iron. This was done by stopping the air supply during normal operation, quenching the whole of the contents of the furnace with water, and then carefully taking out in archeological fashion materials that had been previously marked for identification (Rambush and Taylor 1945). Very good mixing was found to have been accomplished within a vertical distance equivalent to about two furnace diameters. This mixing in a straight, parallel-sided shaft is a beneficial effect of the friction at the burden-wall interface, but when the walls are not vertical or straight, the friction can become much greater, and as has been noted above, some strange configurations have been used in antiquity. In many such cases furnace operation must have been erratic, as burden occasionally" hung up" and then fell, or slow, simply because of physical indigestion. In a furnace built on a slant, the product may have been different in chemistry from upper and lower sides of the shaft, due to friction being stronger and material and gas flows slower on the lower side. The reasons why the flow of gas upward through the fuel bed is distorted laterally were noted in chapter 7 in discussion of how a tuyere works, and the distortion is of course most severe when only one tuyere is used. This explains why, when smelting in bowl and hearth furnaces which have limited possible distance of vertical movement of
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materials, only one tuyere is usually used and the ore is intentionally charged in a one-sided fashion to conform to the gas flow, as has been described. The fuel bed is utilized in a partially horizontal direction, with fuel and ore being manipulated accordingly with a bar. In a shaft furnace with a single tuyere, this placement is of less value because of the inherent mixing of ore and charcoal that occurs in a shaft of reasonable height. On the other hand the mixing itself tends to average out the relationship between ore and gas. Quite small charcoal-fuelled shaft furnaces can be surprisingly productive. The experimental work of Tylecote et al. (1971) on smelting iron ore to blooms was on a furnace 300 mm I.D. with a single tuyere; with an air supply rate of 300 litres per minute the rate of bloom formation averaged 1.2 kg per hour. My unpublished work on a slightly smaller furnace has corroborated this figure. If it is assumed that the furnace was operated for six hours at a time four days per week, the product would be 24 kg of blooms, which after forging could produce about 10 kg of usable objects. Annual production could be 400 to 500 kg of objects. UNPREDICTABILITY OF FURNACE OPERATING RESULTS IN ANTIQUITY
The two serious sources of variation and unpredictability of results of furnace operation in antiquity were the difficulty of controlling the rate of combustion air supply at the tuyere nose and the ratio of ore to charcoal by weight in the furnace charge. The first was largely due to lack of effective ways of measuring air flow rates, and the second was because of lack of recognition of the difference in importance between volumes and weights of materials. All of chemistry is based on weights of reacting materials and only weights have been discussed here, but in antiquity and at least into the Middle Ages in the West, measurements of materials were by volume, and by weight only in the cases of precious metals and gemstones. The importance of quantitative measurement of air flow rate has been noted repeatedly above, so it is startling that it was not until as late as r869 in North America, when chemistry was becoming well understood and widely used industrially, that an authority on blast furnace practice published his demonstration that it was the volume, i.e., the weight, of combustion air that was the prime regulator of furnace operation. For several hundred years before this, pressure had been considered as the important measure of the air supply rate, and in antiquity the measure likely was simply the amount of physical effort necessary on the bellows.
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In antiquity it was of course evident that there was an essential relationship between quantities of ore and charcoal to obtain satisfactory smelting, but there were two unseen variables. One was the bulk density of a material, and the other was the contents of metal in an ore and of FC in charcoal. In the case of ore, its bulk density increases with its content of metal, and there were some visual clues to its purity or metal content. But the metal content of a basketful of crushed ore could still change, not only from one ore body to another but within the same ore body. It also changed with the degree of roasting and the fineness of crushing or grinding. The most serious single variable may have been the bulk density of the charcoal, which varies from one species of tree to another and one part of the same tree to another and according to how the charcoalmaking operation is carried out. Although some visual clues to charcoal quality were recognized early, there could still be variation in the FC content of a basket of charcoal as a whole because of such change in bulk density, and the weight of FC was the important component chemically. As a result a basket of charcoal could vary by a factor of two or more in its content by weight of FC if the smith was inexperienced or inattentive. In this situation furnace operators necessarily became very conservative, a natural reaction in the face of uncertainty, and in order to be able to repeat a successful smelt, they were very particular about which trees were used to make charcoal and how the charcoal-making was done. Since the magic elements that apparently were being effective were elusive, many pointless and useless practices became considered necessary, creating and being encouraged by a mythology. It was thus not easy to transfer a successful smelting operation from one locality to another, whether intentionally or by theft. Which items of practice were essential was not known. In a new location the trees might be different, or the charcoalling different; the ore would certainly have a different metal content, and the bellows connection to the tuyere and the tuyere arrangement would be different. All these factors individually could result in unsuccessful smelting. The situation was much worse for iron smelting than for copper smelting, because of the complicated reactions between iron and the carbon of the fuel.
IO
The Reduction of Metals and the Functions of Slags
PART I: THE REDUCTION OF METALS
Ores may be defined for present purposes as rocks of various kinds that for geological reasons contain concentrations of minerals of metals of use to humankind. These minerals are distinct chemical compounds, often sulphides or oxides but also hydroxides, carbonates, silicates, and others. They are usually of higher specific gravity and different colour than the host rock, and so can be concentrated by crushing the ore and separating the mineral by gravity or visual appearance. In antiquity (and to a large extent even today) the minerals could be economically decomposed or reduced to their constituent metals only by first changing the mineral compound to an oxide if it was not already one. The oxide was then heated in an atmosphere rich in carbon monoxide gas, which combines with the oxygen of the metal oxide to form carbon dioxide gas, which releases the metal. The change to oxide can be made by roasting in air the crushed ore or a concentrate from it on a charcoal or (more usually) a wood fire. This both changes sulphides, for example, to oxides and increases the porosity of the ore by thermal fracturing, the latter giving more ready penetration of reducing gas in the smelting furnace. The host rock material, called "gangue," remains, and its composition is usually such that it requires heat at high temperature to melt or become fluid. A "flux" must be added with the ore charged to the smelting furnace, to decrease the temperature at which the mixture of flux and gangue becomes a slag fluid enough to flow readily in a smelting furnace.
102 Mastery and Uses of Fire in Antiquity The movements of gases and materials within charcoal-fuelled bowl and shaft furnaces have been discussed in chapters 2 and 9, but there are details of the mechanism of reduction of oxide ores of metals that involve these movements and in which slags play an essential part. The mechanisms are now well understood, having been worked out by extensive research in the iron and steel industry in the 19 50S and' 60S; but the account given by Percy in 1864 (302-6) was in fact essentially correct. The following discussion assumes that a shaft furnace is being used. MECHANISM OF REDUCTION
When a crushed ore or a concentrate is added to an operating furnace with an appropriate proportion of lump charcoal, it becomes part of the "burden" (furnace contents) and descends with it. The burden is permeated by a rising flow of products of combustion containing carbon monoxide gas, which reacts chemically with the bits of metal oxide at the surface of each piece of ore or concentrate. This removes oxygen from the metal and forms carbon dioxide gas. The reduced metal occupies less volume than did its oxide, so a small space results which permits both escape of the carbon dioxide and further penetration of reducing gas into the piece of ore. This is assisted by any mechanical porosity that has been created by roasting the ore. A porous skin of reduced metal in the form of microscopic bits and platelets is thus formed, distributed in the gangue in the same way that the oxide was distributed. This reaction can start at quite moderate temperatures (about 300°C for iron) which exist high in the furnace. With continued reaction as the piece of ore descends further into higher temperature gas, the skin thickens towards the centre of the piece until little or no metal oxide is left. Three factors or conditions are involved: (I) Reaction is at surfaces, so smaller and more porous pieces of ore reduce in shorter time than large, dense pieces. (2) Reduction is a chemical reaction which can start only at some minimum temperature, and increases in rate as temperature increases. (3) The time available for reduction is determined by the residence time in the furnace. As reduction of the metal oxide approaches completion, the piece of ore becomes a piece of porous gangue with bits of reduced metal distributed through it. In the case of copper smelting, the metal melts at about 1,083 °c, and if the gangue is still very viscous at this temperature as a result of insufficient fluxing, such pieces can stick together to form a mass that restricts gas flow and eventually can stop furnace operation. The mass of slag and metal must then be extracted from the furnace and when cool, crushed and separated by gravity so that the contained bits of copper or "prills" can be remelted in a crucible.
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However, if flux has been added with the ore in suitable proportion, the gangue plus flux interact and decrease in viscosity to form a piece or drop of molten slag, which permits, and encourages by a surface cleaning action, agglomeration of bits of contained copper. When a furnace temperature of 1,100 to I,200 o C is reached, the slag containing molten copper can form droplets that fall through the fuel bed to the hearth, where the copper forms a pool covered by a layer of molten slag. In the case of smelting iron, a singular and complicating factor is that the reduced metal can have a wide range of melting points, because of the solution of variable amounts of carbon in the iron from the carbon monoxide and the charcoal. If the amount of carbon dissolved has been small, the reduced iron will have a melting point approaching I,53o°C. However, the gangue in the piece of ore, which is self-fluxed by the iron oxide present, can have a fluidity temperature typically of about 1,100 to I,250oC. Then at this lower temperature which occurs at some distance above tuyere level in the furnace, a piece of iron ore becomes a droplet of molten slag containing bits of solid reduced iron. These droplets fall through the interstices of the fuel past tuyere level where the atmosphere is very hot and oxidizing, and the droplets are increased in temperature while the contained iron is protected from being re-oxidized. The lumps of charcoal in the hearth act as a filter, and the bits of iron which are now at welding temperature adhere to each other to form a growing bloom of iron, while most of the slag continues to the hearth. Since copper does not have the property of easily welding to itself, blooms seldom form in smelting ores of copper. It is important to note that several time factors are involved in the smelting process, since the rate of reaction of metal oxide with carbon monoxide increases with temperature, which in turn increases with descent in the furnace. The rate of descent depends on combustion rate at tuyere level, which controls both the time that a particle of ore is within a given temperature range and the slope of the vertical temperature profile in the furnace. This was well illustrated in the work of Bohm (1983), who described the layer by layer extraction and chemical analyses of the contents of an experimental charcoal-fuelled blast furnace smelting iron ores, which had been stopped and cooled while in operation and then excavated layer by layer. Two different ores had been used in separate experiments, one with some porosity and the other considerably harder and denser. The residence times were the same and temperature sensors gave similar vertical profiles of temperature in the furnace. Chemical analysis showed that with the more porous ore the first metallic iron appeared high in the furnace at the temperature level of 300°C, while with the dense ore-reduced iron first appeared at the level of 550°C.
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Mastery and Uses of Fire in Antiquity
In the smelting of both copper and iron, reduction can continue to occur in the hearth of the furnace below tuyere level, where the atmosphere is very reducing and temperature can be high. The source of the iron is the iron oxide in the slag, which in iron smelting comes from the ore itself, and in copper smelting from the fact that iron oxide is a commonly used flux for gangue. In the latter case the reduced iron can dissolve in the molten copper in the hearth, which is undesirable. In iron smelting it simply adds to the bloom and the time for reaction is the whole of that of formation of the bloom. When smelting is done in a bowl or hearth furnace, the sequence of occurrences is essentially the same, but since there is not the automatic vertical descent of pieces of ore in the shaft furnace through increasing temperature, the contents of a hearth must be manipulated from their surface by hand tools (as described in chapter 9) to alter locations of materials with respect to the temperature and gas composition profiles exiting from the nose of the tuyere. THE CARBOTHERMIC SMELTING PROCESS
Also called the ore-carbon process and crucible smelting, the carbothermic smelting process can be used to smelt the oxide ores of iron, copper, and several other metals to solid state masses or to molten metal. The process and its early use have been discussed in detail by Rehder (1998) and are summarized here. Ore is pulverized and mixed with ore, flux, and charcoal in suitable proportions, and the mixture is then heated to temperatures in the range 1,100 to I,400°C depending on the metal involved. A major advantage of the process is that the heating of the mixture can take place in a very oxidizing atmosphere such as that produced by the combustion of biomass fuel by natural draft, so it can readily be carried out in a kiln. The process has been little recognized in the archaeological literature, the earliest archaeological evidence of its use seeming to be for smelting tin in the fourth millennium B.C. (Yener and Vandiver 1993). It was used for copper smelting in the second millennium B. c. (Hegde and Ericson 1985), in China for smelting iron from the fourth century A.D. or earlier (Needham 1958) to the nineteenth century A.D., in Africa (Cline 1937) of unknown antiquity, and in modern times for smelting iron. Tin was smelted at Goltepe in Turkey during the fourth millennium B. c. as a mixture of cassiterite sand and pulverized charcoal, in crucibles apparently heated in a kiln. Molten copper was produced in India in the third to second millennium B.C. by heating balls of a mixture of pulverized ore, charcoal, and flux in a small shaft furnace.
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Metal Reduction and Slags
In China in the nineteenth century iron was still being smelted in quantity by the carbothermic process, and from the enormous slagheaps remaining, apparently for many previous centuries. A good description was given by Shockley in 1904; in summary, tall narrow clay crucibles were filled with a mixture of about pea-sized ore and pulverized coal, and about two hundred were set upright in a rectangular enclosure, with interstices filled with coal. The floor of the enclosure was made of broken used crucibles covered with a layer of small lump coal, the layer of crucibles being left open at its front edge. A layer of coal was placed over the tops of the filled crucibles and the whole was then covered with sherds and a layer of ashes as insulation. Combustion air was supplied across the bottom through the broken crucible floor by either human-powered bellows or natural draft. The former gave a sixteen-hour campaign time and the latter three days. After cooling to handling temperature, the crucibles were broken up, and from the bottom of each was taken an irregular cake or bloom of forgeable iron weighing about five kilograms. After breaking off slag, the bloom was hot forged to bar stock. If too high a ratio of pulverized coal to ore in the crucible had been used, the iron made was a solidified pool of white cast iron. Snockley's measurement of the ratio that gave a product of forgeable iron agrees well with that required by the reaction given below. Cline (1937) supplies the following accounts of iron smelting in Africa: The Baya-Kala in Africa grind [the ore] to a fine dust, wash it ... dry it thoroughly, and mix it with the bark of a certain root. Having filled the furnace with charcoal and adjusted the bellows, they kindle the fire and begin to feed the ore by handfuls. As the fire sinks they add more charcoal. After 5.5 hours they break down the door and take out the bloom. The Ababua smelt iron '" under a leaf-covered shed. They have excavated the floor level down for about 60 cm. long and 40 cm. deep. After mixing the are hematite or limonite - with powdered charcoal and thoroughly drying it, they lay it in the trench, making a bed about 4 cm. thick, 15 [sic] cm. wide, and 70 [sic] cm. long, and cover it with legongo leaves. On this bed they place the tuyere, made of sun-dried clay from a termite hill, and add more charcoal. When they have engaged the larger end of the tuyere with a four-chambered drum bellows, they light the fire; and as the smelting proceeds, pull the tuyere gradually towards the end of the trench. The process takes about four hours. The furnace used by the smelters at Ondulu, Angola, is a long horizontal series of sections arranged on a gentle slope of ground, each section composed of two
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large, flat slabs of ant-hill earth inclining inward toward each other and held apart by small sticks. As each section is added, it is lined with a wall of mixed ore and charcoal; and when ten sections have been constructed and lined forming a little gallery from ten to fourteen feet long - the long central space is filled with charcoal, pieces of ant-hill laid over the top, and the sides plastered with mud. The smelters place their drum bellows at the sides [of the furnace] and work the sections one by one, removing the bloom and moving the bellows on to the next section as soon as each section has been smelted. It takes one day to smelt a kiln of ten sections. The Zulu-Kafir use three pits in a row, each oval, six feet by three in diameter and three feet deep. The ore is mixed with charcoal and smelted over a packed bed of the same fuel, with a pair of bellows working at each end of the pit. The Ovambo use a crucible in the primary smelting of iron ... The Dande manufacture a superior quality of iron in crucibles, in which the ore is mixed with the charcoal and smelted under a forced draught.
In 1864 Percy noted (pp. 840-1) that if a mixture of pulverized black manganese ore and carbonaceous matter is heated in a crucible to a sufficient temperature, molten manganese will be made. (The temperature necessary would be about 1,300oC.) In the late nineteenth century in the United States, iron blooms were made by cutting up the product of heating a layer 25 cm thick of mixed pulverized ore and coal, weighing about 10300 kg, in the low-roofed kiln called a reverberatory furnace. The forged iron was technically satisfactory but could not compete economically with the Bessemer process (Phillips 1891). I developed and was granted a patent in 1979 for a version of the process to smelt iron ore to molten cast iron in a foundry cupola, but while technically successful, the cost advantage turned out to be marginal. The basic process is being successfully used industrially today to make pre-reduced iron pellets for electric furnace refining. Reaction times decrease logarithmically as temperature increases and, to give reasonably short times for copper and iron, are in the range of about 1,100 to 1,400 c. Reduction is by carbon monoxide gas generated internally in the micro-cavities between particles, which reacts at particle surfaces of the oxygen in the ore. The reduction proceeds inward from the surface of the mixture as heat and temperature penetrate. Compaction of the mixture by forming pellets or balls, or tamping down in a crucible or on a hearth, is useful to increase particle contact area and heat transfer rate.
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The gaseous products of reaction must escape from the mass, which they do through the micro-cavities which have increased with change in ore and metal volumes as noted above. For this reason the composition of the external heating atmosphere is immaterial, and can be very oxidizing without effect on the progress of reduction. This is a major difference from conventional reduction described above. The process was first investigated in chemical detail for the reduction of hematite by Innes (1963), who demonstrated among other things that the heating atmosphere could be very oxidizing. Work by others since has demonstrated that the reduction reaction involved for hematite is:
The reactions between other metal oxides and carbon seem to be similar, and a considerable range of metal oxides have been reduced. Almost any form of carbon seems to be satisfactory. Haque et a1. (1992) and others have established quantitativcly for hematite the rclationships of reduction time to heating temperature, reactivity of the carbon, reducibility of the ore, ratio of carbon to ore, particle diameter, and mass or bed depth. Reduction time is an inverse exponential function of the absolute temperature, and I,} 50°C can give times of 10 to 15 minutes. Particle diameter or degree of pulverization is obviously important. When smelting weathered ores of copper and the maximum temperature reached is above the melting points of copper and slag, i.e., 1,100 to I,200 o C, containment in a crucible is necessary since a pool of copper under a layer of slag will form. Such temperatures can be reached in a biomass-fuelled kiln otherwise used for firing pottery, so copper metal and slag can be found in an excavation with no evidence of bowl or shaft furnaces, tuyeres, bellows, or blowpipes. The essential clues would be remnants of crucibles and a kiln. In the case of iron, if the carbon present is in considerable excess of that necessary for reduction only, enough carbon can dissolve in the iron to make it a cast iron that is molten at about I,I50°C, and containment in a crucible is necessary. However, if carbon is in only small excess, so that the reduced iron is low in carbon content and therefore high in melting point, the solid bits of reduced iron can agglomerate to an appreciable extent, if the slag is not too viscous, to form a mass of mixed solid reduced iron and slag. The resulting mass is much like the bloom from a charcoal-fuelled bowl or shaft furnace (to be described in chapter 12), and can similarly be reheated and forged to a sound bar of iron. The really important difference for the
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smelting of iron is that because an oxidizing atmosphere can be used for heating, an ordinary pottery kiln using biomass fuel can thus be used to make a forgeable bloom as briquets heated directly on the floor of the kiln. In the cases of both copper and iron, it would be difficult to determine metallurgically whether a metal artifact had been made from a carbothermic smelt or from the conventional use of lump charcoal and bellows in a bowl or shaft furnace. Because of its directness, simplicity, and ability to use existing pottery kilns using biomass fuel and natural draft, 1 think that the carbothermic method of metal ore reduction was probably used more extensively in antiquity for smelting copper and possibly iron than presently realized. For copper, this could have been either in several crucibles on the hearth of a kiln, or in a vertical biomass-fuelled natural draft furnace holding a single crucible. This could explain two seemingly unrelated archaeological puzzles. One is the many cases world-wide but particularly documented in the eastern Mediterranean and Near East - where there are fields of slag of considerable amount, a notable example being Cyprus, but only minor evidence of furnaces or tuyeres. This could be understandable if pottery kilns or similar structures had been used as smelting furnaces. The other puzzle that would be elucidated is the ubiquity of querns and similar grinding stones that seem to be far in excess of need for flour milling. The latter was brought out in the discussion following a paper by Varoufakis (19 81 ).
It also can be effective to smelt ores with entirely air-dry wood as fuel, in a shaft furnace blown with bellows or with natural draft. This has been demonstrated by reconstruction and successful operation of Roman and later furnaces, excavated in Norway over recent years by A. Espelund (1995). The consumption of wood per kilogram of bloom was apparently very high since it was converted to charcoal in the upper part of the conical furnace, and the blooms of iron made were more porous than usual but forgeable. PART 2: THE FUNCTIONS AND IMPORTANCE OF SLAGS IN SMELTING
It will have become apparent that the presence of a slag that is fluid at furnace operating temperature is an essential part of the smelting of metals. One of the earliest modern discussions of this fact was by Percy (1864,238-42) in his account of the procedures for fire-assaying of the ores of metals including those of iron. Percy emphasized the importance of the slag being both in adequate amount and sufficiently fluid.
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Slags are unavoidable in smelting since very few ores are free from gangue. They are also necessary. Not only arc they the practical way of completely separating the smelted metal from the gangue contained in the ore charged, and eliminating it from the furnace but, as noted above, they promote coalescence of molten reduced particles and then protect droplets of reduced metal from re-oxidation in the lower, hightemperature part of bowl or shaft furnaces. With suitable adjustment of composition they also can remove some impurities such as sulphur and phosphorus from the metal. If by chance a pure metal oxide is to be smelted, some gangue material and flux must be added with it to form a synthetic slag, to perform its functions of metal agglomeration and protection. THE PROPERTIES OF SLAGS
While the most important property of a slag is its fluidity at furnace operating temperature, the provision or creation of the necessary fluidity by addition of other oxides as fluxes has been a difficult problem because of the large number of alternative materials and their combinations. Without chemistry as guide, this was solvable only by trial and error until modern times. Fluidity is the inverse of viscosity, and the relationship between the viscosity of a slag at a given temperature and the chemical composition of the slag is nearly identical in form in metal smelting with that in making glass. This is because the materials involved in both cases arc mixtures of various oxides of metals, and particularly of silica, which contributes "glassiness" or the ability to cool to ambient temperature without containing crystallized material. FORMATION OF SLAGS
To review, a metallurgical slag is composed primarily of the gangue material in the ore being smelted, plus usually an added flux material to decrease fluidity temperature, eroded furnace lining, and the ash from the fuel burned. The last two arc usually a minor proportion of the whole, but all of these materials are mixtures of oxides of a variety of metals, with silica (silicon dioxide) being the most common but also alumina, lime, magnesia, and many others. These oxides have a wide variety of high melting points, but that of silica at I,7IOoC is above the upper limit of the temperatures attainable in antiquity. Another property of oxides of metals is that in their mutual chemical reactions to form more complex compounds, they are "acid," like silica, "basic," like lime, or neutral, like alumina. While each oxide alone may have a high melting point, their compounds can have melting
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Mastery and Uses of Fire in Antiquity
points below (in some cases, well below) those of their constituents. The relationship between composition and melting points of common mixtures are today recorded in rectangular or triangular phase diagrams representing equilibrium conditions. These are the formal basis of fluxing and can be very useful in knowing precisely what to add, and in what amount, to make a slag from the gangue in the particular ore that will be fluid at the furnace temperature necessary for smelting a particular metal. However, equilibrium is seldom reached in practice, so the phase diagrams must be used with caution. VISCOSITY OF SLAGS
This is a large, important, and complex subject, and discussion here is limited to the basic points necessary to understanding the manipulation of metallurgical slags. As discussed under glass-making in chapter 5, the property of glassiness in a mixture of metal oxides results in the material having a viscosity-temperature curve rather than a sharp melting point as in metals. A glass may be clear or coloured depending on the presence of small quantities of other oxides, and this may have minor effect on viscosity. Metallurgical slags are usually partly or wholly of glassy structure at ambient temperature, depending primarily on the amount of silica present. In the glass industry today the melting point of a glass is simply defined as the temperature at which it has decreased to a specific viscosity. As a slag is forming in the upper part of a shaft smelting furnace from the component solid oxides in gangue and flux, a series of interoxide compounds is created as well as some amorphous glass, and as temperature is increased further the compounds dissolve in the glass. This spreads over a temperature range; but once the melt becomes homogeneous and largely free of solid compounds, which can occur at widely different temperatures that depend on the slag composition, it then can act like a true glass possessing a viscosity-temperature curve. To re-emphasize the important point, metallurgical slags are effective and manageable in a furnace only when they are of viscosity low enough (fluidity high enough) at furnace operating temperature to be able to permit movement of other materials, to flow readily amongst them, and in some cases to react with them. The viscosity-temperature curve, which is closely dependent on slag composition, is therefore the property of a molten slag that is of basic importance to good furnace operation. Any given slag can be made as low in viscosity as is necessary, simply by sufficiently increasing its temperature. However, this can be done to only a limited extent in practice even at very high temperatures, be-
III
Metal Reduction and Slags
cause of unwanted other reactions such as reduction of iron from the slag during copper smelting, too high a carbon content in iron smelting, severe erosion of furnace refractories, or simply high fuel consumption. Operating temperatures of smelting furnaces are determined basically by the thermodynamic and metallurgical requirements of the metal oxide being smelted; so slag fluidity at these temperatures must be adjusted by change in the composition of the slag involved. For millennia the creation of a workable slag has been entirely guesswork by experienced operators, but in the last thirty years or so, intensive study of slags and glasses has resulted in the ability to predict or calculate with good accuracy the viscosity-temperature curve of most slags directly from their chemical compositions (Turkdogan 1983 is an excellent treatment). The value of this to the study of furnaces used in antiquity is that from the chemical analysis of a slag, its temperature range of useful fluidity can often be determined, and therefore the operating temperature range of the associated furnace can be estimated. As noted above, this is also applicable with somewhat less precision to estimating furnace temperature from glazed refractories. Viscosity is measured in pascal-seconds (Pas), one of which is equal to 10 poises in the old cgs system. In my experience a slag should have a viscosity at furnace operating temperature of about 0.5 Pas (5 poise) or lower in order to be free-running enough not to interfere with the operation of a bowl or shaft furnace. The chemical composition of the slag that gives adequate fluidity is in principle immaterial, except where there may be wanted or unwanted chemical reactions with the metal being produced. Many of the essentially iron silicate slags used in ancient copper and bloomery iron smelting have viscosities at I,200 o C of 0.1 to 0.2 Pas (Kaiura et al. 1977). A liquid with a viscosity of 10 Pas is sluggish, about the same as that of molasses at 10°C; a viscosity of 0.5 Pas is that of a heavy motor oil; 0.03 Pas is the viscosity of molten copper at I,200°C; and 0.01 Pas is that of water at 20°C. When a slag is tapped from a furnace and cools, as it reaches lower temperatures compounds often form between the oxides and precipitate into the glass matrix. At room temperature these compounds are identifiable under a microscope, and their amounts are in principle predictable from the chemical composition of the slag and from equilibrium phase diagrams, if there are not more than three or four oxides as major components. However, since equilibrium is seldom reached in furnace slags, estimation of the melting point of a slag from published equilibrium phase diagrams of its major component oxides, as has been demonstrated, can produce serious errors. Gale et al. (1985), for example, have shown this by measuring the temperatures of free running of fourteen slags of different compositions and comparing these with the
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Mastery and Uses of Fire in Antiquity
values shown in major component phase diagrams. The samples were from a large ancient field of copper slags and varied considerably in composition. Iron oxide contents ranged from 8.9 to 41.0 per cent, lime contents from 2.3 to 16.7 per cent, and silica contents from 32.6 to 75.0 per cent. The measured fluidity temperatures were lower than those predicted by the lime-iron oxide-silica phase diagram by as much as 500°C. The anorthite-iron oxide-silica diagram (anorthite is a complex compound of oxides) showed less scatter but still deviated by up to 240°C. Besides lack of equilibrium, another reason for the differences is the effects of minor components of the slag such as oxides of manganese, sodium, potassium, phosphorus, and titanium, which originate in gangue and charcoal ash, and which can markedly decrease the melting points of slags, especially in combination. These cannot in a practical way be accommodated in phase diagrams of major component oxides and are a principal reason for the limited practical value of available phase diagrams. In dealing with ancient slags, when in doubt and certainly if the issue is important, the slag fluidity temperature must be measured experimentally. It is also to be noted that the oxide compounds present in a solidified slag are of no importance to the functioning or usefulness of the slag in the furnace, since they are relics that do not exist in a working slag except in one that is starting to solidify and so becoming very viscous. However, the presence of bits of reduced metal and/or compounds such as sulphides and arsenides can be useful in understanding a particular furnace practice by giving a clue to the raw materials used. A cautionary note on the chemical analyses of slags from antiquity that contain considerable amounts of iron oxide is that over long periods of time, and particularly in humid surroundings, some of the ferrous oxide normally present is oxidized to ferric oxide. This can be misleading, since the slag as originally taken from the furnace usually contains very little ferric oxide.
I I
The Smelting of Copper
Pure copper is a soft, very ductile, only moderately strong metal, but it can be increased in strength and hardness by cold working such as bending and hammering. This is much more effective when the copper has been alloyed with arsenic or tin to form bronze. Arsenic is in fact a more efficient work-hardening agent in copper than tin (Buchwald and Leisner 1990), but in antiquity during the second millennium B.C. the usc of tin slowly replaced that of arsenic, possibly because of the toxic effects of arsenic oxide vapour formed during smelting or melting. Other clements such as antimony, bismuth, lead, silver, and gold occur in small quantity in many ores of copper, but they will not be discussed here since they have little effect on furnace design or operation, and their metallurgy has been well reviewed by Tylecote et al. (1977). Copper occurs as metal in the natural state, in geographically widely separated locations and usually in small quantity. Nearly all of the copper used by humankind so far has been smelted from orcs of copper. These arc characterized by several facts: they are relatively scarce, the copper content of the earth's crust being less than one-thousandth that of iron. They are widely distributed geographically and usually contain minerals of other metals such as arsenic, lead, antimony, zinc, and particularly iron, which can be in some quantity. The concentration of copper in the ore is often under 5 per cent, and the mineral compounds containing the copper are most often sulphides, which in antiquity required conversion to oxides before smelting to copper. This combination of features has meant that smelted copper metal of low impurity content is not easy to make, and
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that except for iron, smelting procedures to avoid or remove impurities can be complex. The smelting of copper is in most cases involved with the risk of producing iron at the same time. This is because iron compounds not only occur in considerable amounts in the most common ores of copper but iron oxide is a powerful and widely available flux that when added to a copper smelting furnace can convert the gangue in the ore to a fluid slag. Iron metal can be reduced from such a slag and can then dissolve in copper, which will harden the copper and decrease its workability. Since the amount of iron reduced will usually increase with smelting furnace temperature, smelting of copper should be done at the lowest temperature that gives molten copper and adequate slag fluidity. This will be discussed in more detail below. The copper in most of its ore bodies is present as sulphides, often accompanied by iron sulphides, and outcrops of such ore to the surface will permit weather to oxidize the sulphides to oxides, carbonates, and hydroxides. Over geological time, rain can dissolve the copper compounds more rapidly than the iron ones and carry them down through crevices, leaving a surface layer or cap of mostly iron compounds called "gossan." This has a characteristic appearance, and so can be a marker of an underlying copper deposit. Under the gossan there can be a layer of weathered copper minerals lower in sulphur content, but with that content increasing with depth below the surface and so decreased weathering. The layer of low sulphur content and enriched copper ore is usually shallow, and many such deposits were mined out in antiquity during the early stages of the ability to directly smelt them to copper. Continued mining of these deposits then encountered ores of increasing sulphur content, and since sulphides cannot be smelted directly to metal in charcoal furnaces, the furnace product from such ores contained both molten copper and an increasing amount of a molten material that solidified to a black, stone-like mass that is today called a "matte." (This situation and its solution is discussed below). The amount of copper smelted in antiquity evidently increased along with human population, and it seems to be agreed among modern investigators that most of the total amount of copper produced in antiquity was from sulphide ores. SMELTING OF WEATHERED ORES OF COPPER
The copper in weathered copper ores mainly occurs as oxides, hydroxides, carbonates, and mixtures of these minerals and can be smelted with charcoal, usually after roasting in air to increase the extent of conversion to oxide. Smelting can be done in a crucible in a kiln or in a
II
5
Copper Smelting
bowl or shaft furnace. The latter were on the average quite small throughout antiquity, typically 300 to 400 mm LD., usually using a single tuyere with bellows air supply but more than one blowpipe with human breath. An example of a simple method of crucible smelting described by Gowland (I899) uses a crucible directly as a furnace, and was employed in Japan for unknown hundreds of years. The crucible is placed in a hole in the ground which has been lined with an 8 mm thick mixture of clay mixed with broken charcoal, to just fit the crucible and decrease heat loss. A clay tuyere supplied with air by bellows is placed so that its nose or outlet is at the inner top edge of the crucible, directed downward at an angle of about 20 degrees. Then, in Gowland's words, A roughly made thick semicircular lid of clay is placed over the blast pipe so that it almost covers the back of the furnace. A small fire is then made in the cavity in order to dry it thoroughly. When dry it is filled completely with charcoal, upon which, when thoroughly ignited, a layer of fresh charcoal is placed, then upon this a layer of ore, then another layer of charcoal, and so on, until a conical heap of ore and charcoal covers the furnace. The bellows are then worked vigorously, when the ore is gradually reduced and the charcoal consumed, until after about one hour the entire heap sinks into the furnace cavity. The unburnt charcoal and slag which cover the molten copper are then raked off with a wooden rake. The metal is not removed by ladling, but in the following manner: A little water is sprinkled on its surface with a straw brush, causing a thin layer of metal to solidify; this is at once raised up with a hook, removed on a shovel, and thrown into water.
Smelting of weathered copper ores was also described in chapter IO, including the carbothermic process which has the considerable advantages of requiring only a moderate amount of charcoal as reductant, the use of biomass instead of charcoal as fuel, and natural draft as combustion air supply. SMELTING OF SULPHIDE ORES OF COPPER
The roasting of ore to increase its friability, porosity, and reducibilty before smelting evidently was practised extensively in antiquity. As weathered copper ores near the surface of an ore body became mined out, or an available ore body had not been sufficiently weathered by atmosphere, appreciable amounts of sulphur would be present. It could not have escaped notice that these ores on being roasted would give off the sharp and acrid smell of what we know today as sulphur dioxide gas, created by the oxidation of sulphur in the ore.
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Mastery and Uses of Fire in Antiquity
However, roasting seldom removes all the sulphur from the ore. On smelting such roasted ore, in which the copper is present as a mixture of oxide and sulphide, the product would be a considerable amount of molten copper, but there also would be a certain amount of molten matte, which is heavier than slag but lighter than copper. On solidification of the molten materials tapped from the furnace, there would be a layer of slag on top, then a layer of matte, then copper metal. These layers are readily separated, the slag being discarded and the copper saved; but the matte contained no visible copper and was apparently useless. Matte is a mixture of iron and copper sulphides, its copper content varying widely depending on the ratio of iron to copper in the ore typically 15 to 60 per cent, the remainder being largely iron and sulphur; since it contains little or no slag it is in effect a very rich are of copper. While the copper in matte is not visible as such, its density is higher than those of most orcs or rocks, so in antiquity it was probably considered valuable. If it is treated as an ore by being crushed and roasted, much of its sulphur will be removed, and during smelting of the roasted matte much of the copper will be recovered as metal. Evidently this was eventually done, giving good recovery of copper from the original sulphide ore. When the practice was adopted of treating the matte separately as an ore, the use of a high silica flux (which is acid) such as sand was necessary in the smelting furnace to absorb the iron oxide (which is basic) in the roasted matte to form an iron silicate slag. The consumption of charcoal per kg of copper produced is lower when smelting roasted matte than when smelting an oxide ore, since the copper is concentrated and there is smaller slag volume to melt per kg of copper. Also one matte smelting operation can deal with the matte accumulated from several primary smelting operations, depending on the composition of the local ore and the success of the roasting process in removing sulphur. The choice of kind and amount of flux necessary to make a slag that is fluid at furnace operating temperature was a continuing problem in copper smelting, since all ore bodies differ in metal content and in gangue composition, and this usually varies further within each ore body. A roasted ore low in copper mineral content, whose gangue was often siliceous, might not contain enough iron oxide to flux the gangue to a slag, and so some would need to be added for its smelting. On the other hand, a matte or rich ore containing a higher ratio of iron oxide to copper could need the addition of a siliceous flux such as a sand to absorb the iron oxide and decrease the risk of its reduction to iron. The smelting of a roasted matte by itself always requires a silica flux for the
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Copper Smelting
appreciable amount of iron oxide present. The slags would usually consist mainly of fayalite (iron silicate), with fluidity temperatures in the range of about 1,050 to 1,200 o C; but as noted in chapter 10, a wide range of slag compositions that are low in silica or in iron oxide have melting points in this range because of the presence of other oxides in the are. Those low in iron oxide will of course give lower reduced iron in the copper. IRON IN COPPER
Nearly all successful smelting of copper in antiquity would have on occasion produced some metallic iron, reduced from iron are added as a flux or from iron in oxidized sulphide ores. The iron would appear floating on molten copper as silvery bits or as a molten film, depending on the carbon content of the iron. As iron oxide starts to be reduced in a charcoal furnace at temperatures between about 300 and 400°C, so iron oxide in are descending through the reducing gases in the upper part of a bowl or shaft furnace becomes partly reduced to bits of solid metallic iron. This will happen to a degree depending on the amount of iron oxide, the residence time in the furnace, and the fact that iron oxide requires nearly three times the energy supply for its reduction as does copper oxide. So at the temperature level in the furnace where reduced copper becomes molten and slag has become fluid, bits of solid reduced iron can be present which have dissolved little or no carbon from the fuel and so have melting points between about I,}OO to 1,530oC, well above the furnace temperature sufficient for smelting copper. The fall of droplets of reduced and melted copper and particularly of slag through the furnace burden will carry the reduced bits of iron with them to the hearth of the furnace, all being superheated on passage through tuyere level. Provided that furnace temperature is not far above the melting point of copper, the bits of iron will have little opportunity to dissolve carbon from the charcoal through which they fall, because of the short contact time and being at least partly enclosed in slag. The accumulating pool of material in the hearth will then be composed of four layers in order of their densities: slag on top, then matte, then reduced iron as a collection of solid particles, and then molten copper lying in the hearth. The slag in the hearth is often 50 per cent or more in iron oxide content and is in close contact with charcoal, so some further reduction of iron can take place from the slag, and it will tend to be higher in dissolved carbon content. If furnace temperature increases for any reason, all of the reduced iron in the hearth will increase in carbon content to
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Mastery and Uses of Fire in Antiquity
eventually, if temperature is high enough, make a cast iron with a melting point not far above that of copper, and so become molten. Iron, whether solid or as molten cast iron, will dissolve in molten copper in amount increasing with temperature; in turn, any metallic iron not dissolved will dissolve some copper. The solubility of iron in molten copper increases rapidly with temperature, from 5 per cent at 1,200 oC to 20 per cent at 1,400oC. As noted above, the iron content of the copper therefore will be lowest at the minimum furnace temperature that will produce both a fluid slag and molten copper at just sufficient temperature above its melting point to be tapped out. In the case of first-stage smelting of roasted copper sulphide ores usually containing iron and some residual sulphur, reduction of iron and its course in the furnace will be similar, with the exception that the layer of matte that forms from the sulphur and reduced iron lies between the slag and the iron in the hearth. The matte can also dissolve reduced iron, in amount depending on its sulphur content. In all cases, as temperature of the molten copper decreases during its solidification after tapping out of the furnace, its solubility for iron decreases to a level well below I per cent, and any excess iron appears as precipitates in the solidified copper, in the forms of particles, spheres, andlor dendrites from microscopic to visible in size. These are often with the microstructure of white cast iron because of solution of more than 2 per cent of carbon in the iron. Very large amounts of iron have been found on occasion in ancient copper, up to 35 per cent as reported by Craddock and Meeks (1987), which must have come from a seriously overheated furnace. Such material is mechanically useless, and copper with large amounts of iron is not common in antiquity because it is not difficult to decrease iron content. On simply remelting copper that contains iron to just above its melting point, iron above about 4 per cent can float to the surface for mechanical removal. This iron will be of little use because of its dissolved copper content. The remaining iron can be removed by oxidizing the remelted copper with air from a bellows or blowpipe, helped by stirring. This oxidizes the iron to iron oxide, which can then be absorbed as an iron silicate (fayalite) slag by addition of silica sand. Most of the many analyses of copper from antiquity show iron contents well under 0.5 per cent, from either careful smelting practice, or the use of such a refining process, or probably both. These points were clearly demonstrated by Merkel (1983 and in more detail 1990), where in smelting copper ore with an iron ore flux in a 300 mm I.D. shaft furnace, the use of an excessive space velocity of 10 to 12 metres per minute resulted in a maximum furnace temperature of I,400°C, which produced more than 10 per cent of iron in the
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Copper Smelting
copper. This was decreased to 0.014 per cent by repeatedly remelting the copper and stirring it under a "gentle air blast." When the smelting slag is low in iron oxide content, the amount of iron reduced of course will be less, but the fluidity of the slag may be lower. However, as noted above, there are slag compositions that can have sufficient fluidity at copper melting temperature that little or no iron oxide flux need be added. An example is a slag from a copper smelting furnace in northern Chile, C-14 dated to late in the first millennium B.C. (Graffam 1992), that contained only 3.8 per cent FeO; others were noted in chapter 10. In such cases furnace temperature can then be allowed or encouraged to increase with minor risk of too much iron being reduced, with the purposes of either increasing the fluidity of such slags if necessary for smooth furnace operation or to increase molten copper tapping temperature for foundry purposes. Iron that has been reduced higher in the burden and has dissolved less than about 2 per cent of carbon will have a melting point of more than about 1,390°C, well above that of copper. Then, if it has had time to separate by gravity to form a layer on solidifying molten copper, it will likely be as a mass of fine solid particles. Such iron if separated and forged would have very poor mechanical properties because of its copper content. Amounts of about 3.0 per cent copper that can be retained by moderately rapid solidification will make the iron hot-short (non-ductile when hot) and difficult or impossible to forge. Copper also prevents forge-welding completely (Greg and Daniloff 1934), making it nearly impossible to consolidate such iron into a sound bar. Small amounts of arsenic dissolved in iron from impure copper not only do the same but make iron non-ductile both hot and at room temperature. Also as little as 0.005 per cent lead is today well known to markedly decrease the ductility of iron. Both of these elements are not uncommon in ores of copper, so in general iron produced as a direct by-product of copper smelting is very much inferior in working properties than those of clean iron produced by the reduction of iron ore directly. In the many cases where, during copper smelting, enough carbon has dissolved in reduced iron to make it a cast iron, as noted above, the iron appears in the solidified metal as rounded or dendritic inclusions. It seems to have been generally overlooked that if such cast iron were to be extracted from a copper artifact, the carbon in it could provide a useful independent C-14 dating of when the copper of the artifact was smelted. It may be noted that Tylecote and Boydell (1978) melted equal amounts of cathode copper and pure iron powder in clay-graphite crucibles, and iron taken from the surface layer and solidified was forged
120 Mastery and Uses of Fire in Antiquity at 900°C with no comment made on difficulty. The microstructure was then one of pearlite with some elongated particles of copper, the pearlite indicating the solution of 0.6 to 0.7 per cent carbon from the graphite in the crucible walls, and the copper being the amount not dissolved in the iron. However, the metals used were of high purity and the forgeability of the iron did not represent that of iron from smelted ores, which would contain variable amounts of other metals. COPPER SMELTING AS ORIGIN OF USEFUL IRON
The subject of copper smelting as the origin of useful iron has had continuing discussion in the archaeological literature, apparently beginning in the early 1960s with Wertime, Pleiner, and Cyril Smith (Wertime 1980). Reduced iron has been a problem with copper smelters from antiquity to the present, and the industrial metallurgical literature of the nineteenth and twentieth centuries had much to say about it, without ever being able to find a use for the unwanted iron because of its very poor mechanical properties. In antiquity copper was not removable from iron, and this is still not economically feasible. However, the millennia of copper smelting that preceded the ability to smelt clean iron directly from iron ore provided ample time to explore the properties and possible uses of the more tractable varieties of by-product iron, particularly those of moderate copper and low carbon content. These could be reasonably workable. For example, a few unpublished iron artifacts from Kommos in Crete from seventh century B.C. contain small amounts of copper showing as coloured threads of copper carbonate and hydroxide 9 (personal examination); and a nodule of iron from the thirteenth century B.C. contains 6 per cent of copper by analysis. Small iron artifacts containing moderate amounts of copper were found at Timna (Gale et a1. 1990). Very likely many samples of such material are sleeping in museum basements, since iron artifacts, particularly older ones, are often not analysed and then seldom for copper content. This prevents modern tracing of the change backwards through time and place, towards when and where iron completely free from copper was produced directly from iron ore. COPPER SMELTING WITH NATURAL DRAFT
Iron can be smelted in quantity in a shaft furnace with lump charcoal fuel and natural draft as air supply, and temperatures above 1,450oC can demonstrably be attained, as will be noted in chapter 12. There
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should therefore be no question of the ability to smelt copper ores to molten copper in similar natural draft furnaces operated at lower temperatures, and it is surprising that there is apparently not more evidence in the archeological literature of antiquity of smelting copper by such a method.
12
The Smelting, Forging, and Properties of Iron
PART I: THE SMELTING OF IRON
Considering the metallurgical interaction between copper and iron during smelting, it seems likely that the first human awareness of smelted iron as a possible replacement for very scarce meteoric or native iron was of iron unexpectedly appearing during early copper smelting. Its mechanical and forging properties would have been poor and variable because of its copper content, and the metal not as useful as copper or bronze. It took a long time to discover that if the iron ore used as flux for copper smelting was smelted by itself using different operating parameters for the furnace, the iron made could have strength and toughness much superior to those of bronze. The first appearance of smelted iron in the archaeological record is apparently about 3,000 B.C. (Craddock 1995), but whether it contained copper is not clear. The basic and simple reasons for the technical complexity of the smelting and working of iron are as follows: 1
2
While carbon from the furnace fuel will not dissolve in copper and so has no effect on its properties, it dissolves readily in iron at temperatUfes above about 730°C and then becomes a powerful hardening and strengthening alloy element in the iron. As the temperature of iron is increased further, both the rate and the extent of its solution of carbon increase, so the carbon content and the resulting strength and hardness of the iron are dependent in a very sensitive way on the furnace temperature.
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3 In a given furnace, its temperature increases as the ratio by weight of ore to fuel in the furnace burden is decreased. 4 As iron increases in carbon content, its melting temperature decreases from r,54ooc with no carbon to r,I4ooc at 4.3 per cent carbon. The working out of these relationships that occur invisibly within the smelting furnace seems sufficient to account for the long time taken for experienced copper smelters to learn how to smelt iron from iron ore. THE SMELTING OF IRON ORE
During the smelting of iron ore, carbon from the fuel dissolves readily in iron and then becomes a powerful hardening and strengthening alloy in proportion to the amount dissolved. This made it necessary that, to produce iron of controlled properties, its smelting be a two or a three stage procedure. This is because a single stage smelting of iron ore will always have one of two or occasionally three results that depend on grade (iron oxide content) of the ore used; the ratio by weight of ore to charcoal in the furnace; and the rate of supply of combustion air, necessarily supplied by bellows or natural draft. These are: r The production of only a molten slag with no reduced iron. Some molten slag as well as a mass of solid state reduced iron of from nearly zero up to about 2 per cent carbon content, as well as some molten slag. 3 A pool of molten high carbon steel or cast iron under a layer of molten slag. Under some circumstances in a shaft furnace a mixture of molten cast iron and a bloom too can be produced, which was taken advantage of in the second millennium A.D. in the Japanese Tatara and the German Stuckofen furnaces.
2
However, throughout antiquity and into the early Middle Ages in the West, the second result, the "direct" or bloomery process which produced a solid-state rough mass or bloom of iron, was adopted. This was because although the bloom was of no direct use as taken from the furnace, it was workable in a second stage operation of intensive forging at high temperature that converted it into a strong, malleable iron bar largely free from slag. Since this bar of iron was the product of two separate stages of processing, the latter requiring a reheating hearth that could consume considerable charcoal, it was not "direct," but custom has called it so.
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The third result, the production of a molten cast iron, would occur whenever the furnace temperature became too high, for example, due to an error in the ratio of ore to charcoal added to the furnace. The cast iron could be left in the hearth to solidify or be tapped out to solidify in a depression in the earth. In either case the iron would be a "white" cast iron (described below) which was extremely hard, with no ductility, almost unforgeable, and so discarded to be called "furnace rubbish" by archaeologists thousands of years later. The smelting of iron directly from its oxide ore to avoid contamination by copper can readily be done in the same bowl or shaft furnace fuelled by lump charcoal and with a single tuyere supplied by bellows that was used for several millennia for copper smelting. However, five changes in furnace operation were necessary that apparently took more than a millennium to work out. These changes were as follows: r Only crushed, clean iron ore could be used. No flux was usually necessary since the ore could flux its own gangue. 3 The ratio of ore to charcoal was decreased to moderately below that for copper smelting. 4 Bellows and not blowpipes must supply the combustion air. 5 The iron produced was accepted as being in the form of a solid-state mass or bloom containing some trapped slag and voids, lying in the hearth of the furnace just under the tuyere nose. After extraction from the furnace, the bloom had to be reheated and forged at 1,100 to r,200oC to squeeze out as much slag as possible and weld shut the voids. The mechanical properties of the product increased with the extent of slag removal, and the remnant slag gave directional mechanical properties, but they made possible very good forge welds. 2
MECHANISM OF IRON REDUCTION
While the sequence of events in a bowl or shaft smelting furnace was given in chapter 10, further details specific to the smelting of iron will be described here. It will be remembered that the rate of heat generation and the rate of descent of the stock column in a shaft furnace are determined by the rate of air supply, i.e., the space velocity. If the ratio by weight of ore to charcoal in the furnace charge is decreased but air supply rate is constant, a smaller quantity of ore is subjected to the same quantity of heat so the temperature of the furnace is increased, which results in increased rate of solution of carbon in reduced iron. When the iron and slag reach the hearth, they carry an increased amount of
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Smelting and Forging of Iron
heat which increases hearth temperature. Therefore at some still lower ratio of ore to charcoal, enough carbon will have dissolved to make a molten cast iron, to be found in the hearth instead of a solid state bloom. The transition from making low carbon iron from a high orecharcoal ratio, to high carbon iron from low ratio of ore to charcoal, is continuous. When higher ratios of ore to charcoal are used so that average furnace temperature is lower, reduction of ore becomes slower, less carbon is dissolved in the reduced iron, and thus its melting temperature increases. Bits of reduced iron encased in gangue or slag can fall past the high temperature tuyere zone without being melted, but are increased enough in temperature to weld together on contact. Just below tuyere level a mass of iron thus starts to grow and is increasingly supported by the charcoal in the hearth, with slag draining off to the bottom of the hearth. The mass is now called a bloom, and contains some trapped slag, occasional bits of charcoal, and appreciable porosity. Since the ore is charged in pieces of various sizes and reduction proceeds in from surfaces, the percentage of reduction of each piece to iron at each level in the furnace varies, as do the brief opportunities for solution of carbon from the charcoal. On being deposited on a growing bloom, this creates the variegated micro-distribution of carbon content within a bloom, which is observed to always be the case whenever there is enough carbon present to form visible pearlite or cementite. This micro-variable carbon content has caused very many mistaken identifications of iron artifacts as having been carburized during or after forging. It seems likely that if the ore charged were to be screened so that all pieces were closely the same size, the variability of carbon content in a bloom would be much less. This could be tested by careful experimental furnace runs, but to my knowledge none of this kind has been published. In the hearth below tuyere level the charcoal present creates strong reducing conditions, and as furnace temperature is increased by a lower ratio of ore to charcoal, iron oxide as FeO in the slag will be reduced to iron to a corresponding extent. The slag thus varies in FeO content to maintain chemical equilibrium with the carbon content of the iron, lower FeO content slag accompanying higher carbon content iron. This is shown in figure 10, which has been re-plotted from the results of smelts nos. 14 to 20 inclusive in Tylecote et al. (1971). While that study noticed the general relationship, it is more closely quantitative than was apparently realized. The equation of the line is: log %C = 0.63 - 0.034 x %FeO
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Mastery and Uses of Fire in Antiquity
o·at o.o2+---~----~--
o
;?o
__----~--~r---~ ~
60
F#.Q-.C.()JJ!~~~4.. S' !"'3-,~ Figure 10 Carbon Content of Iron Bloom versus FeO Content of Slag
Analysis of a slag for FeO can give a good indication of the carbon content of the iron made; for example, a slag with less than about 10 per cent FeO would indicate that a molten cast iron had been made, and less than about 5 per cent that it was a higher carbon content cast iron. The smelting mechanism outlined above seems to fit well with these and other observed results, including the smelting hearth operations by Percy (1864) and by Eglestone (1879) described above. Of course variations can be created by change in other factors. For example, increased shaft heights, residence times, and charcoal reactivity all tend to increase carbon content of the iron, as does decrease in charcoal lump size and ratio of iron to gangue in the ore. FeO is a powerful flux for silica, a major constituent of the gangues of most ores, but in general as the FeO content of a slag decreases, the temperature at which it is fluid enough to flow readily through a taphole increases. However, cast iron is made at higher furnace temperatures, so the situation on slag fluidity is complex; but a low FeO content in a slag does not always mean high fluidity temperature as has been discussed. It may also be noted that when a low carbon content bloom is made by using an increased ratio of ore to charcoal, the iron oxide content of the slag can exceed the iron oxide content of the ore used, for example, as in table viii of Tylecote et al. (1971). This means that recovery of iron from the ore then depends on slag volume and so can become low.
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Smelting and Forging of Iron
Correspondingly, ore of low iron content makes more slag per unit of iron, and the slag level in the hearth can then rise in a short time to and then into a tuyere nose, which can limit the air flow rate and so the furnace operating rate and temperature, and thus limit the size of the bloom made. With such ores, periodic slag tap-outs must be made to control slag level. As noted in chapter 10, the question of whether a particular furnace has or has not a slag-tapping practice is frequently simply a question of the relationship between the gangue content of an ore and the volume of the furnace hearth below tuyere level, and so is a poor basis of classification. It is clear that the total amount of iron reduced at a given ore to charcoal ratio, and so therefore the size of the bloom or the quantity of cast iron made, increase with the total amount of combustion air blown measured at tuyere entry. Since some minimum space velocity depending on furnace heat loss rate must be maintained, bloom size increases with the internal cross-section of the furnace and the number of hours of operation. A further point is that with suitable adjustment of ore to charcoal ratio and of tuyere placement, and with a wide range of ore lump size as charged, a solid state bloom of iron of moderate carbon content and molten cast iron can be made in the furnace at the same time. This is the explanation of the practice in Europe and Japan several hundred years ago noted earlier in this chapter, but whether this occurred in antiquity is not known. Incidentally, nickel present in iron ores such as some laterites and manganese (which is not uncommon in iron ores) will be reduced with the iron and can reach appreciable amounts, depending on the ore composition and increasing with the carbon content of the iron. A more specific note on the production of molten cast iron from iron ore in bellows-blown charcoal furnaces is that it is easy to do and will almost invariably occur whenever the space velocity is above about 5 to 8 m per minute, depending on how well insulated the furnace is, and whether the ratio by weight of iron ore to charcoal is lower than about 0.5 to I. THE" BLAST FURNACE"
While I am concerned here with pyrotechnology only in antiquity, it may be noted that the so-called "blast" furnaces used in Europe from the early Middle Ages into modern times were at the start simply the bloomery furnaces that were in widespread use making iron by the direct process. The production of cast iron was normally carefully avoided as a waste of charcoal and furnace time, but it was easily made if wanted. The impetus to make it intentionally was the appearance in
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Mastery and Uses of Fire in Antiquity
Europe in the period about 1,000 to 1,200 A.D. of the ability to decarburize molten cast iron to make a solid-state bloom of low carbon iron nearly identical to that made by the direct process. This ability is called "fining," to be outlined below, and its attractions were considerable. One was that the recovery of iron from the ore was nearly doubled. Another was that when the smelting furnace was producing molten cast iron, it could be operated continuously until the furnace refractories failed. A third was that the cast iron could be cast into bars called "pig iron," which could be stored or sold to be remelted and "fined" later. A fourth was simply economies of scale of operation. As the demand for iron increased, furnaces were slowly increased in hearth area and height, past the ability of human muscle to operate bellows and forge hammer; at this point the water wheel, which had been in regular use for centuries for milling flour, was adopted. The earliest archaeological evidence seems to be in Sweden at Lapphattyn, dated about 1,250 A.D. (Gordon and Reynolds 1986). Only the hearth remained, its area 0.15 m 2 • Bellows were driven by a water wheel, and the iron was fined in adjacent forges. By the fourteenth century contemporary accounts marvelled at the tremendous noise the larger bellows made, and that seems to have been the source of the term" blast" furnace. SMELTING IRON WITH NATURAL DRAFT AND WITH BELLOWS
Natural draft was used for smelting iron to blooms in shaft furnaces in Africa for a long but unknown length of time, and Cline (1937) has described such furnaces and their methods of operation still in use in the late nineteenth and early twentieth centuries. These furnaces tended to be fairly tall, two to six metres, and 0.5 to 1.5 metres in diameter, but were not in widespread use, the reason possibly being that a bellowsblown furnace could be operated at higher rates of air flow with the forced air supply, and so could be smaller and more productive as long as human labour was available. Iron was also similarly smelted in Burma, as described in Percy (1864, 271), and in late Roman times in what is now Poland (Nosek 1985); and I am of the opinion that natural draft smelting furnaces were used more extensively in antiquity than is presently realized (the archaeological remnants can be minimal). The technical details of such operations are given in appendix 3. The description given by Bellamy (1904) of the operation of an African natural draft iron smelting furnace is particularly useful, because it describes a recently living practice and gives ample quantitative detail. It is also interesting and instructive to compare its results with those of a
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Smelting and Forging of Iron
modern replica of a shaft furnace operated by Tylecote et al. (1971). The principal design differences were that the furnaces were of different internal shape, and that the Bellamy furnace used natural draft with multiple deeply penetrating tuyeres while the Tylecote furnace had a single tuyere penetrating only about 20 mm, using forced air from a blower. The furnace described by Bellamy was still being actively used in 1904 by a community of the Yoruba tribe in what is now Nigeria. The amount of detail reported was considerable, including dimensions of furnace and tuyeres, weights and analyses of raw materials, times of addition, and analyses of products. The ore used was a hematite sand that was concentrated by gravity in water, from a roasted ferruginous shale. Total iron content was 59.9 per cent, and the gangue was mostly silica. A flux was added with each furnace charge, which was crushed slag from a previous smelt which contained 36.8 per cent iron and which decreased the iron content of the ore plus slag mixture to 54.4 per cent. The charcoal used was not described in any detail. The furnace was built to be reusable, of clay with walls about 500 mm thick. The shape was circular in plan and moderately conical in elevation, with the inside diameter at tuyere entry level being about 1,000 mm and at the top about 230 mm, the height from hearth to top being about 1,500 mm. The hearth had a cup-shaped bottom, with a hole in the centre about 80 mm in diameter, to be used as a slag tap hole. In the wall of the furnace around its circumference near its base were six rectangular slots, through each of which three tuyere pipes, each about 36 mm I.D. and 600 mm long, entered the furnace at an angle of about 45 degrees. These were sealed in place with clay, their noses reaching nearly to the centre of the furnace at the start of a smelt. The total tuyere internal cross-sectional area was close to 18,000 mm 2 • A hole in the side of the furnace at ground level, to be used eventually for extracting the bloom made, was plugged with clay. To make a smelt, this hole was plugged, some lighted charcoal was put into the furnace, and it was then filled with charcoal. After about two hours combustion, 2.3 kg of slag from a previous smelt was added with 18 kg of charcoal, and after another two hours some slag was tapped through the bottom hole. Another identical charge of slag and charcoal was immediately made, and two hours later another slag tap was made. Another charge of slag and charcoal was now made but with the addition of 2.3 kg of iron ore. Ten charges in all were then made at intervals of about 2.5 hours of 2.3 kg of slag, 18 kg of charcoal, and amounts of iron ore increased with each charge to about 12 kg at the last. A slag tap was made just before each charge. About five hours after the last charge, remaining slag was tapped out and the opening in the wall was cleared to remove the bloom made.
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Mastery and Uses of Fire in Antiquity
The time from the first ore charge to the last slag tap was about thirty hours, during which a total of 180 kg of charcoal and 74 kg of ore was charged, with 23 kg of slag retained from previous smelts. The iron bloom made weighed about 34 kg, so 5.3 kg of smelting charcoal were consumed per kg of bloom made. The average rate of growth of the bloom was 1.1 kg per hour, and it contained 1.67 per cent carbon, 0.25 per cent silicon, and 0.01 per cent or less of other clements. The average ratio of ore plus slag to charcoal in the charges was 0.50 to 1. The charcoal burning rate while smelting was 180 kg over thirty hours, or 0.10 kg per minute, so the average air flow rate was 0.10 x 4.0 = 0-40 cubic metres per minute. The furnace of T ylecote et al. was a cylindrical shaft 300 mm I. D. with walls 350 mm thick which included insulating firebrick, and the furnace was thoroughly preheated, as was that of Bellamy. A smelt, no. 12 in the Tylecote series, was made using forced draft, with an ore containing 58 per cent iron and a ratio of 0.50 are to charcoal, and no added slag. The measured air rate was 300 litres per minute, the charcoal consumed in smelting after preheating was 4.1 kg per kg of bloom, and the production rate of bloom was 1.0 kg per hour. The bloom made contained 1.8 per cent carbon. The shapes and internal dimensions of the two furnaces were appreciably different, Tylecote's being straight sided and Bellamy's being both larger and tapered, but the influence of the deeply penetrating tuyeres in Bellamy's furnace would make it perform as though smaller in average diameter, as discussed in chapter 7. The two furnaces had unusually thick walls and correspondingly low heat loss rates, but the African one was about 50 per cent thicker and would require longer time for preheat to reach the same wall temperature. The operating parameters are surprisingly similar, in ore grade, ratio of ore to charcoal, rate of bloom made per hour, and air flow rate. An interesting fact is that the production rate of medium carbon content steel blooms ready for forging, in tonnes per hour per unit volume of furnace, was more than one-third of the rate of the best modern blast furnace making molten cast iron which still has to be fined (decarburized by oxygen) to steel. However, in the bloomery process, fuel consumption was much higher, and recovery of iron from the ore was much lower. The only significant difference in operating parameters between the Bellamy and Tylecote furnaces is in the lower charcoal consumed per kg of bloom made in the Tylecote furnace, and this was probably due to a lower heat loss rate through its walls since half of the wall thickness was composed of modern insulating brick. Also the charcoal used in the Bellamy furnace was made in a native heap or pit, while the Tyle-
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Smelting and Forging of Iron
cote charcoal was made in a modern commercial plant and likely had higher FC content. It may be noted parenthetically that Tylecote et al. tried three times to smelt iron using natural draft, with different sizes of tuyeres, but all three attempts were unsuccessful because of too low air flow rate. The low rate was simply because a Venturi orifice was used to measure the air flow rate, and without the operators realizing it, the orifice (with high resistance to air flow) was in series with the furnace tuyeres and so limited the flow rate. THE CARBOTHERMIC PROCESS OF IRON SMELTING
As was noted in chapter 10, iron ore can be smelted to a solid state mass or bloom of iron, or to a molten cast iron, by pulverizing and mixing ore and charcoal in suitable proportion of carbon to iron and then heating the mixture to temperatures in the range of 1,100 to 1,400°C. A major advantage of the process, as was pointed out, is that heating of the mixture can take place in a very oxidizing atmosphere such as that produced by the combustion of biomass fuel. A bloom can be produced that is nearly identical with that made in a bloomery furnace, and can be forged to bar iron that would be similarly difficult to distinguish. THE FINING OF CAST IRON
The term "fining" is commonly used for the process whereby a molten cast iron containing up to 4 or more per cent carbon is decreased in carbon content by the use of oxygen, since carbon oxidizes preferentially to iron. The oxygen can be in air, in furnace gases, in iron ore, or in a slag as FeO. In chapter 5 it was noted that melting cast iron in a reveberatory furnace always decreases its carbon content. Alternatively, the cast iron can still be molten from a smelting furnace, or may be allowed to solidify as a bar of pig iron and then remelted in a charcoal hearth furnace. In the latter case, the end of the bar is slowly pushed into the high temperature zone close to a tuyere nose in a forge furnace full of charcoal. The iron melts into droplets that are decreased in carbon content by preferential oxidization. As this occurs, the droplets increase in melting point and fall to the hearth as particles that, facilitated by slag in the hearth, become welded together. These small masses can be gathered into a larger mass by manipulation with a bar and lifted from the remaining liquid as a bloom, to be forged either while still hot or when reheated to forging temperature in a separate hearth. The product made by fining is a bloom roughly similar in shape
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and slag content to one made in a bloomery furnace, but usually with less porosity and slag and with more uniform carbon content. There seems to be no firm evidence for the use of fining in the West until about the end of the first millennium A.D. The origin of fining could have been in analogy with the well-known practice of copper refining, in which something in the copper that hardens it undesirably (iron) can be removed by blowing air through or across the molten copper. Similarly, the oxidation of a molten hard (high carbon) iron results in a softer iron being formed. Carbon is also removed from iron as a bloom of it is being forged, since the temperature of re-heating must be in the range 1,100 to I,300°C to both melt the slag in the iron so it can be extruded and to make the iron sufficiently plastic that it can be forged with reasonable effort on the hammer. Very oxidizing combustion atmospheres necessarily accompany such temperatures, and carbon is removed from the surface of the iron to a varying and sometimes considerable extent, as will be described in part 2. Carbon also can be removed from solid cast iron by heat treatment for longer time at a lower temperature in a suitably oxidizing atmosphere, as will be discussed in part 3. CONTROL OF IRON SMELTING
To make iron blooms by the direct process which contain the relatively narrow range of carbon contents that give the most useful working and mechanical properties, furnace temperature has to be controlled within narrow ranges specific to the carbon content desired. Since in antiquity most of the actual causes of variation in furnace temperature were unknown, as was the degree of their individual importance and even the existence of carbon as an alloying element, the ability to intentionally make iron of particular properties (carbon content) could have been acquired only slowly and never too surely. Broadly, the iron smelter in antiquity was in the position of the character in the operetta The Mikado who complained of a game of billiards that it was being played "on a cloth untrue, with a twisted cue, and elliptical billiard balls." The desirable forgeable bloom would have been recognized when it was obtained, but given the serious limitations of the volumetric measuring methods available for all raw materials including the essential combustion air, the carbon content of the blooms made would vary over a considerable range. CARBON CONTENTS OF IRON IN ANTIQUITY
The archeological record gives evidence that this is evidently what happened. I took a total of 507 analyses of iron artifacts, dating from
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Smelting and Forging of Iron
1,100 B.C. to 500 A.D., from the several books published by Tylecote and from Pleiner (1980). After eliminating the cast irons which were so easily made, these showed an average carbon content of 0-48 per cent, with a range from 0.001 to 0.90 per cent and a small proportion higher. About 10 per cent scattered through time had quenched microstructures, often at quite low carbon contents which gave only very moderate increase in hardness. This gives support to my suggestion that the hardness attainable by simple cold work hardening was sufficient for most purposes. 1 then found that a similar survey had been made by Schaaber et a1. (1977) of samples from late antiquity in central and southern Europe. The 484 samples showed an average carbon content of 0.53 per cent, and the close agreement of the two averages suggests that about 0.50 per cent is a reliable value for the average carbon content of iron made in antiquity. The published analyses of iron are very largely of finished objects, and due to the loss of carbon during forging of a bloom, the average carbon content of the blooms must have been higher. The amount of loss varies, as described above, but might be in the order of 30 to 50 per cent of the initial carbon content. The average bloom made would then have contained in the order of 0.70 to 1.0 per cent carbon. Clearly there is no necessary connection between the carbon content of an iron artifact and the time of its manufacture. The agreement of the two average carbon contents, one from the whole of antiquity and one from late in the period, also suggests that the range of carbon contents was stable over time and place and was a condition of the degree of control of the smelting and forging process. The primary means of controlling the carbon content of smelted iron are to control the ratio of charcoal to ore by weight and the specific air supply rate, and throughout antiquity such control was erratic. The width of the range of actual carbon contents was probably a measure of the ability to control the process with the very limited technical knowledge available. However, in particular cases, control of carbon content could still be within a narrower range than the average. This would occur, for example, where a smith used a particular furnace, consistently used charcoal made by the same man from the same species of wood, used ore from the same deposit, and employed the same bellows and operator supplying combustion air. This would result in iron from smith A being noticeably and fairly consistently softer (or harder) than iron from smith B working somewhere else.
Only the carbon content of iron has been discussed in detail since it is the principal alloying element, but in practice iron ores are seldom free from other elements that in small quantities change the appearance of the ore little or not at all and so in antiquity were undetectable. These,
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when reduced with the iron during smelting, might enhance its properties or, more often, seriously degrade them. Common beneficial clements are manganese and to a lesser extent nickel, the former being particularly useful because it can neutralize the effects of sulphur, which otherwise can seriously decrease the hot forgeability of iron and steel. However, sulphur in charcoal-smelted iron is usually low, and in antiquity would be high enough to give trouble only if sulphur was present in the ore. Phosphorus and arsenic can occur in iron ores in appreciable amounts in some deposits, and each can be reduced with the iron and have undesirable effects on its properties. Both clements in small quantities can harden and embrittle iron, particularly at low ambient temperatures. The supposed useful effects of phosphorus on decreasing the melting point of cast iron have been seriously exaggerated in the archaeological literature, since a smaller increase in carbon content will have the same effect. An appreciable fraction of all iron made in antiquity was for these reasons poorer in toughness and weldability than would be expected from its carbon content alone, which is often the only clement analysed for in an artifact. The subject is important, but its assessment waits on more extensive analyses of ancient iron artifacts. PART 2: FORGING THE BLOOM
An iron bloom as extracted from a smelting furnace can be of various sizes and shapes, and can vary in texture from a loosely adherent mass of small nodules of varying carbon content in a matrix of frozen slag, to a one-piece lump of high density and moderate slag content. As noted above, except when the iron is very low in average carbon content, it is always variable in local carbon content over both short and long internal distances, from nearly carbon-free to 2 or more per cent carbon. The objective of the smelter operator was to make a single dense bloom of hardness (average carbon content) suitable for the intended use of the forged product, and of low slag content, so that it could be forged with a minimum amount of effort into a useful bar or object. If, instead, reduced irregular masses were produced by the furnace, they would have to he broken up, sorted by hardness and fracture appearance to select forgeable iron, and those pieces then forge-welded together to make a piece or bar of useful size and desired strength and ductility. Success in making forgeable blooms in one piece naturally varied with the experience of the smelter. The blooms from a fining operation can be less variable in average carbon content, contain fewer voids, and be of more uniform size than those from a smelting operation, since their formation can be under better and more direct control.
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If the bloom is in one piece, it is a mass or lump with a very rough surface that has pieces of charcoal and of frozen slag adhering to it, which can be knocked off with a hammer. The interior, as can be seen on a saw-cut face, contains voids and entrapped solid slag and occasionally bits of charcoal. The amounts of void and slag depend on furnace operating temperature, the iron content of the particular ore used, and the density and size distribution of its pieces as charged to the smelting furnace. For example, with marginally high enough furnace temperature and usc of a very loose-textured or amorphous ore such as some bog ores, the bloom may be so fragile as to require very careful forging and may contain considerable slag. In antiquity when the bloom weighed more than about 10 kg, it was cut with an axe into two or more pieces while still hot, in order to be forgeable with human muscle power on the forge hammer. Blooms weighing more than 15 or 20 kg would indicate that a water-powered forge hammer was available. Good discussions of bloom weights are in Tylecote (1987) and Mott (1961). The slag contained in a bloom can be expelled by forging only when the slag has become of sufficiently low viscosity to be squeezed Ollt, and the temperature range within which this can be done includes those temperatures at which iron is not only reasonably plastic but can readily be welded to itself. Forging to expel slag can then also weld cavities shut. The extent of forging necessary to make a bloom that is solid and low in slag content is obviously increased by higher slag content and porosity; higher carbon content iron, being harder and stronger, requires more force from the hammer to deform. The strength of hammer blows also must be controlled to suit the initial density of the bloom, at first light enough to consolidate without disintegration, and then increasingly heavy as the mass becomes stronger and more coherent, to expel as much slag as possible. At the finish of this operation some slag always remains, stretched out into thin stringers, and the strength and ductility of the iron increase as the quantity of the remaining slag is decreased and the stringers are thinner. A wide variety of microstructures and carbon contents of blooms were made in antiquity due to the difficulties involved in smelting furnace control. The smith on the hammer had to acquire by experience extensive tactile and observational skills to be able to make bar stock of acceptable and reasonably uniform mechanical properties, from different blooms and with minimal loss of original bloom weight as scale formed during reheating for repeated forging. FORGE-WELDING OF IRON
When iron is heated in a charcoal forge fire, the surface is oxidized at an increasing rate as temperature rises, as can be readily observed by
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periodic examination. This is because, as noted in chapter 7 and shown in figure 6, the atmosphere in a bed of burning charcoal such as a forge fire is oxidizing to iron at all temperatures useful in its forging and welding. The oxide scale formed is in three adjacent layers of oxygen content decreasing from the surface, i.e., hematite, magnetite, and wustite in thickness ratios of approximately I: 3: 100 (Kazantsev 1977). The scale is brittle below about I,300oC and cracks off the bar as it is deformed during forging. When an unfinished bar is re-heated for more forging, a fresh layer of scale forms, so that many re-heatings to forge a finally thin object can result in not only considerable loss of iron as scale but considerable decarburization of the iron. This as noted above is a form of "fining. " As one example among many, such decrease in carbon content during forging under primitive operating conditions was given by Bellamy (19°4), where an average bloom carbon content of 1.67 per cent was decreased to 1.02 per cent in a tool forged from it - a loss of 39 per cent of the original carbon content. However, even a thin layer of scale can prevent the making of a continuous hot weld between two pieces of iron. One solution to this problem is to dust the two hot pieces of iron with pulverized silica sand just before forging. This converts the iron oxide scale to an iron silicate slag of moderate melting point, so that when the hammer is applied, most of it is squeezed out. Since bloomery or wrought iron contains some of this kind of slag, it can be forge-welded to make good welds. This was a valuable property of all iron made until the middle of the nineteenth century, when the open hearth and Bessemer processes made molten, essentially slag-free steel. The slag in a bar of bloomery or fined iron can seldom be completely squeezed out during the making of a weld, and can then be detected in a metallographic sample as shorter or longer straight lines, typically with interruptions, and often sharply defined different carbon contents on each side. In iron that has been repeatedly folded and forge-welded to decrease the heterogeneity of carbon content of the original bloom, such as in the Japanese sword-making practice, there is always some residual slag, and the very thin remnants are then revealed as the" grain" visible on the sides and particularly on the back of the sword. In a Damascus sword forged directly from a cake of Wootz steel, which contains essentially no slag since it has been molten, the grain or figure visible is instead due to patterns of distribution of iron carbides, and the steel is accordingly stronger at the same carbon content. Weight losses during forging are due to iron that is re-oxidized to a scale during heating to forging temperature, slag squeezed out of the bloom, and carbon oxidized from the surface layers of the iron. The
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first two make up very nearly all of the weight loss, increasing with the slag content of the raw bloom and the number and lengths of re-heatings necessary during the forging. The number of re-heatings increases as the raw bloom is of lower density and as the surface area per kg of the final product increases. For example, the manufacture of thin plate or sheet for utensils or for armour will be accompanied by quite high losses to iron oxide scale. Loss can be decreased by dipping the piece into a thin clay wash before each return to the forge fire, which would be particularly effective when making sheet; but there seems to be no way of establishing whether this was a practice in antiquity. The total loss can obviously vary considerably, from less than 40 per cent to more than 70 per cent; but as noted above, for any particular final product the loss will decrease with increased skill and experience of both the smelting furnace operator and the smith or forge operator. For these reasons the value of modern attempts to replicate quantitatively such complex and laboriously acquired forging skills is questionable. High loss of a raw bloom to finished bar, i.e., low yield, is costly in both materials and labour. For example, if a bloom were made with a charcoal consumption in the smelting furnace of 6 kg per kg of bloom, and 4 kg of charcoal were consumed in the forge fire, a final yield from bloom to finished object of 30 per cent would mean that the charcoal consumption per kg of object would be 30 kg. CARBURIZATION OF IRON
Many years ago the assumption appeared in the archeological literature that the variations in carbon content seen metallographically within a bloom of iron or in a bar forged from it were due to carbon absorbed by the iron from the fuel charcoal while being heated for forging. The premise, which seemed obvious at the time, was supported on a theoretical basis by the well-known equilibrium diagram connecting iron oxide, carbon monoxide-dioxide ratio, and temperature. Although the assumption is still being made today to interpret sources and methods of production of ancient iron artifacts, it is clearly in error, as I have shown discussion in detail elsewhere (Rehder 1989) and by the inherent heterogeneity of blooms discussed above. In many cases such older interpretations must be reviewed and conclusions modified. Iron placed in a forge fire to be heated to forging temperature inevitably will be oxidized and decarburized at its surface, not carburized. However, as figure 6 indicated, there is a location at some distance from air entry into a charcoal fuel bed where the temperature is about 1,OOO°C and the local atmosphere is carburizing. A piece of uncoated
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iron held in that position (if it could be found) for a sufficient time would in principle be carburized. This would then require holding the position for hours in a fuel bed that is being continuously burned away and replenished, and the location of the position will change during the course of every pulse of the bellows - obviously a procedure with little chance of success. It is important to note that the above discussion concerns a piece of iron simply inserted into a forge fire for heating. If, however, the iron is wrapped with a source of carbon in a clay envelope, or placed in a closed clay box or pot along with charcoal or any organic material, and the whole is heated directly in a fire or in a kiln to above about 730°C, the iron will absorb carbon at its surface at a rate and to a depth that increases rapidly with temperature of holding. The holding times for appreciable depth of carburization are considerable - for example, a depth of two millimetres requires thirty hours of holding at 850°C and about seven hours at 950°C. Higher temperatures cause undesirable grain-coarsening in the iron. If only a shallow depth of carburization is required, such as on the teeth of a file, a coating on the iron of a clay slurry mixed with pulverized charcoal and dried is enough to permit carburization to occur. The explanation is that enclosure keeps out all or most of the furnace atmosphere allowing a very reducing atmosphere to develop within the enclosure from the carbon in the organic matter. The earliest recorded description of this practice was by Theophilus (1979) in 1,100 A.D., but Pliny the Elder in 79 A.D. (1962) mentioned twice in his Natural History that files were used in shaping and finishing the bronze castings of statues. These could not have consistently cut bronze unless they were made of iron that had an appreciable carbon content and was cold work hardened as described below; quench hardening was certainly known by Roman times. The earliest use of intentional carburization of iron is not known. Biringuccio in the sixteenth century A.D. described a procedure for carburizing iron in which pieces of soft iron are immersed in molten cast iron and held for some hours. The carbon content of cast iron is in the order of 3 to 4 per cent and it is molten above about I,250oC, so carburization of the soft iron of higher melting point proceeds; but I know of no evidence that this method was used in antiquity. COLD WORK HARDENED IRON
While quenching iron that contains carbon can increase its hardness to considerably above that of work hardened bronze, it was shown in part I of this chapter that this process was apparently used to only a
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minor extent throughout antiquity. The reason seems to have been that cold work hardening of an iron edge will give satisfactory hardness for most purposes. I have shown (Rehder 1992) that iron can be cold work hardened in the same fashion as bronze and that at the average carbon content of iron in antiquity, its strength and hardness then can be considerably higher than those of the best work hardened bronze. The work-hardening technique would be familiar from its use on bronze and would be more easily controlled than quenching. The edge could still be sharpened by a sandstone, which is not possible with a quenched edge. It certainly was used on iron, but the extent of its use cannot be known accurately because such work hardened edges corrode much more rapidly than the body of the iron, and in most cases have long disappeared from artifacts that arc otherwise in good condition. PART
3:
THE COMPOSITION AND
PROPERTIES OF IRON AND THEIR CONTROL
The word "iron" is of course the name for a chemical element, and its more general use has been for iron of quite low carbon content. Until late in the nineteenth century such low carbon iron was usually modified by the word "malleable." "Steel" has usually designated an iron that can be appreciably increased in hardness by quenching in water from a red heat; this becomes useful only at carbon contents above about 0.20 per cent. The term "wrought iron" is of fairly recent origin and usually means a low carbon content iron that contains stringers of slag. These result from its being made by the direct process or being "fined" from cast iron as described above. It therefore includes all iron made in antiquity. The term "steel" also tends to be applied to wrought iron with enough carbon content to be quench hardened; this can be confusing, but there is no commonly used specific term. In iron cooled in air from above a bright red heat such as after forging ("normalized"), as the carbon content of the iron is increased from zero up to about 0.90 per cent carbon, its strength and hardness increase and ductility decreases proportionately. It will have tensile strength of about 320 MPa at 0.10 per cent carbon and 950 MPa at about 0-90 per cent, with hardness about 90 and 280 Vickers Hardness Number (VHN) respectively, and 35 per cent and 15 per cent elongation. Carbon also gives iron the property of becoming much harder and stronger by quenching in water from a bright red heat, its hardness reaching a maximum of about 900 VHN at a carbon content of about 0.70 per cent. Below about 0.20 per cent carbon the increase in hardness from quenching is small enough to be seldom worth the expense or trouble.
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Mastery and Uses of Fire in Antiquity
Iron of very low carbon content is almost as soft and ductile as copper, though a little stronger, and is easily hot forged and forge welded to itself. Low carbon content iron is desirable for general use because its toughness and strength are combined with excellent forge ability and weldability. However, in antiquity the low carbon content made it the most difficult kind of iron to smelt consistently because of the narrow furnace temperature range between making low carbon iron and not making any iron at all. The forgeability and weldability of iron both become poorer as its carbon content is increased. At high carbon levels from about 1.0 to 2.0 per cent carbon, the hardness of normalized iron continues to increase, but strength increases much less rapidly, and forgeablity and weldability become considerably poorer. These irons, or rather, very high carbon steels, are used today for special purposes because of their high strength and hardness. At carbon content of about 1.5 to 1.8 per cent, they constituted the famous Wootz metal of which the fabulous Damascus swords were made. This was the first known cast steel and contained almost no slag because it had been melted. Its origin in time is not clear, but apparently it was late in antiquity. CAST IRON
Cast iron is high carbon iron with a wide range of compositions which are defined for iron-carbon phase diagram reasons as containing more than 2.1 and usually less than about 4.3 per cent carbon. As noted above, it will be made whenever in a bloomery furnace smelting iron ore is operated with too Iowa ratio of ore to carbon in the burden for the combustion air rate. There is archaeological evidence of white cast iron that was cast into useful shapes in the West in the fourth to third centuries B.C. (Solntsev 1969), and in China from about 500 B.C. The latter will be discussed after a general description of the properties of the various kinds of cast iron. As the carbon content of iron is increased, its initial solidification temperature decreases; for example, at 2.1 per cent carbon it is 1,38ooc and at 4.3 per cent carbon 1,150oC, this latter being not far above the melting point of copper at 1,083°e. Solidification is in two different modes. One is as a "white" cast iron that contains the carbon entirely as the compound iron carbide (cementite). It is extremely hard (900-1,000 VHN) and results in a finely facetted silvery fracture and no useful ductility. The microstructure of white cast iron is a mixture of cementite and much softer ferrite, in proportions depending on carbon content and with an average hardness of 410 to 550 VHN as cast in the composition range of 2.5 to 4.0 per cent carbon (for reference, window glass has a hardness of about 400 VHN). Because of its moderate melting tempera-
14 I
Smelting and Forging of Iron
ture, white cast iron can readily be cast into sand, clay, or metal moulds at temperatures ncar those of bronze founding. In this way objects of high hardness can be made directly without forging, and objects of thicker section and suitably shaped, such as an axe head or chisel, can be remarkably shock resistant. In 1986, I cast a hatchet head in white cast iron, with a slot for a wooden handle, and after sharpening, the edge the hatchet was able to cut up to ISO mm diameter logs of wood with no evidence of damage or edge chipping. A small square cast bar, its end sharpened at an angle, made a durable chasing tool that was effective on both bronze and mild steel. Quite recently it was demonstrated that white cast iron containing up to about 3 per cent carbon can be hot forged if the correct technique is used (Wadsworth and Sherby 1980). I have since forged without difficulty a 20 mm square bar of 2.7 per cent carbon white cast iron to a chisel edge one mm thick, the technique being primarily that necessary for the successful forging ofWootz. This involves a low forging temperature of 700 to 800°C, not too hard hammer blows, and patience. Deformation rate is slow, so many reheatings arc necessary, but formation of scale is moderate because of the low temperature. This demonstration is important since it lends credence to reports such as those mentioned in Cline (1937) that the Hausa in Africa cast swords in white cast iron that were then forged to final shape, and the detailed report of Dixey (I920) describing the smelting of ore to molten cast iron, which after solidification was forged to knives and hoes. Since molten cast iron was without doubt made accidentally in antiquity many times, there are likely yet undiscovered iron artifacts from antiquity of both cast and forged white cast iron. The other mode of solidification of cast iron is due to the presence of other elements, particularly silicon, which is reducible from slag at high furnace temperatures. In sufficient quantity silicon causes the carbide to decompose during solidification to carbon as graphite and to ferrite (iron free of carbon). When most of the graphite is as microscopic flakes in the microstructure, the iron is called "gray" cast iron because of the colour of its fracture. While it has little ductility, it is strong, wear-resistant, and machinable. When the graphite from solidification is in rounded nodules, the ductility of the iron can be very good, and it is called" ductile" iron. Heat treatment of both kinds of iron can modify their matrix structures to give a wide range of mechanical properties. MALLEABLE IRON
This section is included for readers interested in ancient Chinese metallurgy, as the material was not known in antiquity except in China.
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Mastery and Uses of Fire in Antiquity
If white cast iron containing a moderate content of silicon is heated to and held at temperatures between about 800 and I,OOOoC in an atmosphere low in oxidizing potential such as in a closed container, the primary cementite decomposes into iron and free carbon. The latter precipitates as microscopic rounded masses of graphite called temper carbon, the rate of decomposition increasing particularly with the content of silicon and increase of temperature of treatment. The decomposition rate also decreases with increase in the thickness of the casting. Time for complete decomposition of primary cementite can vary from an hour or so for a thin casting with carbon content of 3.0 and silicon content of 1. 5 per cent held at 925 DC, to hundreds of hours for a moderately thick casting with carbon 2.5 and silicon 0.10 per cent held at 800 c. When the primary cementite is entirely decomposed and the iron is then cooled to below about 730DC, the matrix structure decreases in solubility for carbon and carbon is precipitated as a finegrained mixture of lamellar cementite and ferrits called pearlite. To decompose this cementite, cooling rate must be very slow or the iron must be held for some time at just below 730DC. If, however, high temperature heat treatment of white cast iron is in a furnace atmosphere of controlled oxidizing power, carbon will be removed with only moderate oxidation of the iron, so that the iron acquires ductility. The result, called "white-heart malleable iron," is no longer made, but it is mentioned here because comment has been made in the archaeological literature, particularly in reference to ancient Chinese iron, that it is difficult or impossible to distinguish white-heart malleable iron from low carbon iron fined from cast iron or made by the bloomery process. The comment is puzzling, since in fact steel fined from cast iron and bloomery iron, which are both forged products as used, always contain variable but appreciable amounts of slag stringers. On the other hand, white-heart malleable iron contains very little slag since the iron was molten before being cast, and nearly all slag had floated to the surface and was skimmed off. There can on occasion be what look like small slag stringers in white heart cast iron, but these can be identified through the use of polarized light as graphite flakes resulting from too high annealing temperature. D
IRON IN ANCIENT CHINA
Smelted iron appears suddenly in the archaeological record of China in about 600 to 500 B.C., apparently as cast iron. Smelted copper appeared much earlier but just as suddenly, with no artifactual evidence of development of smelting process in either case. Moreover, the earli-
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Smelting and Forging of Iron
est smelted copper in China was millennia later than in the Near East; in the case of iron, by 800 B.C. iron smelting was fully developed in the Near East and iron was being widely used. It is hard to believe therefore that copper and iron smelting in China had independent origins, since the sequences were the same and cast iron is easily made in bloomery furnaces. It is generally accepted that the Chinese were making objects of cast iron about 500 B.C., using their well-developed foundry technology for casting bronze. While it is very unlikely that the Chinese originated the smelting of cast iron, the important thing about the artifactual evidence in China is that it shows continued development of the use of cast iron as castings, the use of fining, the heat treatment of white cast iron in a pottery kiln to make it a malleable and ductile iron, and then the manufacture of steel essentially by various methods of carburization, into the first millennium A.D. The development of these varieties of iron and then of steel has been discussed in detail by Needham (1958), but because of difficulties of translation of the ancient Chinese literature and of its interpretation, Needham's account appears somewhat inconsistent as to detail and dating. More recent publications in English from China on the development of the metallurgy of iron in particular are not easy to follow because of repetition and internal contraditions. An example is Ancient China's Technology and Science (1986), with no author named, published by the Institute of the History of Natural Sciences, Chinese Academy of Sciences (pp. 392-4°1). IMPLICATIONS OF THE DIFFICULTY OF MAKING IRON
The uncertainties involved in antiquity in controlling the operation of a smelting furnace were discussed in chapter 9, and the foregoing description of the details of the process of smcIting iron adds to them. They seem to adequately account for the long development period that the archeological record shows for the smelting of iron. They also suggest clearly that there were associated and ensuing effects on the geographical diffusion of techniques for ironmaking, in that successful transmission, either intentional or by theft, would have been peculiarly affected by chance. As one example only, simply using a different species of tree for making charcoal in a new location could markedly change the resuIts obtained by smelting, even if all other things were somehow reproduced identically. This difficulty could influence the whole geographical distribution of the ability to make iron. It could also have a
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Mastery and Uses of Fire in Antiquity
restrictive effect on the number of centres of successful iron production and would seem as well to increase the likelihood of successful development of ironmaking in independent locations and over periods of time. Of course strong political and economic factors were also involved in the acquisition and control of such a useful metal as iron, and these would have interacted with the technical issues. Iron ores are well distributed, and various centres of power existed that were intent on finding out how to make iron locally, to avoid the necessity of trade or to create it. But the technological puzzle was dominant because it was well tangled, there were few keys, and even when tentative solutions were found, the technology was difficult to transfer.
I3 Fuel Consumption by Pyrotechnology in Antiquity
The several categories of use of high temperature heat in antiquity consumed different quantities of fuel per unit weight of product made, and it is of interest and practical value in archaeology to determine them. My objective here is to generate some carefully considered estimates of fuel consumption for various kinds of product, for use in comparisons and as bases for estimates of amounts of biomass consumed by pyrotechnology in antiquity. These estimates are of course dependent on the quantitative artifactual information available, which is variable in amount and accuracy. While the figures are necessarily averages, the method of arriving at them will be made clear so that as more accurate data becomes available in particular cases, closer estimates can be made. Since the fuel employed was either some form of biomass used directly, or charcoal made from biomass, useful comparisons between processes can be made only when quantities of charcoal are converted to quantities of biomass necessary to make them. Although the yield of charcoal from biomass varies, used here is the conservative figure of 0.10 kg of usable charcoal from 1.0 kg of air-dry biomass. POTTERY AND OTHER CLAY PRODUCTS
Clay and other ceramic products, as was noted in chapter 5, were made in a wide variety of furnace configurations, sizes, maximum temperatures, and accompanying thermal efficiencies using some form of biomass directly as fuel.
146 Mastery and Uses of Fire in Antiquity
As an example, take the heat content of clay to be 0.84 KJ per kg per °c. To reach a firing temperature of 800°C, 1.0 kg of pottery requires 0.84 x 800 = 670 KJ of heat, and if the thermal efficiency of a particular kiln is 4.0 per cent, 670/0.040 = 16,750 KJ = 16.75 MJ of heat will be required from the fuel. If the heat content of the air-dry fuel is 14 MJ per kg, then 16.8/14 = 1.20 kg of fuel must be burned per kg of pottery fired. This figure agrees well with published average modern figures (Vimal and Bhatt 1989), but the average thermal efficiency of kilns in antiquity more likely would have been 1.0 to 2.0 per cent; the fuel consumption then could have averaged from 3.0 to 4.0 kg per kg of product. Higher temperature firing would also have increased fuel consumption. PLASTER
The conversion of gypsum to Plaster of Paris (also as noted in chapter 5) requires quite low temperature, within the narrow range of 128 to 163°C. This is in the range of cooking of food, and a simple oven would suffice. No detailed estimate is made here, but fuel consumption may be guesstimated as about 0.3 kg of biomass per kg of plaster. LIME-MAKING
The manufacture of lime and the heat necessary per kg of quicklime were discussed in chapter 5. Cato described a Roman lime kiln that I estimated would have contained about 36,000 kg of lump limestone; I then estimated the amount of biomass fuel necessary to convert the contents to quicklime as 2.8 kg per kg of lime made. However, the smaller kilns probably used in earlier antiquity would have had lower efficiencies of use of heat and correspondingly increased fuel consumption. Although sufficient artifactual information on lime kiln sizes throughout time is lacking, average fuel consumption might have been three to five times as high, or about 2.8 x 4 = I I kg of fuel per kg of lime made. As noted in chapter 5, operation on a continuous basis would result in much better fuel economy, and fuel consumption would then be about 0.68 kg per kg of lime, which agrees well with similar continuous kilns that have been operated into the present day (Vimal and Batt 1989). But I know of no evidence that continuous withdrawal lime kilns were used in antiquity. GLASS-MAKING
The fuel used for glass-making in antiquity was biomass wherever there
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Fuel Consumption by Pyrotechnology in Antiquity
is record, and the efficiency of use of the heat developed would have been very poor. This was because the furnaces were quite small, the mass of structure and crucible was large per kg of glass made, in most cases the melting was done twice to attain uniformity, and the temperature of the flue gases had to be at least as high as the temperature of the molten glass, carrying off large quantities of heat. The increased fuel and labour necessary made glass much more expensive than ceramics. THE SMELTING OF METALS
Since copper and iron attained wide use in antiquity, the fuel consumption in smelting them and preparing them for use is an important issue. Ores were usually roasted before smelting, to make them more easily reduced or, particularly in the case of sulphide ores of copper, to convert as much as possible of the metal sulphides into oxides reducible by charcoal. Roasting was carried out in heaps or windrows with interlarded biomass as fuel, and biomass was also used for preheating smelting furnaces. Charcoal was the fuel used for the smelting operation itself, but it was made from biomass so all of the heat used from ore as mined to finished metal was derived originally from biomass. This makes total biomass used per unit weight of metal produced a useful figure. COPPER SMELTING
The smelting of ores of copper was discussed in chapter II, but the wide range in their content of copper and the fact of whether they were sulphides or weathered oxides make large differences in the wood and charcoal needed to produce a kilogram of copper metal. The uncertainty as to the compositions of the ores or concentrates actually smelted in most locations in antiquity makes estimation of fuel consumption for copper production a kind of poorly informed guesswork. The only artifacts available for the purpose are the considerable number of fields of furnace slag scattered across the world, identified as resulting from copper smelting by their general composition and residual copper content. There are probably slag fields directly associated with the mine that supplied the ore used, and from this combination a useful estimate of the amount of biomass consumed per kg of copper made might be possible. But I know of no such circumstance on which the pertinent data are available, and in any case one could not safely generalize it to other fields of slag. The ores used for copper smelting in antiquity ranged from weathered outcrops of sulphide ore to the sulphide ores themselves, but the
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Mastery and Uses of Fire in Antiquity
consensus of archaeo-metallurgists seems to be that most of the copper smelted through antiquity was from sulphide ores. These seem to have been typically of moderate to low copper content, and certainly the ability to use ores as low as 2 per cent copper was developed. A conjecture of an average of 5 per cent copper could be made in order to develop an approximation of the amount of biomass fuel used per kg of copper made, or more usefully, per kg of slag inescapably made. Crookes and Rohrig in 1869 published a textbook on copper smelting giving extensive operating data on many commercial copper smelters in various parts of Europe. Fortunately for comparison with conditions in antiquity, the furnaces used were quite small, about 1.6 m in hearth diameter or about twice those typical of antiquity. Charcoal was often used as smelting fuel; roasting of ore and matte was in heaps using wood fuel as in antiquity; and much of the ore was sulphide containing about 5 per cent copper. The principal difference from antiquity was that the furnaces were operated for several weeks at a time because of better refractories in their construction and higher demand for copper, and this would give higher efficiency of use of charcoal than in the shorter smelting times probably used in antiquity. Since differences in furnace size and campaign length can be allowed for, it should be practical to give the operating results published by Crookes and Rohrig and then modify them appropriately for conditions in antiquity. Such modification can be changed if more data become available. The copper resulting from ore and matte smelting was remelted and refined by oxidation with air, because of iron and other alloy contamination. Recovery of copper was 95 per cent. Per kg of refined copper made, 10.9 kg of charcoal were used for smelting and 21.7 kg of wood for roasting; 29.5 kg of slag were made. At IS per cent yield, charcoal required 72.7 kg of wood, so the total wood (biomass) used was 94.4 kg. Then one kg of slag would represent 3.2 kg of biomass and 0.034 kg of copper made. Smaller furnaces and shorter smelting runs in antiquity would probably double the charcoal required for smelting. Wood for roasting should be increased by 30 per cent to include that for furnace preheat in shorter runs. Recovery of copper should be decreased to about 85 per cent, and the amount of slag would be little changed. When these changes are made, one kg of slag would represent 5.9 kg of biomass and 0.030 kg of copper made. This transforms to one kg of copper requiring the consumption of 194 kg of biomass, which is rounded to 200 kg for inclusion in table 3 at the end of this chapter. The principal reason for change in fuel consumed in smelting copper is the content of copper in the material being smelted, and this varies widely either from different ore bodies or from an ore being concen-
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Fuel Consumption by Pyrotechnology in Antiquity
trated by crushing and sorting. The 5 per cent ore discussed above would have a gangue content of about 85.6 per cent, which is seventeen times the copper content, and it and flux for it must be heated to furnace operating temperature. However, if the ore were to be crushed and copper mineral and gangue separated by hand to make a concentrate for smelting that contained 25 per cent copper, the gangue content would be only 1. I times the copper content. This would decrease the biomass represented by one kg of slag to about 33 kg and 0.25 per cent. The saving in fuel by concentrating the ore is a strong reason for doing it, but it introduces confusion into a modern estimate from a field of slag of the biomass consumed. IRON SMELTING
Iron ores are much more common than those of copper and usually higher in metal content. Orcs with 50 per cent iron as mined are not uncommon, and some deposits have iron ore nodules of 60 or 65 per cent iron ore that are disseminated in earthy material and easily separated. The majority of iron ores are oxides, hematite containing 70 per cent iron, and magnetite containing 72 per cent iron; hematite can become hydrated or carbonated to make limonites and siderites of lower iron content. Large quantities of iron occur as sulphides, but these have never been economical to smelt for their iron content even when roasted to oxides, because of the difficulty of removing all of the sulphur, which is a serious contaminant of metallic iron. Iron ores usually have been roasted before smelting from antiquity into the early twentieth century, because of the increase in their reducibility described above. The production of a useful bar of iron in antiquity was thus a three-stage process involving roasting of the ore, smelting it to a bloom of iron, and then forging the bloom to expel slag, consolidate the iron, and shape it to a useful object. Each stage required fuel, as wood for roasting and furnace preheating and as charcoal for smelting and the forge fire. The estimate here of fuel consumption for iron smelting is based primarily on the series of well-controlled experimental smelts made by Tylecote et al. (1971), averaging the results of their smelt numbers 14 to 3 I inclusive, during which iron of carbon contents from very low to 1.8 per cent were made. The furnace, described earlier, was unusual in that it was very well insulated, which would have decreased fuel consumption. The ore used was from several sources, most often a roasted siderite (iron carbonate) containing 34 per cent iron. After thorough preheating of the furnace with charcoal, each smelting run averaged three hours in length, which was about the
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residence time and therefore the time for processing one complete column of burden or charge. The average weight of bloom was 3.7 kg, average carbon content was 0.32 per cent, average charcoal consumed during smelting (i.e., excluding preheat charcoal) was 11.7 kg, and smelting charcoal per kg of bloom averaged 3.2 kg. It is necessary to add the charcoal for filling the furnace before charging ore, which brought the charcoal per kg of bloom to 6.2 kg. This figure would of course have been lower if the furnace had been operated for a longer period for each smelt, as would undoubtedly have been the case in antiquity. The furnace was preheated with charcoal amounting to three times as much as used for the smelting itself. In practical terms this is unrealistic since charcoal is labour intensive; wood or brush can preheat satisfactorily and was very likely used in antiquity, so is not included here. It is assumed for present purposes that the furnace was preheated with 10.0 kg of wood. The average length of smelt is also assumed to have been six hours in a practical production setting, to make a bloom twice as large. This results in a charcoal consumption, including furnace filling, of 4.7 kg per kg of bloom. However, Tylecote's furnace had 330 mm thick walls that included modern insulation, giving unusually low heat loss rate and so decreasing fuel consumption. A furnace with much thinner walls, about 60 to 90 mm and containing no insulating material, would be in my opinion more representative of ancient (and indeed of nineteenth century A. D. African) practices and would require more fuel. At the end of chapter 2 I compared my own unpublished bloom making in a similar sized furnace to Tylecote's. With only a 40 mm thick wall my furnace had three times the heat loss rate of the Tylecote one, and the charcoal required per kg of bloom was twice as high. When the variety of furnace sizes, ore grades, and lengths of smelt undoubtedly used in antiquity are taken into consideration, it would thus seem that an estimate of an average 8 kg of charcoal per kg of bloom, with 16 kg of wood for ore roasting and furnace preheating, is reasonable and conservative. The charcoal consumed in the many reheatings of a bloom for forging to acceptable bar and the yield of finished iron bar depend on the initial density of the bloom, its size, the size of bar to be made, the number of reheatings necessary, and especially on the skill of the smith. Blooms forged to thin bars or particularly plates will require a maximum amount of charcoal per kg of final bar even with experienced smithing. There can therefore be considerable variation in charcoal consumption by the forge per kg of finished bar iron. Reliable data from antiquity on yield and charcoal consumption is not available since nothing of this nature was then weighed, and mod-
lSI
Fuel Consumption by Pyrotechnology in Antiquity
ern attempts in replication lead to unrealistically low yields and high charcoal consumption because of production of small blooms, use of charcoal for preheating, and inexperienced smithing. For example, a recently published account (Crew 1991) showed a yield of bar from bloom of 30 per cent, and a total charcoal consumption of 136 kg of charcoal per kg of bar iron. This would require an unrealistically large amount of air-dry wood - 907 kg - to make I kg of bar iron. Assuming that about 4 kg of charcoal are used in the forge fire per kg of bloom forged to bar iron by a smith fully experienced in such work on bloomery iron, the total consumption of charcoal per kg of bloom is 8.0 + 4.0 = 12.0 kg. If the average yield from bloom to bar is taken as 50 per cent when processed by a smith experienced in bloomery iron, then the charcoal consumption per kg of bar iron will be 24 kg. It may be noted that an early fourteenth century English bloomery and forge which left good records (Mott 1961) made one bloom per day of unstated weight, and the total charcoal used to smelt and forge it to a saleable 13 kg iron bar was given as 9.6 kg per kg of bar. Considerably more would have been required if the bloom had been forged to plate or to weapons. Recently in a collection of data (Rostoker and Bronson 1990) figures for total charcoal for production of 15 to 41 kg of bar iron per day from a bloomery furnace have been given as 8.1 to 8.7 kg of charcoal per kg of bar. These latter data seem to be as roughly consistent within themselves as might be expected, but they are from a period a whole millennium later than the end of the time covered here. In antiquity the scale of operation of ironmaking was undoubtedly smaller and so less fuel efficient. In Roman times 8 to 12 kg seems to have been a maximum bloom size, limited by the fact that only human muscle was available for bellows and particularly for forging; in the fourteenth century A.D. and later, hammers and bellows were run by water-power. To be conservative, for present purposes an average charcoal consumption in antiquity for iron-making is taken as about 20 kg per kg of bar iron made, and 16 kg of wood. At 15 per cent recovery of charcoal from wood, the total wood required would be in the order of 150 kg per kg of bar iron. Modern operation of replica bloomery furnaces indicates that the slag made per kg of bloom is in the order of 3 to 4 kg per kg of bloom, depending of course on the iron content of the ore. There would be possibly another 2 kg or so of oxide scale and expelled slag from forging, depending on the extent of forging. This slag is scattered in small bits and is usually discarded with the furnace slag. Iron furnace slags are difficult to use as indicators of either the amount of iron made or of the fuel consumed, for several reasons. Usually they are quite high in
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iron content; the quantity per unit of iron smelted depends on the grade of ore used, since the iron oxide in the ore is its own flux; and there is a variable mixture in a slag field of smelting slag, forging scale, and the usual non-smelting detritus. These factors, combined with the basic uncertainty of the actual quantity of slag in a field, would seem to make such estimates close to exercises in futility. SUMMARY
Table 3 summarizes the biomass fuel consumptions of several uses of furnaces in antiquity. These are considered to be approximately right, but the figure for smelting copper has a broader error range for the reasons given above. Table 3 Biomass Fuel Consumption by Furnace Product
Kg per kg of Product Product
Biomass
Clay products
1.0 - 3.0
Plaster of Paris
0.2 - 0.4
Lime
8.0 - 14.0
Glass
10 -15
Copper
200
Iron
150
While the fuel consumption per kg of product for metal smelting is much larger than those for pottery and lime, it must be kept in mind that in antiquity the weights of pottery and of lime made per unit of population were larger (probably much larger) than the weight of metal products made.
I4 Fuel Supply and Deforestation
Here I make a plea for correct names of actions. To grow forest on clear land is to "afforest" it, a word that is in the dictionary. To clear land of forest or trees is to "deforest" it, another word to be found in the dictionary, with similarly clear etymology. The word "deafforest," which is too commonly used, is not only in no dictionary to my knowledge but is self-contradictory. GROWTH RATES OF FOREST
Deforestation is the name for the removal by humans of the larger and older kinds of biomass such as trees. This has been done primarily to make space for human habitation and for the growth of other kinds of biomass that can feed and clothe the human body, but trees are also cut for structural materials and fuel. Deforestation is a concomitant of civilization, and the ancient Greeks from as early as 500 B.C., and later the Romans, complained bitterly about the disappearance of the forests that were an essential source of building timber and fuel (Evans 1983). The condition was, however, highly dependent on the difficulty and cost of transporting forest products, both timber and charcoal. In antiquity transport was slow and costly, involving small, animal-drawn wagons on very poor or non-existent roads and slow, small ships often travelling only in daylight. Consequently barbarians a hundred kilometres from Athens were burning down forest for land clearance for settlement at the same time that Athenians were bewailing shortages of timber and fuel. Major destruction of forest for human settlement went
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on throughout antiquity into the Middle Ages, and continues today in supposedly enlightened times. In the considerable discussion in the archeological and anthropological literatures of deforestation in antiquity, fuel used for pyrotechnical purposes has been given some prominence as a supposed major factor (e.g., Hughes 1983). The subject is large and complex and there is not space in the present chapter for anything approaching a full discussion. Comment here is confined to an outline of some of the basic factors involved, followed by two recalculations of well-known published estimates of fuel consumptions and corresponding forest areas destroyed for pyrometallurgy in antiquity. These involve in one case the smelting of lead and in the other of copper, both requiring charcoal as fuel. The determining factor in the land area necessary to continuously supply wood for charcoal making at some specific rate in kilograms per year is the rate of growth of forest and coppice. Curiously, this has not been sufficiently taken into account in most of the current estimates of area of land required to support pyrotechnology, though the growth rates for different types of soil and climate arc today well known (e.g., Whittaker and Woodwell 1971). Growth rates in the Mediterranean area in antiquity may have been on average moderately different from those of today, but the modern rates are necessarily used here. At least three of the factors influencing deforestation were markedly different in antiquity from today. These were then the lack of use of fossil fuels, low annual yields of agricultural products per hectare of land, and high cost of transportation. A principal factor in the extent of deforestation was the ratio of the yield per hectare of food and fibre to that of forest. Yields of food and fibre may be taken from the Domesday Book of twelfth century England (Maitland 1921), when conditions were similar to or possibly a little better than in antiquity. These showed that a population of 1.38 million occupied nine million cleared acres, i.e., 2.6 hectares per person, of which 1.5 hectares were sown and I. I hectares were pasture. The average yield of grain was six bushels per acre or 0.52 m 3 per hectare, and if the bulk density of grain is taken as 600 kg per m 3 , the yield was 0.032 kg per m 2 per year. However, the normal rate of growth of the forest that the fields replaced was, from Whittaker and Woodwell, probably at least 1.5 airdry kg per m 2 per year, nearly fifteen times the productivity of grain and without need for labour or attention for sowing or weeding. In antiquity the impact of the need for cleared land for plough and pasture must have been particularly noticed during the estimated seven-fold expansion of the population of Attica during the eighth and seventh centuries B.C. (Snodgrass 1980), if 2.6 hectares of woodland had to be cleared per added person.
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Deforestation
The effect of high transportation costs per tonne of material moved was to make all economic affairs involving materials narrow in scope and of limited stretch. A detailed study of the supply of charcoal for the iron smelting industry in England in the nearly modern times of the seventeenth and eighteenth centuries (Hammersley 1966) has noted that the maximum distance that charcoal was carried overland was five miles (eight kilometres), the cost of longer distances justifying coppicing, or where possible, moving the furnace that needed the charcoal. It is evident from the literature that the story of deforestation is tangled, and in antiquity the record is further obscured by a serious lack of quantitative information on practical and technical matters. For example, the population of even such a prominent place as Periclean Athens is uncertain. While our interest here is in the consumption of fuel in antiquity for pyrotechnical purposes, and what part it may have played in the contemporarily perceived deforestation, in order to assess the extent of this part, it is necessary to select some geographic area and then to find out something quantitative about population density. We also need to know the area of arable land and of forest within economic transport distance, the quantities of structural timber and products of pyrotechnology consumed annually, and the amount of fuel necessary to make these products. The acquisition and sorting out of such information requires major effort, but at least a start can be made here by making new estimates, believed to be realistic, of the average area of forest consumed per annual kg of pyrometallurgical product. The fuel consumption figures in table 3 of chapter 13 are in terms of air-dry wood per kg of product, and if an estimate of amount of product made per year can be arrived at, the annual amount of wood necessary for each product can be found. This can then be translated into the area of forest necessary to grow biomass at a rate that will satisfy the annual consumption rate in perpetuity; effectually the forest is farmed like any other crop. When the necessary area is found for a particular product of pyrotechnology, it can then be compared with the area of total arable land available, to determine what fraction of the total was involved. ESTIMATES OF FOREST CONSUMPTION
Two examples from antiquity illustrate both the procedure of estimation and the small size of forest areas necessary for pyrotechnology and metallurgy in particular. Both cases have been previously published, and the base or starting figures used have been unchanged here for direct comparison, though in my opinion they are seriously questionable.
156 Mastery and Uses of Fire in Antiquity LEAD
The first example concerns the famous lead-silver mines at Lavrium in Greece. A published estimate (Wertime 1981) states that for the production of 1.4 million tonnes of lead from which 3,000 tonnes of silver were extracted over a period of two hundred to three hundred years, one million tonnes of charcoal were consumed. The acreage of forest cut down to make the charcoal was given as 2.5 million acres (10,100 km2), which is an area many times larger than the total of arable land within one hundred kilometres of Lavrion. However, the growth rate factor had evidently been ignored. Taking the time elapsed as about 250 years, the resulting average rate of charcoal consumption was therefore about 4,000 tonnes per year. Then at a yield of 15 per cent in making charcoal, this represents consumption of 26,700 tonnes per year of wood suitable for charcoal making. Such wood is typically about one-third of the weight of standing forest, so a forest growth rate of 80,000 tonnes per year was necessary to supply the charcoal in perpetuity. At an average growth rate for temperate climates of 1.6 kg of air-dry biomass per square metre of land per year (Whittaker and Woodwell 1971), this requires 50,000,000 m 2 • In other words, 4,000 tonnes of charcoal could be derived in perpetuity from 50 km 2 of land, the area of a circle 8 km in diameter. In simpler terms, 0.08 kg of perpetual charcoal can be obtained from one square metre of land. To check this figure for forest area necessary for perpetual supply of charcoal, data from "iron plantations" operated in the midsoutheastern United States in the nineteenth century (Temin 1964) seem applicable. The climate was apparently similar to that of ancient southern Greece, and on average, thirty cords of secondgrowth wood suitable for charcoal making were obtained per acre per year. This is a figure that agrees well with that of Crown surveyors of coppice for charcoal making in England in the eighteenth century (Hammersley 1966). This represents about 40,500 kg of wood, since the forest was largely softwood with a bulk density of probably about 1,340 kg per cord. From this wood 9,640 kg of charcoal were made per acre per year. On a twenty-year regeneration time, this is a growth rate of 1.5 air-dry kg per m 2 per year, a good check on the modern figure of 1.6 used above. The perpetual supply of charcoal was thus 0.12 kg of charcoal per m 2 per year. The figure is 50 per cent higher than that reached above for Lavrium in antiquity, but this is largely accountable for by the fact that the average yield of charcoal making in the American figures was 24 per cent, compared to the 15 per cent yield estimate used for antiquity. The 50 km 2 area is indeed small compared to the
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land area available and puts a completely different light on the muchdiscussed question of the charcoal supply for Lavrium. COPPER
Another well-known example concerns copper production in Cyprus. An estimate of charcoal consumption and of forest area thereby destroyed published by Constantinu (198 I) takes as base figures estimates that four million tonnes of slag from copper smelting accumulated over a period of about 3,000 years and that this represents the production of a total of 200,000 tonnes of copper. The smelting of one kg of copper was assumed to have required 300 kg of charcoal, and this, after some calculation, is taken to have resulted in the destruction of 150,000 km 2 of forest, or sixteen times the total area of Cyprus. Using the estimate of four million tonnes of slag accumulated over 3,000 years, the average amount of slag made over time was at the rate of about 1,330 tonnes per year. Assuming that the copper ores used were mostly sulphides that over the same period averaged 5 per cent copper, from chapter 13 it is seen that one tonne of slag represented the production of 0.035 tonne of copper and the consumption of 5.3 kg of wood for roasting and for charcoal making. Then 1,330 tonnes of slag per year represents the production per year of 46.6 tonnes of copper and the consumption of 7,050 tonnes of wood. As wood suitable for charcoal making constitutes about one-third of the weight of standing forest, forest is cut at the average rate of about 21,150 tonnes per year. At a growth rate of 1.6 kg of air-dry forest per m 2 per year, this requires an area of 13.2 km L to supply in perpetuity the charcoal required. This area is only about 0.15 per cent of the total area of Cyprus. These figures for production of metal have been averaged over a long period of time, and the production rate of copper in particular undoubtedly increased from a low to a higher figure during the period. Also, there is no way of knowing what the average grade of ore used was or the proportion that was oxide or sulphide, so it is uncertain what ratios of slag to metal are applicable. However, the most conservative figures have been used, and even when the production rate may have increased in later antiquity to ten or even fifty times the average, the forest area necessary to supply the copper industry would still have been little more than 3 per cent of the land area available. It is important to note as well that fuel used for pottery, brick kilns, lime making, and domestic use would have come in considerable part from the two-thirds of the forest harvested that was not used for
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charcoal making and so is invisible statistically. Even if this consumption per year was twenty times that of copper, the fuel consumption would add to that of copper by only about 80 per cent, to require as a maximum a total forest area of 5 or 6 per cent of the area of Cyprus. It thus seems unlikely that the view is correct that Cyprus was deforested largely by fuel for pyrotechnology or that it was even a major factor. Habitation, food, and fibre were probably much more important factors in land clearance; and while the free-ranging goat undoubtedly slowed regrowth, the goat population was a factor of the human population. An instructive parallel may be found in English experience in the iron industry in the seventeenth and eighteenth centuries. A study by Riden (1977) has shown that the maximum output of charcoalsmelted iron occurred in the period 1720-45 before coke was used, when an average of 27,100 tons per year of cast iron were made. During this time an average of 1. 5 tons of charcoal were used per ton of iron made (Hammersley 1966), and about an equal amount was used for fining the cast iron to wrought iron and forging it, so that average consumption of charcoal per year was about 81,300 tons. (Note that charcoal consumption per ton of iron was much lower than was given in chapter 13 for a period three millennia earlier). Using the forest productivity figure developed above on the basis of experience of 0.12 kg of charcoal per m 2 per year (Temin 1964), which is 0.48 tons per acre per year, the area of forest necessary to supply charcoal for the iron industry in perpetuity was 169,000 acres. This is about 0.40 per cent of the land area of England. Increasing use of land for agriculture, roads, and living accommodation over time increased the price of charcoal sufficiently to encourage trial of coke as replacement. This change apparently was made primarily for economic reasons, but it then turned out that coke gave higher furnace temperatures that permitted the use of lime as flux for slag to absorb the increased sulphur in the iron from coke (Rehder 1987). In Germany, where the ratio of forest to cleared land was at the time appreciably larger than in England, replacement of charcoal by coke for smelting was delayed for more than half a century. The estimates above are intended to increase the accuracy of the bases on which estimates of the impact of fuel consumption by some pyrometallurgical processes of antiquity are made. However, major uncertainty lies in interpretation of slag fields in terms of the composition of the ores used, the actual volume of slag, and the amount of it that is simply furnace detritus. This can be determined for a given slag field only through drilling it on a statistical pattern and weighing and analysing the cores, which to my knowledge has never been done. The
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quantitative effects on deforestation of other pyrotechnologies and of population densities remain to be explored. For example, in antiquity there would seem to have been a much larger weight of pottery and lime made per person than of metal; both are voracious consumers of fuel, but the key questions arc how much pottery and lime were made per year per person, and what was the population, on a time basis. CONCLUSION
The fuel consumption figures given above, and the forest areas necessary to supply them in perpetuity in the two examples from antiquity, are in my opinion roughly the right size and supported by reliable data from much later times; they also support the proposition that pyrotechnology was not in general the major cause of deforestation in antiquity. This is not intended to suggest that there were not local and periodic shortages of fuel in antiquity, since these have been perennially the case to the present day, even when on the average there has been sufficient or ample supply. The grazing of the free-ranging goat certainly could have inhibited regrowth, but this was a factor of uncertain size since the goat population would have been in proportion to the human population, and the latter was on the average small per square kilometre. Also, on the considerable time scales involved, changes in climate and associated growth rates were large. The principal factor in deforestation in various parts of the world seems to have been clearance for creation of arable land, pasture, and housing. Braudel (1990) has described the massive clearance of forest for this purpose in Europe of the eleventh century A.D., and today large areas of rainforest are being burned for the same purpose.
15 Artifacts from the Operation of Furnaces
Many of the artifacts from furnace operation must have become evident from the foregoing discussions, so this short chapter is intended as a reminder and check-list. The notes here are necessarily incomplete, because of limited space and the large number of ways in which pyrotechnology was practised over millennia and by a large variety of peoples. The artifacts concerned can be divided for convenience into four categories: fuels, furnaces in which they were burned, combustion air supply and its accessories, and products made. Since this last category contains items so numerous in kind and number that a separate treatise would be necessary to deal with them adequately, they will not be discussed here. FUELS
In antiquity biomass clearly was used directly as fuel in much larger quantity per unit of human population than was charcoal. This was mostly because the low efficiency of use of the heat content of the fuel meant that large quantities of fuel were necessary for a relatively small amount of useful heat. As well, two particularly large uses of biomass fuel were for the firing of clay products such as pottery, roofing tiles, and bricks and for the making of lime, all products that were used in considerable quantities per unit of population. Fortunately biomass was usually in ample supply, not just as forest but as waste from agriculture, and required only air-drying and cutting to size for a firebox.
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The combustion of biomass usually leaves little direct artifactual evidence except ashes, but these can be instructive. They are composed largely of metal oxides, amounting to totals of from 0.2 to about 5.0 per cent, averaging 1.2 per cent of dry wood; but a few kinds of biomass can have considerably higher contents of ash. The relative amounts of the different oxides in ash vary widely with species of biomass and the soil in which they were grown, so chemical analysis of ash can often indicate the species of biomass burned, and this in turn could indicate its heat content. The amount of ash found could indicate the quantity of heat generated during the last firing, although it may be an accumulation from more than one firing. As noted in chapter 3, on the average one kg of ash represents the development of 1.2 GJ of heat from combustion of 80 kg of air-dry wood. If the species of the biomass is known from the composition of the ash, these figures could be more closely determined. Charcoal, on the other hand, is a manufactured and very durable material, and when found unburned should be carefully screened to determine its lump size distribution, which is important in analysing furnace operation. Its "proximate" analysis can give its volatile matter, fixed carbon, and ash contents, which are also important for combustion analysis. Chemical analysis of its ash could suggest or identify the species of biomass from which the charcoal was made. KILNS
During excavation of kilns it is important to measure the floor areas of fireboxes and heating chambers, since the ratio of areas of firebox to heating chamber is one of the factors necessary for estimation of the maximum temperature that might have been attainable in the kiln. The height of the top edge of the flue opening above the base of the firebox, its internal cross-section area, and the means of controlling the rate of air entry to the firebox are also necessary to estimate air flow rate due to natural draft. However, the flue height is not often available because of collapse of the kiln or use of its materials for other purposes at a later time. The material of which the walls are made, and their thickness, arc necessary for estimating heat loss rate as part of estimation of potential maximum temperature. Kiln floors should be closely examined for evidence of what kind of material was heated, such as glaze from pottery, pieces of lime (which would have since been changed back to calcium carbonate by atmosphere), and glass, slag, or metal spilled from crucibles. Crucibles themselves and contained remnants of slag or metal are of course important, as are their fabric, size, and shape. This is because crucibles containing
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a reduction mixture of pulverized ore and charcoal can be readily heated in kilns to produce molten metal and slag. Outside surroundings of a kiln and rubbish pits can contain discarded faulty product and slag or metal spills from crucibles. Maximum kiln temperature that was actually attained can be estimated from the inner faces of wall material, particularly if their material possessed a softening temperature such that at maximum furnace temperature some viscous surface flow or dripping occurred. Chemical analysis, particularly of the latter, can lead to a good estimate of its flow temperature, though measuring it directly by reheating a sample in a laboratory furnace is even better. This temperature would have been 50 to 100°C lower than that of the hot gases in the kiln, due to heat loss through the wall. Partial or complete collapse of a kiln can be due to the wall material being of too low softening temperature, and/or the intensity of firing being high due to a badly matched area of firebox and heating chamber. BOWLS, HEARTHS, AND SHAFT FURNACES
These were used almost entirely with charcoal fuel. Charcoal fuelled furnaces must be examined in detail to gain full understanding of their operation. The data necessary for shaft furnaces include shaft inside diameter, vertical inside wall profile, wall thickness and material, height from hearth floor to top of shaft, and location of most severe erosion by heat, which will usually be about 20 to 50 mm above tuyere entry. For tuyeres, data should include height of entry above furnace floor, angle of entry, number, placement, inside diameter at nose, length, extent of penetration inside inner wall, and material of construction. For bellows, their type, size, number, and method of operation are necessary to estimate their output of air. The method of connection of tuyeres with bellows, and nature and tightness of joints, will affect leakage of air. If no bellows are found, the size of openings at or near the base of the shaft should be checked to sec if they arc large enough for natural draft, and blowpipe nozzles should be looked for, which are characterized by outlet openings less than about 10 mm. Other features to note are the location and size of slag and metal tap-holes. Samples of tapped slag should be photographed to show flow patterns and must be analysed chemically to permit estimation of temperature-viscosity curves and possibly determine what fluxes were used. COMBUSTION AIR SUPPLY
Three sources of air supply were used in antiquity: blowpipes, bellows, and natural draft. When used for fuel beds of charcoal, natural draft
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requires large total tuyere area, as noted in appendix 2, and large furnaces such as a metre in diameter require long tuyeres. For smaller furnaces the tuyeres can be simply short ceramic cylinders not much longer than the furnace wall thickness. Natural draft shaft furnaces fuelled with lump charcoal were used in Africa until into the present century for smelting both copper and iron orcs, and the simplicity, effectiveness, and low physical effort are such advantages that it seems likely that such furnaces were used more generally in antiquity outside Africa and possibly at an earlier date than presently estimated. The pertinent artifactual evidence is easily missed or misunderstood. The furnace remains of a natural draft shaft furnace may be only a low ring of earth or stones, with spaces or notches that . . were once aIr opemngs. As discussed in appendix 2, the direct action of wind can be used as air supply for charcoal fuelled furnaces which are short in height, provided both that the total tuyere area is sufficiently large and that all tuyere openings face a wind with a consistent velocity greater than about 20 km per hour. Two notes are necessary about slags from smelting operations. The first is that care must be taken to ensure from context that what looks like a slag has come from a furnace operation, since partly or completely fused mud brick from fires from warfare can resemble and occasionally have very similar compositions to furnace slags. The second note is that the most important property of furnace slags is adequate fluidity at furnace operating temperature, and this is sufficiently dependent on their chemical composition that a fluidity-temperature curve exists. There has been much modern industrial development of this relationship (Turkdogan 1983) which is much more accurate than use of an equilibrium phase diagram but has been little used in archaeology or archaeometry. Details of mineral compounds in solidified slags are not very relevant to furnace operation, since these compounds seldom exist at the temperatures at which a slag has the necessary fluidity.
Appendices
Appendix
I
COMBUSTION IN FUEL BEDS OF CHARCOAL
To simplify discussion of the process of combustion, it will be assumed initially that the fuel is lump charcoal that is held as a bed in a circular refractory container, supported except where noted by a grate through which combustion air is passed upward. A grate can give a flow of air that is reasonably evenly distributed across a fuel bed cross-section, and so makes analysis of combustion phenomena easier. However, in practice grates were and still are very seldom used in bowl and shaft furnaces for melting or smelting metals, because of their short service life. THE PROCESS OF COMBUSTION
The combustion of pure carbon in dry air consists of two chemical reactions that occur in succession. The initial reaction is that of carbon with the oxygen of the air to form carbon dioxide, with the generation of a large quantity of heat. This takes place with extreme rapidity, and for practical purposes may be considered as nearly instantaneous. The gas from initial reaction, now very high in temperature because of its large content of heat, then passes through more fuel and the second reaction takes place. This is the reduction of the carbon dioxide to carbon monoxide by carbon, which goes at a much slower rate than the creation of carbon dioxide, is susceptible to many influences, and absorbs heat. Because of charcoal's high reactivity, in reasonably deep beds of charcoal this second reaction goes to completion as will be noted below. When the two reactions are added together
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Appendix
1
to determine the final result of the complete combustion of carbon by air in a sufficiently deep bed of charcoal, it is found that one kg of carbon requires 4.85 m J of air to produce 2.08 m 3 of carbon monoxide and 3.83 m 3 of nitrogen, with the net generation of 10.4 MJ of heat. Charcoal always contains some residual VM, but it is its FC content that is primarily involved in combustion, most of the VM present being driven off in the upper part of the furnace. Therefore if the FC content of a charcoal is for example 80 per cent, one kg of charcoal will require 4.85 x 0.80 =3.9 m l of air for its combustion and produce 10.4 x 0.80 =8,3 MJ of heat. Charcoal furnace operating data that includes reliable figures on both air consumption at the tuyere nose and the FC content of the charcoal is scarce; but those of Birkinbine (1879) in commercial furnaces, and in the small bloomery furnace runs of Tylecote et al. (1971) discussed in chapter 12, confirm these figures, as does data from modern commercial furnaces, Constantine (1975) in Australia, and Braga et al. (1982) in Brazil. A practical figure to use for analysis of an ancient furnace operation would then seem to be about 4.0 m of air per kg of charcoal. Then 1.0 m) of air will burn completely 0.25 kg of charcoal, and produce 2.1 MJ of heat energy. Since in a bed of charcoal of sufficient thickness carbon is always in chemical excess, it is important to notice that the rate of supply of combustion air then quantitatively determines the rate of production of heat. EFFECTS OF FC AND OF HUMIDITY IN COMBUSTION AIR
The fact that the FC content of charcoal can vary because of poor practice in its manufacture must be considered. If, for example, the FC content is 60 per cent, one kg of charcoal will require 2.9 m 3 of combustion air and generate 2.1 MJ of heat. One m} of air will then burn 0.34 kg of charcoal but still produce 2.1 MJ of heat. In fact the consumption of air per kg of charcoal is given by 20.6/ %FC and so is a measure of the FC of the charcoal being burned. In analysing the operation of modern replica furnaces, it is therefore important to know the FC content of the charcoal used, since most charcoal readily available today in small quantity is for barbecuing which does not require high FC and is cheaper to make. If the air used per kg of charcoal in smelting is less than expected, it is probably due to the charcoal used. Air can contain water in amounts increasing with temperature, ranging from very little to more than 360 grams per m 1 • Water is decomposed by and consumes hot carbon, approximately one kg of carbon per kg of water. It has been well recognized by all operators of fuel-fired furnaces that they consume more fuel to reach a given temperature in humid summer weather than in dry winter weather. The interest here in humidity is its effects on modern replica operation.
169
Combustion in Fuel Beds of Charcoal
In 1944 Hiles and Mott summarized their outstanding experimental work on the combustion of lump fuels in beds, conducted in England over a period of many years before the 1940s. These experimental results are directly applicable to furnace conditions in antiquity because the space velocity of 13.1 m that was used in the experiments on charcoal happened to be in the same order of size of that used in antiquity. Charcoal with an average lump diameter of 25 mm was burned on a grate in a circular furnace 340 mm inside diameter and 305 mm high. Figure 6 is from Hiles and Motts' results, with air entry from the left; the spatial distributions of gas composition and of temperature are well illustrated. The maximum temperature reached was 1,420°C, which is only about 73 per cent of the AFT of the charcoal. This was because the fuel bed was short and open on top, so the rate of heat loss from the nearly I,OOO°C surface temperature of the bed was large. It was shown clearly in experiments with other lump fuels that while combustion followed the two-step reactions described above, in practice the oxidation of carbon to carbon dioxide was never complete to the theoretical 20.9 per cent, the maximum being about 18 per cent for a hard-burned, low reactivity coke. Charcoal because of its higher reactivity gave a maximum of 10 to 12 per cent carbon dioxide and reached this within a distance from air entry of two to three fuel lump diameters. The production of heat by the initial formation of carbon dioxide increases the temperature of the products of combustion in proportion to the amount of carbon dioxide created. If charcoal is being burned by dry air at ambient temperature, the AFT which I calculate for 10 to 12 per cent carbon dioxide with allowance for dissociation is 1,880 to 2,0000C. An average of 1,9400C would be a practical figure. As noted above, the reduction of carbon dioxide by carbon to carbon monoxide is not only relatively slow but is controlled by several factors, an important one being that one volume of carbon dioxide produces two volumes of carbon monoxide. The rate of reduction increases with the chemical reactivity and surface area of the carbon and decreases with decrease in temperature. The considerable amount of heat absorbed by the reaction decreases gas temperature so that the reaction is self-limiting, effectively ceasing when the temperature falls below about 1,0000C. There are practical limits, however, to the sizes of these factors. As combustion air entry volume, which determines gas space velocities, is increased sufficiently, a value will be reached above which the physical lifting force of the air will begin to lift and fluidize the whole fuel bed, thus altering completely the mode of combustion. About 60 per cent of the fluidization velocity has become accepted as a practical limit on the rate of modern coke-fuelled blast furnace air supply, and for charcoal fuel the limit is apparently at about 50 m per minute (Constantine 1975). However, this rate could be reached only in the late nineteenth century A.D. when sufficiently powerful blowers became
170
Appendix
1
available. In antiquity, space velocities evidently were much lower, in the order of 5 to 15m per minute. There are also limits on fuel size, beyond which its surface area per unit volume of furnace content becomes sufficiently low that all carbon dioxide is not reduced before the reduction reaction is stopped by too low gas temperature, and there is then not enough carbon monoxide produced for satisfactory reduction of ores. A practical example of this effect was given in chapter 7. The effect is of course independent of the wall effect that limits fuel lump size, as dicussed in chapter 2. In charcoal furnace practice, fuel lump size is usually less than about 40 mm and typically is in the range of 10 to 25 or 30 mm, depending on furnace size. It is worth noting again that the reduction of carbon dioxide to carbon monoxide is not only relatively slow but the extent of its completion is determined by factors involving the actual fuel being used and by the rate of air supply, i.e., kinetics control the actual progress of the reduction reaction. As a result, furnaces using solid lump fuels such as bowl, hearth, and shaft furnaces can operate at gas compositions and temperature that are far from chemical equilibrium values. This is a well-known fact in modern blast furnace operation, which must be taken into consideration in understanding gas compositions and temperatures in different parts of real operating furnaces. A technical detail that should be noted is that the carbon monoxide content of combustion gases measured at the top of a deep charcoal bed never reaches the theoretical 34.7 per cent, because of the presence of moisture and some VM in all charcoal (Metals Handbook 1964). Typically a maximum of 30 to 32 per cent is reached, with some hydrogen and methane present. Still another detail concerns modern replication of operation of forge furnaces, which are hearths. In antiquity charcoal was always used as fuel, with associated distributions of temperature and gas composition. If the fuel is changed to bituminous coal for modern operation, combustion is largely of the coke that is continuously made from the coal, and this is a fuel of lower reactivity than that of charcoal. As a result maximum temperature ncar the tuyere nose will be higher, and the reduction to carbon monoxide and the associated decrease in temperature will occur over a longer distance from the tuyere nose. TEMPERATURES ATTAINABLE
The maximum temperature attainable in a bed of burning carbon is at the point or level of maximum carbon dioxide content of the products of combustion, and as noted above, with charcoal fuel the AFT is typically about 1,940oC. In the combustion run by Hiles and Mott, whose results are shown in figure 6, the maximum temperature reached, about 1,420°C, was only
171
Combustion in Fuel Beds of Charcoal
about 73 per cent of the AFT of the charcoal. This was partly due to the heat lost through the 50 mm thick refractory wall but largely due to the fact that the fuel bed was ony 300 mm deep with a 1,00oC surface. This was enough to develop the full combustion chemistry, but the temperature of the top of the bed was 1,000oC, which gave rapid heat loss by radiation upward. In wellinsulated charcoal fuelled furnaces, maximum temperatures in excess of 1,600°c have been measured, which is more than about 83 per cent of the fuel AFT. VOLUME OF BED AT TEMPERATURE
As noted above, as the rate of supply of combustion air is increased, there is an increase in the vertical extent, i.e., the volume, of the fuel bed that is at high temperature. This is due to the effects of both increased distance to complete the reduction of carbon dioxide and the higher heat release rate produced by the increased rate of combustion. There are good experimental data to demonstrate and quantify this important point; that of Draper et al. (1979) also included the effect of increased combustion rate on the volume of fuel bed that was at elevated temperature. Results for two air rates, which are well above those used in antiquity but illustrate the point, are given in table 4. Table 4 Effects of Air Supply Rate on Temperature and on Its Extent
in a Coke Fuel Bed
Specific
Tuyere velocity m/sec.
Max. temp.
76
9.1
1,805
760
107
12.7
2,020
930
air rate
m/min
°c
Length of zone abolJe 1,600°c, mm
The temperature of 2,020 o C is close to the AFT of the coke used as fuel, which I estimate as 2,070oC. The effect of increasing the space velocity, i.e., the heat generation rate, on the maximum temperature attained is clear. The relationship between the space velocity and the vertical distance over which high temperature extends is also clearly visible. These effects were also shown in work by Evans et al. (1959). Both coke and briquetted brown coal char in two size ranges were used as fuels, so that the effects of fuel reactivity and lump size could be seen. A cupola 300 mm I.D. was used, with a lining thickness of 100 mm and air introduced through four
172
Appendix
1
50 mm I.D. tuyeres. The length of fuel bed above tuyere level that was above a temperature of I,500oc was measured at two space velocities. At high air rates of 70 to 150 m per minute it was shown clearly that the length of the high temperature zone was considerably higher for the low reactivity coke and in the larger lump size and also increased rapidly with air rate similarly to that shown above in table 4. What is of direct interest here is that at rates below about 50 m per minute that include those used in antiquity, the values for char became identical for both lump sizes, i.e., lump size was not a factor. By using values for reactivity for char and charcoal given by Reeve et al. (1975),1 made estimates for charcoal shown as figure 3 in chapter 2. The lines in this figure will vary moderately with the actual reactivity of the fuels involved. Projected down to space velocities of 5 to 15m per minute, which is the order of size of rate apparently used in antiquity, the length of the zone above 1,500oC with hardwood charcoal as fuel would be in the order of 60 mm. This is short and would mean that to obtain a given degree of superheat, higher average temperature and a lower ore to fuel ratio would be necessary at higher air rates. It also emphasizes the difficulty of locating the highest temperature level in an experimental reduction furnace. In summary, both the kinetics of combustion reactions and the experimental evidence show that the rate of supply of combustion air in terms of its space velocity in the fuel bed, which is the rate of heat generation, is the paramount factor controlling furnace performance in both maximum temperature attained and its distribution. In antiquity the development of sufficient power to give increased air supply was probably the controlling factor in development of shaft furnace capabilities. CHARCOAL VERSUS BIOMASS AS ENERGY SOURCE
A comparison of the heat economies in the use of biomass fuel directly and charcoal made from it is interesting because of the interaction between losses during charcoal making and differences in the thermal efficiencies of the furnaces used. As noted in chapters 3 and 5, biomass was burned directly as fuel largely for heating objects or materials for which the oxidizing products of combustion were not a disadvantage. The efficiency of transfer of the heat content of the fuel to the work being heated was low because of excess air in combustion, moisture content of fuel, high flue gas temperature, and periodic operation, being typically in the range of about 0.1 to 3.0 per cent. If the heat content of air-dry wood is taken as 15.0 MJ per kg and the average thermal efficiency is 1.5 per cent, the effective heat in the work will be 15.0 x 0.015 =0.225 MJ per kg of fuel burned.
173
Combustion in Fuel Beds of Charcoal
If one kg of the same wood is made into charcoal, about o. I 5 kg of usable charcoal will result, which will have a heat content of 0.15 x 28·5 = 4.3 MJ. There will have been a loss of 15.0 - 4.3 = 10.7 MJ, or 71 per cent, of the heat content of the wood, due to partial self-combustion and to loss of all of the VM to atmosphere. However, charcoal is usually mixed with the work to be heated and is burned in a bowl or shaft furnace with considerably better thermal efficiency due to much better heat transfer. In the relatively small furnaces used in antiquity, the efficiency would have been between about 10 per cent and as much as 40 per cent in taller shaft furnaces. Then with a heat content of charcoal of 4.3 MJ per kg of original wood and a thermal efficiency of average 25 per cent, 4.3 x 0.25 = 1.07 MJ of heat would appear in the work being heated. This is 1.07/0.225 = 4.8 times the effective heat from the original one kg of wood. But thermal efficiency is not the only criterion of use of a particular fuellfurnace combination, the reducing power of the products of combustion of charcoal being essential for the reduction of oxide ores of metals. In antiquity the question of efficiency of use of fuel could be noticed only in a very general way, since there was no concept of heat as a quantity nor any practical method of measuring it. The fact that a given kiln would require a much larger pile of softwood than hardwood to fire pottery to a given hardness would be obvious; but comparison with charcoal as fuel would not be possible because of the quite different purposes for which the fuels were used. With this background we can discuss the use of charcoal for firing a kiln. In this case the heat transfer rate would actually be lower than with biomass fuel because of the lack of a visible flame (unless very badly made charcoal is used). Since the high losses to and through walls and to the high temperature flue gas remain, the net efficiency of use of the heat in the fuel would not be higher than if biomass had been burned directly. When the loss of heat content during making of the charcoal is also taken into account, much more wood would have had to be cut to produce the same heating result than if it had been burned directly. There is a separate aspect to the comparison of wood with charcoal for smelting of ores. Wood cut into short lengths has a sporadic record of use in shaft smelting furnaces, both as a partial replacement of charcoal or of coke and as the only fuel used. The latter has been replicated by Espelund (1995) from an eighteenth century A.D. Swedish iron smelting practice, but I know of no detailed record of its use in antiquity, although it would seem certainly to have been tried. When air-dry wood is added to the top of a shaft furnace, it is converted to charcoal at well above tuyere level during its descent in the furnace, this charcoal then acting as it would if it had been added to the top of the furnace. Carbonization of the wood to charcoal results in a decrease in bulk density of about 40 per cent. Since the charcoal in a smelting furnace in
174
Appendix
1
antiquity occupies about 90 per cent of the furnace volume, if air-dry wood is instead used as fuel there will be a large decrease in volume of solids in the furnace over the length necessary for carbonization. This is probably the reason that the Swedish furnace replicated by Espelund not only had an internal profile of an inverted cone, much wider at the top than at tuyere level; the burden as it descended in the furnace had to be frequently manipulated by a pole from the top to ensure adequate mixing of ore and charcoal.
Appendix
2
PRESSURE DROP IN TUYERES AND FUEL BEDS AND POWER REQUIRED
The flow of gas through a container or guide of any shape or size creates friction with the walls, which requires force to overcome and shows itself as a pressure drop in the gas. The quantitative factors involved have been well developed in the chemical industry. Flow through a pipe creates a pressure drop, as does a restriction or bend in the pipe, or a nozzle or orifice through which gas escapes. A bed of broken solids also resists the flow of a gas through it, creating a pressure drop. In a furnace there is then an additive series of pressure drops to be overcome to create gas flow from bellows to the top of the fuel bed. The work effort or power required to move air and the resulting gaseous products of combustion through the system increases with the pressure drop and the rate of gas flow. A tuyere is an orifice, and the flow of air through it at the velocities encountered in a practical furnace is amenable to a simple mathematical treatment. However, flow through a bed of broken solids is a phenomenon that is complicated by the fact that as flow rate increases, there is a transition from smooth, fully laminar flow to turbulent flow, which requires more complex mathematics. In practice the pressure drop in a tuyere is usually much greater than that in the associated fuel bed, so the calculation of the latter can be simplified. TUYERES
The pressure drop in air being passed through a nozzle or orifice such as the nose of a tuyere can be calculated by the simple orifice formula (Rehder 1982) as:
176
Appendix 2
dP, = 4.08 x
10 8 x
Q2
(I)
n 2 x d4
where dP,
= pressure drop in tuyeres, Pa
Q = air flow rate, m 1 per minute n d
= number of tuyeres = tuyere I.D., mm.
The strong effect of orifice area on pressure drop is notable. Equation rewritten as: dP
I
can be
=2.52 x lOS x Q2 (A,)2
where A,
= total area of tuyeres, mm
2•
Increased penetration of the fuel bed, desirable in large furnaces, requires increased tuyere exit velocity, and equation 2 shows that the pressure required, and therefore the power on the bellows, increases as the square of the velocity. If a tuyere is long compared to its inside diameter, the resistance to flow of air in a pipe must be added to the nozzle resistance. Where the I.D. of the orifice and of the pipe are the same, it can be shown that the increase in resistance or pressure drop is 0.017 times the ratio of the length of pipe to its I.D. For example, if the length of pipe is 200 mm and the l.D. of pipe and orifice are 20 mm, the ratio is 10, the increase in pressure drop is 0.17, and the total pressure drop is that of the orifice multiplied by 1.17. If the pipe length is 2,000 mm, the ratio is 1.7 and the multiplier is 2.7. Also a right angle bend or elbow in a pipe will add a pressure drop equal to a straight length of the same pipe 30 times its I.D. To determine the total pressure required from a source of air such as a pump or bellows to force air through a tuyere, the total path from bellows exit to tuyere nose must be included. FUEL BED
The pressure drop in gases passing through packed beds has been extensively studied, since it is both an industrially important and a technically complex subject. Many equations have been developed for predicting such pressure drop, but it must be kept in mind that in practice the predictions have uncertainties of 20 per cent or more. For application to gases passing through beds of fuel such as in shaft furnaces, the formulation of Ergun (1953) has come to be widely accepted as a guide. Its components are the void fraction of the bed, the average diameter of the particles of which the bed is composed, the average
177
Pressure Drop in Tuyeres and Fuel Beds
gas temperature, the space velocity or Q/A, the length or height of the bed, and the average diameter of the particles of which the bed is composed. The equation was published originally in English units, and the examples given were for coke-fuelled blast furnaces at space velocities of 50 metres to more than 60 metres per minute, which produce turbulent gas flow. However, at the much lower space velocities used in furnaces before the late seventeeth century A.D., the gas flow is transitional between laminar and turbulent. The criterion is the modified Reynolds number of the system, which is a function of the mass flow rate of the gas and of the average particle diameter, flow below a number of 10 being considered as laminar and that above 100 being considered turbulent (Leva 1959). This means that for an average particle diameter of 16 mm (6 to 26 mm), which would approximate charcoal size in early blast and bloomery furnace practice, at furnace operating temperature the gas flow would be transitional between space velocities of about 4 to about 20 metres per minute. This happens to encompass the space velocities that evidently were used from antiquity into the eighteeth century in Europe. The exponentials of the factors for gas velocity and particle diameter in Ergun's equation must therefore be changed to be applicable to such furnaces. The procedure which I developed to determine these exponentials required experimental work with beds of charcoal (Rehder 1990), and the results required adjustments to those in the paper which are made here. MODIFIED ERG UN EQUATION
For an open top shaft, with specific air flow rate less than about 40 metres per minute and bed height less than about 12 metres, Ergun's equation can be shown to be expressed as follows:
dPh
= 1. 5 X
11 - e] x
where dPb = S= Q= A= H", e= Dp = tg=
+273]
X
Sr.7 X
H
pressure drop in bed Q/A = space velocity, m/min air flow rate, m 3 /min cross-sectional area of bed, m 2 height of bed, m void fraction in bed average charcoal lump diameter, mm average temperature of gas in bed, °c
Charcoal as used in practice is made up of a mixture of sizes, and void fraction averages about 0.45. Also the average vertical temperature of fuel beds of charcoal deep enough to have recaptured most of the heat generated at the tuy-
178
Appendix 2
eres seems to be about 900°C. Under these conditions equation 3 can be simplified to:
dPb = 35
X
S1.7
X
H Dpf.4
The equation must be modified for specific instances where average gas temperature and the void fraction of the charcoal used are other than 900°C and 0.45 respectively. Because many of the factors involved are difficult to measure accurately and are usually not steady during actual operation, accuracy of prediction is limited; thus no more than two significant figures are meaningful. POWER REQUIRED FOR AIR SUPPLY
In order for a bellows or air pump to force combustion air through connecting pipes to wyeres then through tuyeres, and then as products of combustion through a bed of lumps of fuel, their several resistances or pressure drops in series must be overcome. Ignoring a small negative pressure drop that is due to a natural draft effect (discussed in appendix 3), the power required is given by: W
=0.0166 X
dPt x Q
where W =watts dP t =total pressure drop, Pa. This is the power necessary at the outlet of a bellows, and to determine the mechanical effort necessary, this must be increased by the mechanical efficiency of the bellows and by the amount of leakage in the system. The total pressure drop is that in tuyere(s) and fuel bed, and since the pressure drop in the tuyere is usually the major component of the total pressure drop, the power necessary varies nearly as the cube of the rate of air flow. EXAMPLE
The well-documented furnace described by Tylecote et al. (1971) can be used to demonstrate the order of size of pressure drops in and power required in a small charcoal fuelled shaft furnace. The furnace was 300 mm I.D. giving a crosssection area of 0.0707 sq m, and the air flow rate was 300 litres per minute, giving a space velocity of 4.2 m per minute. The fuel bed height was 1.0 m, the average particle size of the charcoal was 35 mm, and a single tuyere 20 mm I. D. was used. Applying equation 1, the pressure drop through the tuyere, ignoring its short length, would be 230 Pa. From equation 3a, the pressure drop in the fuel bed would be 2.8 Pa, so the total pressure drop would be 232.8 Pa. Then from
179
Pressure Drop in Tuyeres and Fuel Beds
equation 4, the power necessary at the entrance to the tuyere would be 1. I 6 watts. It will be noted that the pressure drop through the tuyere is many times that through the fuel bed, and this phenomenon is characteristic of fuel beds of charcoal. It also should be noted that the actual power necessary to operate bellows in antiquity could be in the order of 5 to 10 times this figure, due to the losses and low mechanical efficiencies mentioned above. However, it has been shown by Reay (1977) that an average 70 kg man can generate power all day at the rate of 120 watts, so the furnace could very easily be blown with one man on bellows.
Appendix 3
NATURAL DRAFT IN FUEL BEDS
As soon as air reacts with any fuel, the temperatures of both fuel and of gaseous products of combustion increase rapidly and considerably, as discussed in appendix 1. The density of the gas decreases as its temperature increases, creating a rising force proportional to the gas temperature. This can draw more air into the fuel bed so that combustion continues. If the fuel bed consists of small lumps of charcoal, all the oxygen in the air will be consumed within a short distance from its entry as described in appendix I, and if the fuel bed is contained in a bowl or shaft, the temperature, composition, and movement of the products of combustion can be predicted within a useful degree of accuracy. Natural draft furnaces that are not dependent on wind velocity have been used from an unknown antiquity in tribal Africa for smelting both copper and iron ores in fuel beds of charcoal (Cline I937). Because of the simplicity of their construction and lack of bellows or blowpipes, in my opinion naturaldraft charcoal fuelled shaft furnaces probably were used in antiquity to a larger extent than is realized. Archaeological remains of such furnaces can be as simple as a low ring of earth or stones. However, if the fuel is biomass, which forms fuel beds with high and variable degrees of void fraction as described in chapter 3, pressure drop of air moving through the firebox is variable as fuel is consumed and periodically added, and in addition the pressure drop through the kiln differs with ware distribution which can change with each reloading. Because of these differences in combustion patterns between biomass and charcoal fuels, discussion here of the use of natural draft will be separated by fuel type.
181
Natural Draft in Fuel Beds
Some years ago, I published a paper on the use of natural draft (Rehder 1987), and some corrections to that paper will be made here. My later experimental work on beds of charcoal (Rehder I990) resulted in changes in the exponentials of air rate and of fuel lump diameter in the equation for pressure drop in charcoal fuel beds. Also an error in tuyere area calculation in the appendix was not caught until after publication. This led to the mistaken conclusion that there was no effect of stock column height in fuel beds of charcoal. Corrections are made below. A discussion in some detail on the effects of wind velocity on the design of natural draft furnaces follows. NATURAL DRAFT IN FUEL BEDS OF CHARCOAL
The negative pressure drop or draft in a burning fuel bed is conventionally found by considering the hot products of combustion as being in an empty stack or chimney, with outer air temperature at 25°C and ambient absolute pressure at 101.4 KPa. The draft is then given (Combustion Handbook 1965) by: dPs
= I2.0 X Hs x ( 1
-
(298)
( ts + 273) where dP s = pressure drop in stack, Pa Hs = height of stack, m ts= average temperature of gas, °c
If the average temperature of the gas is estimated as for example
900°C,
then: (6)
If there is empty stack above the top of the fuel bed that contains hot products of combustion, then H is greater than fuel bed height, but temperature at the top of the bed in charcoal fuelled shaft furnaces is generally moderate and usually the shaft is kept nearly or quite full of burden, so this factor will be ignored here. It is important that any furnace to be operated by natural draft be well sealed against leaks of air drawn in through defects in the furnace wall, since this air bypasses normal combustion, short-circuits some of the negative pressure, and adds oxygen to the products of combustion. This is a much more serious matter with natural draft than with a bellows-driven air supply and can prevent successful operation of the furnace. A simple, thorough plastering with mud over the exterior surface of the furnace would be a sufficient seal since the pressures involved are low.
182
Appendix 3
For natural draft to draw gases through a furnace, its pressure must exceed the total of the resistances or pressure drops of both fuel bed and tuyeres. The draft pressure is shown in equation 6 to be a direct function of fuel bed height, and equation 3 in appendix 2 shows that fuel bed resistance is also a function of bed height. Since the multiplying factors of bed height are different in the two cases, a higher fuel bed will create a higher net draft pressure, but not quite in proportion to the increase in height as will be shown. Pressure drop in tuyeres must be kept as low as possible, so the total tuyere area of a natural draft furnace is considerably larger than that of the same size furnace being blown by blowpipe or bellows. The necessary tuyere area for given furnace parameters and desired gas flow rate can be found by noting that the pressure drop in the tuyeres must equal the net draft due to the difference between natural draft and the fuel bed resistance at the space velocity desired in the furnace. Equations 2 and 3a in appendix 2 and equation 6 above arc applicable; as an example, consider a furnace 0.40 m I.D. (0.13 m" area) with a fuel bed L5 m high, using charcoal of 35 mm average diameter and requiring a space velocity of 6.0 m per min. From equation 3a the pressure drop in the fuel bed is 7.6 Pa and from equation 6 the draft pressure is 13.5 Pa. The pressure drop available for the tuyeres is then I3.5 -7.6 = 5.9 Pa. Then from equation 2, noting that the air flow rate is 6.0 x 0.13 = 0.78 m} per min., the total tuyere area necessary is found to be 5,096 mm'. If this is divided into four tuyeres around the base of the furnace, each tuyere will be 40 mm I.D. These equations can be condensed into one, but the step procedure above is simpler to use. Such condensation does, however, show that the effect of fuel bed height is such that doubling it increases draft pressure sufficiently so that the area of tuyere necessary is decreased by one quarter. The advantage of this is that the velocity of air through the tuyeres is then increased proportionately, giving better bed penetration. An analysis of natural draft furnace operation such as given in the example above can be applied in reverse to interpret archaeological remains. For example, if the I.D. of a furnace is discernible and the number and size of tuyeres can be known from traces of wall openings or from tuyere remnants, then by making a reasonable assumption of furnace height and the size of charcoal used, the space velocity and the productive capacity of the furnace can be approximated. If the space velocity was in the order of 3.0 m per minute, copper probably would have been smelted, and if about double this, iron could have been smelted. Air flow velocity through the large tuyere area necessary for natural draft will be low, about 2.7 m per second in the example above, and penetration of air into the fuel bed will be shallow. As furnaces are made larger in diameter, the centre of the burden or fuel bed can then become inactive over an increasing proportion of the cross-sectional area and a "deadman" will appear as a cone resting in the centre of the hearth. In the case of copper smelting, this
183
Natural Draft in Fuel Beds
would only decrease the smelting rate that might be expected from the furnace diameter, since the active part of the hearth is then an annulus; but in the case of smelting iron, no single bloom of appreciable size could form, and small blooms or nodules would likely form at each tuyere. The solution for the smelting of iron - the use of long tuyeres that penetrate deeply into the fuel bed to deliver air directly to near the centre of the furnace - was discussed in chapter 7. During furnace operation such tuyeres are progressively fluxed away, and so concentration at the centre decreases through the smelt; but the system has been used effectively for smelting of iron in Africa. Modern experience has shown that if granular solids are to be simply heated, natural draft furnaces can be very large in diameter. As an example, iron carbonate (siderite) ores, which can be decomposed to iron oxide at temperatures above about 600°C, have been processed in modern times in large quantity in simple shaft furnaces such as the Gjers kiln (Forsythe 1922). These are cylinders typically 10 to 12 metres high and 6 to 7 metres in diameter, lined with firebrick, open at the top where burden is added, and with openings around the base through which finished product is drawn and combustion air enters. Ore to be roasted is mixed with a few per cent of lump coke as fuel, and operating temperature is controlled by dampers on the air inlets at the base of the stack. Output rate is 100 to 140 tons per day per kiln, and space velocity is in the order of 1.0 m per minute, which is sufficient for the moderate temperature necessary. Since no reduction of ore is desired, carbon monoxide is not necessary in the products of combustion, and maximum heat release from fuel can be obtained by use of low reactivity coke as fuel. Lump size and void fraction of the ore are the important factors in minimizing pressure drop since it constitutes the major portion of the volume of the burden; but a limit on increased lump size is set by the need for the heat to fully penetrate each lump during its residence time in the kiln. Limestone can be successfully decomposed to quicklime using the same technique and furnace with waste biomass, including straw, as fuel. Higher temperatures, above about 1,OOOoC, are necessary, and lime-making in such furnaces has been common practice for many hundreds of years. There is no technical reason why copper ores could not be smelted to molten copper using charcoal fuel in similar small or large natural draft furnaces, modifying the hearth to collect molten copper for periodic tapping. Such practice would be identifiable by small amounts of copper in the fused lining or slag, combined with a large tuyere area and absence of evidence of bellows, conditions which were fulfilled in the South American furnace mentioned in chapter 8. EFFECTS OF WIND ON NATURAL DRAFT FURNACES
The effects of wind on structures are complex, and much experimental work has been done in the interests of the architecture and stability of buildings. A
18 4
Appendix 3
furnace is a structure that is affected by wind, not from the viewpoint of structural stability but from the positive and negative air pressures developed around its circumference. The architectural studies are an essential guide, and use will be made here particularly of Aynsley et al. (1977) in estimating the positive and negative pressures developed that can affect air flow through a furnace. Systemization of these effects is difficult because of the number of variables involved, so the following discussion takes an empirical approach. In general, a wind creates a positive pressure on the wall of the furnace directly facing the wind, and negative pressures at the sides, back, and top of the furnace. Their size and horizontal distribution are quite different for round structures and those with plane sides. In a round furnace that has, for example, four necessarily quite large tuyeres around its circumference near the base, the positive pressure on the tuyere in front is exceeded by the total of the negative pressures on those on the sides and back. This net negative pressure not only can create an air flow horizontally across the bottom of the furnace but decreases the vertical pressure of the natural draft due to hot gases within the furnace. The negative pressure created by aspiration of the wind across the top of the furnace adds to whatever net pressure exists inside due to temperature difference; this increases upward flow of gas within the furnace. The sizes of these effects are more difficult to calculate for round than for square furnaces, but they increase with the square of the wind velocity, and the net effect becomes important only at quite high wind velocities. For example, at 20 km per hour the pressure to move gas through such a furnace might be increased by about one-third. This increases furnace operating rate, and the dependence of rate on wind velocity can be a disturbing factor in practical furnace operations of an otherwise normal natural draft furnace. Use of wind as an assistance therefore requires a location where its velocity does not vary widely or frequently. Such furnaces, which have been placed in windy locations, but are basically natural draft furnaces of dome height with tuyeres all around their circumference, may conveniently be termed "wind-assisted" furnaces. WIND FURNACES
Furnace design can be modified to take specific and maximum advantage of wind pressure, and it then could be considered a true wind furnace, entirely dependent on a steady and appreciable wind velocity. If the frontal cross-section of the furnace is made square or rectangular with flat sides, and one face is at right angles to the wind direction, the distribution of pressures is simpler than with a circular cross-section and with side and back effects more clearly delineated. The pressure on each face is then given by:
185
Natural Draft in Fuel Beds
where dP = pressure, Pa C p = empirical factor depending on face V = wind velocity, km per hr. Taking into account the local C p , the pressures on the faces are multiples of the square of the wind velocity in km per hr. For the front face at right angles to the wind, that of the total of the sidewalls and back is -0.087, and on the top is -0.056 pascals. The multiples that increase gas flow rate through the furnace are then (0.037 + 0.056) = 0.093, and those decreasing the flow rate total 0.087. There is clearly advantage to be obtained by eliminating tuyeres from the sides and back of the furnace, and if this is done the pressure due to wind to move gas through the furnace is 0.093 x VL pascals. In addition to this there is the normal negative natural draft pressure given by equation 6 above. To utilize wind pressure to its maximum, the furnace should be rectangular in plan, with its long face at right angles to the wind direction. The width of the furnace from front to back need be only enough to accommodate air penetration from the tuyeres, and at the moderate tuyere velocities generally used in antiquity, 0.3 m or so should be adequate. The tuyeres should be just far enough apart so that their cones of combustion touch one another - about one to two tuyere diameters. The height of the fuel bed above the tuyeres should be 0.5 to 0.75 m to recover heat and also to give adequate residence time of materials. The production rate of the furnace at a given wind velocity will be in direct proportion to its length. There is considerable advantage in embedding such a furnace in the slope of a hill facing the wind, since the front face can be open to the wind while the ends and back are given very good heat insulation by the surrounding earth. Consider as an example the wind furnaces in Sri Lanka, described in useful detail by Juleff (1996). They are about 2 m long, 0.35 m high above tuyere level, and 0.3 m front to back, with fourteen tuyeres penetrating the front wall about 0.15 m above the hearth. The hearth area is about 0.66 m\ and the furnaces were well insulated by being embedded in a hillside and were preheated for replicate operation. The average lump size of the charcoal used was not reported and this has an appreciable effect, but an average 25 mm diameter is assumed from inspection of the coloured photograph of the top of the furnace in action, which would have a void fraction of about 0.45. Because of the short height of the fuel bed, its surface temperature during operation was high, measured as about 900°C, and the combined heat loss rates from the top of the furnace in the form of radiation and enthalpy of products of combustion would be at least 50 per cent of that generated. As discussed in chapter 2 and appendix I, the basic measure of heat generation rate
186
Appendix 3
in a furnace fired with lump fuel is the air supply rate per unit area of furnace cross-section, which is called the space velocity and is the space velocity through the fuel bed in m per minute. A space velocity of about 4.3 m per minute in a well-insulated and preheated furnace 0.75-1.0 m high will smelt iron very satisfactorily (Tylecote et al. 1971); this has been corroborated by my own unpublished experimental work. With the considerable heat loss rate from the top of the Juleff fumace, a space velocity of about 7.0 m per minute would be necessary, and with a hearth area of 0.66 m", this is an air flow rate of 4.6 m per minute. The pressure from wind to move air through the furnace was noted above as 0.093 V\ and to this is added the negative pressure due to the hot gas column at an average temperature of about I,200°C, which from equation 5 is 3.35 pascals. The resistances to the movement of gases through the fuel bed are those of the tuyeres as a total, and of the fuel bed itself. The resistance or pressure drop in the fuel bed from equation 3 in appendix 2 is 3.22 Pa. The resistance in the tuyeres from equation 2 in appendix 2 is 53.3 X Ios/A,'. Then: (8)
From this, at V = 40 km per hr, total tuyere area At should be 5,980 mm\ or 427 mm 2 per tuyere = 23 mm I.D. Similarly at V = 20 km per hr, the tuyeres should be 33 mm I.D. Since it would be impractical to change tuyere size as wind velocity changes, a compromise in tuyere diameter would have been found by trial and error at about 28 mm I.D. This is close to the size scaled from the small drawing in Juleff's paper, of about 25 mm at the tuyere nose, and the agreement is reasonable. Since the furnace and its contents are massive and well insulated, transient changes in wind velocity would have minor effect on furnace temperature. Experience has shown that for iron smelting it seems desirable that the nose of the tuyere penetrates into the fuel bed a centimetre or two, with a bloom forming just under its tip. In the case of a lateral row of tuyeres quite close together, as in the Japanese Tatara furnace blown by bellows, their interference produces a mixture of medium to high carbon steel blooms and a low carbon cast iron, which seems to have been the case with Juleff. However, if a molten metal product is desired, as when smelting copper, there need be no tuyeres as such but simply evenly spaced vertical slots in the furnace wall, whose total area is that necessary. The bottoms of the slots should be sufficiently above the floor of the hearth for it to hold metal and slag for tapping. An excellent example of such an ancient pure wind furnace is in the Bergbau Museum in Bochum, Germany, in which the back wall was also sloped backward at an angle of about 45 degrees. In summary, wind furnaces are effective smelters of iron and of copper, requiring only some reasonably constant wind of more than about 20 km per hr.
187
Natural Draft in Fuel Beds
They are scarce in the archaeological record since the necessary geological and weather conditions are not common, and ordinary natural draft furnaces are just as effective smelters and are independent of weather, except, as noted above, they are at a disadvantage in strong winds. NATURAL DRAFT FOR FUEL BEDS OF BIOMASS
Kilns and furnace structures of similar geometry discussed in chapter 4 were in antiquity almost invariably fuelled by biomass, and the matters of primary importance were the generation of a luminous, high temperature flame of suitable length as described in chapter 3, and its guidance through and around the articles of work to be heated before it escaped through a vent or smoke-hole. The driving force to move gas through the firebox and kiln is the draft as defined by equation 5 above, and the resistances to be overcome are those of air entry as an orifice, the firebox and fuel bed, the work chamber filled with work, and the vent or flue as orifice. These can in principle be estimated by use of the equations discussed above, but there are other considerations that complicate matters. One is the fact that most uses of biomass fuel in antiquity were on a batch or periodic basis, such as the firing of pottery and other clay products, in which the work being heated is stationary, so heat and temperature are cycled through the work. However, in charcoal furnaces with work mixed with fuel, the work moves by gravity through the furnace as fuel is consumed, being subjected to an increasing temperature regime of a stable vertical pattern and then extracted at the furnace base; i.e., the work is moved through a temperature change. The variables are more difficult to control in the more massive kiln structure. Another factor complicating analysis of biomass-fuelled furnaces is that, as discussed in chapter 3, in a firebox containing biomass, combustion air goes both through and over the heap of fuel, and the proportion going over the heap increases and decreases as the fuel is consumed before being replenished, with corresponding changes in open space and so in pressure drop through the firebox. Again, the temperature of the gas leaving the vent will be low at the start of firing, but at the end must be at least as high as the maximum temperature to which the work is to be fired, in order that pieces of work near the vent will reach desired temperature. The pressure drop through the flue opening and the negative pressure of natural draft will change accordingly. The comments so far in this section have been from the viewpoint of making an attempt to analyse from whatever remnants of a kiln found archaeologically how it operated and what temperatures may have been possible. For the reasons just given, such analysis cannot be as full as for charcoal fuelled shaft furnaces; but on the basis of modern knowledge and experience, we know at least
188
Appendix 3
that effective design approximations exist, consisting primarily of certain proportions of volume between firebox, work chamber, and height and diameter of vent. VOLUME OF COMBUSTION AIR REQUIRED FOR BIOMASS
If we know the volume of air necessary to burn one kilogram of biomass, we can begin to analyse the operation of a natural draft furnace. While this is not difficult to arrive at for oven-dry biomass, in practice the amount of air required is affected considerably by both water content of the biomass and the inescapable excess air, as was discussed in chapter 3. The quantity of air necessary to burn biomass can be calculated from its "ultimate" composition, which is given conventionally on an oven-dry basis in terms of carbon, hydrogen, oxygen, and ash contents and heat content. It is assumed that combustion is complete to carbon dioxide and water vapour because of the excess oxygen present as well as the existence of excess air external to the fuel bed. Using analyses of five temperate zone softwoods and three hardwoods that averaged 52.0 per cent carbon, 6.45 per cent hydrogen, 40.8 per cent oxygen, and 0.73 per cent ash, with heat content of 2I.L MJ per kg (Tillman et al. 198r p. 43), the air required is 6.21 m l per kg of dry wood. The heat developed per m 1 of air is 3.3 MJ. This is higher than with charcoal fuel because combustion is entirely to carbon dioxide. However, all biomass contains some water, and excess air is in practice always involved. In an experimental study of the combustion of maple and of pine woods on a grate in a stove, with natural draft air supply at a space velocity of 5.2 m per minute (Malloch and Baltzer 1935), the moisture contents of the woods as fired varied from rO.5 to 44.2 per cent, an average of 25.2 per cent, which is considered to be about the average for air-dry wood. The excess air consumed varied from 43 to 72 per cent, an average of 57.5 per cent. Using the figure above of 6.21 m J of air per kg of dry wood, the air required per kg of wood as fired was 6.21 x (1 - 0.252) x 1.575 '"' 7.83 m 3 per kg. When the maple and pine woods are taken separately, they were only 5 per cent higher and lower respectively. Thus it seems reasonable to take 7.0 to 8.0 cubic metres as the quantity of air that on the average will be necessary to burn on a grate with natural draft air supply, one kilogram of air-dry wood as received. When wood is burned as a heap or mass on the floor of a firebox, excess air will be higher in variable and uncertain amount, so 9 to 12 cubic metres of air per kilogram of wood may then be necessary. It is clear that the quantity of air necessary per kilogram of biomass and the heat developed per cubic metre of air can be quite variable, proportional to the variability of water content and conditions of fuel spacial arrangement.
Appendix 4
A FURNACE TO RELIABLY MAKE A BLOOM OF IRON
A bloomery furnace is basically simple and can easily be constructed of ordinarily available materials. Only small amounts of clay or materials of high fusion points are needed, since high temperature occurs only in a short vertical area of the lining, and for the few hours that such a furnace is usually operated, stiff ordinary clay is satisfactory for grout and for patching. A furnace such as that described below has been operated without difficulty to make several blooms, which formed at the rate of about 0.5 kg per hour. It should be noted at the outset, however, that the primary factor that can cause poor or unexpected results is lack of attention to the rate of air supply. This must be held to at least the minimum of the range suggested, and not exceeded. The furnace can also produce molten cast iron and smelt copper and other ores, with suitable adjustments to the ore-charcoal ratio and rate of air supply (space velocity). BASIC DESIGN
The primary objective is taken to be the production of a bloom of iron weighing a kilogram or two in a few hours of operation. The productive capacity of a shaft furnace is a function of its space velocity and inversely of its heat loss rate; thus the capacity of the air blower or bellows is the determining factor in a given furnace. The simplest, most readily available air supply is an ordinary household canister-style vacuum cleaner with a 550 to 600 watt rating. Most
19 0
Appendix 4
of these, when connected in reverse to blow instead of suck air, will supply in the order of 250 to 300 litres of air per minute against the resistance of a connecting hose and a 20 mm l.D. tuyere (the resistance of the fuel bed being relatively minor). If at all possible, a flow-meter should be installed and the air rate to the furnace adjusted by a damper over the inlet to the blower. A space velocity in the furnace of minimum about 6.0 metres per minute is recommended, because the furnace walls are not intended to be thick for ease in dismantling and so have moderately high heat loss rate. For 250 to 300 litres per minute of air, the furnace internal cross-sectional area then must be in the range 0.042 to 0.050 m\ which is an I.D. of 230 to 250 mm. The wall thickness suggested is about 15 per cent of the 1. D. or 40 mm, which will result in a moderate weight of furnace body so that it can be easily taken apart for access and repairs. A shaft height from tuyere entry to top of about 700 mm works well and should not be decreased. A single tuyere is to be used that is 20 mm but not more than 25 mm LD. With these conditions the furnace will form a bloom at the rate of about 0.50 kg per hour. CONSTRUCTION
The furnace can be conveniently constructed in three separable parts: a hearth that contains the tuyere in its upper edge and a stack in two parts that sits on the hearth. Each part is preferably contained in a metal shell for support and for convenience in handling. The advantages of this arrangement are several. When a smelt is finished, the stack can be lifted off in two parts, and the bloom is immediately accessible. The hearth is then easily cleaned out and patched if necessary, any repairs or adjustments in position made to the tuyere, and refractory erosion for a short distance above the tuyere repaired. This arrangement is shown in figure I I. The refractory of the furnace wall must resist high temperature over only a short vertical distance from tuyere level to about 100 mm above it, erosion being deepest at about 50 mm height. Several practical alternative materials are suitable as linings. A castable refractory mix made from silica sand and about 10 per cent of Portland cement should be satisfactory for runs up to several hours. Just enough water should be used to make an easily pourable mixture; quick-setting cement, if available, will save time in setting. If a reasonably refractory coal-measure clay is available, it can also be used as refractory lining, rammed in a layer against the wall of the container. With the stack made in two sections, the upper one can be rammed with any earth that contains enough clay to bind it, since temperatures there are quite moderate. A convenient procedure is to find three iron pails of 320 to 350 mm I.D and about 350 mm high, with as straight sides as possible. Leaving the handles on (which will be very useful), remove and discard the bottoms of two and make a few dents in the walls with a hammer, to act as keys to retain the refractory lining
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A Furnace to Reliably Make a Bloom of Iron
Figure I I General Arrangement of Experimental Furnace
to be applied. A sheet of stiff plastic or of metal is then rolled into a cylinder 230 to 250 mm O.D., pinned to retain its size, and centred into one of the bottomless pails sitting on the floor or on a board. Refractory mix can then be poured into the annulus between liner and wall, taking care to keep the liner centred. When the mix has set, the liner can be used in the other pails. If clay is to be used instead of a pouring mix, it is simply rammed firmly to the correct thickness. The third pail with the bottom left intact is to be the hearth, and au-shaped notch to accept the tuyere is cut into the top of one side. A hole about 20 mm J.D. is also made low in the side about 45 degrees either way from the tuyere and about 175 mm from the top of the pail, to make a slag tap hole. If castable or rammed refractory lining is to be applied to the pail as described above, before the mix has set a wooden or cardboard short cylinder, its O.D. that of the tuyere, is impressed into the tuyere notch at a depressed angle of about 15 degrees (a slope downward of one in four). To form a slag hole with castable refractory, a greased wooden or metal rod about 20 mm in diameter is set in the similarly sized hole low in the bucket wall. The inner end of the rod is supported temporarily so that it touches the forming sheet of the inner wall and is at an angle upward to the outside of about 20 degrees. After the lining material has been poured and set, that for the tuyere is removed, the rod for the slag hole is withdrawn and the hole cleared, and the bottom of the hearth is filled with silica sand to a level about 150 mm below the top of the pail. The surface of the sand should be shaped so that there is a gentle slope towards the slag hole, and the hole is filled with loose sand and plugged from the outside with a little clay. If desired, a single layer of 1I0 mm thick house brick can be loosely laid in first in the bottom of the pail to save sand. The slag hole is provided simply in case it is needed to drain molten slag that may be filling the hearth and risking getting into the tuyere. This could re-
19 2
Appendix 4
suit from using low grade ore, and/or making a larger bloom by extending the running time, which will develop more slag. The tuyere can be of ceramic or of iron such as a plumber's pipe "nipple." It should be 20 mm to a maximum of 25 mm I.D. and at least 150 mm long, and particularly if it is of iron it will become quite hot in operation from heat radiated and conducted from its nose. A "T" fitting should be used at its outer end, so that a removable plug in its continuation will permit cleaning slag from the tuyere nose with a rod if necessary during a smelt. The connection from the stem or base of the T to the air hose connecting to the blower will become hot and, if the air hose is rubber or plastic, it may need to be cooled by a rag wick into a pot of water. A spare tuyere will be useful in continuing a heat if the one in usc becomes seriously narrowed by slag. It may also be desirable for mechanical strength to fasten a bracket to the hearth wall to support the tuyere, or to support it at the T by a stool. ASSEMBLY
There are now four parts to be assembled: the hearth, two sections of stack, and the tuyere; and the usefulness of the handles that were left on the pails will now be evident. First, the tuyere is bedded into the prepared notch in the top of the hearth with clay or stiff ordinary mud, setting it so that it points along a radius of the hearth and protrudes from the inner wall about 25 mm. The space just above it will be filled with clay to be level with the top of the refractory in the pail. Then, after placing a layer of soft clay or mud 2 to 5 mm thick around the top of the refractory lining of the hearth section, the first section of the stack or shaft is set firmly and concentrically on it so that the interior wall join is smooth, with no ledge to cause a hang-up of burden during operation. The second section of stack is then added to the first with a similar grout joint. These joints will come apart easily for disassembly after finishing a smelt. When all mud and refractory is dry, the air delivery hose is connected to the tuyere. If a flowmeter is not available, a useful detail to add to the air delivery system near the tuyere connection is a water-filled glass U-tube to act as a manometer. When the blower is running and the furnace is filled with fuel and burning, a reading is taken as a base. During a smelting run if the pressure increases appreciably, it is probably an indication that the tuyere is becoming partly blocked with slag and must be cleaned or replaced to restore the original rate of air flow. If the pressure decreases, there is either a hang-up of burden or more likely a leak in the air delivery system. RAW MATERIALS
The two necessary materials are charcoal and iron ore. The charcoal must be lump charcoal made directly from wood, not briquets made of charcoal fines
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A Furnace to Reliably Make a Bloom of Iron
held together by a pitch binder which melts. It must be crushed and screened so that the maximum sized piece is 25 mm and the fines below about 3 mm are removed. Both soft and hardwood charcoal are satisfactory, the latter giving a little higher furnace temperature and carbon content in the bloom, but a mixture will be found quite satisfactory. The charcoal must be kept dry, since it easily absorbs water which is then weighed as fuel, increasing the actual ratio of ore to dry fuel by weight. Obtaining ore will be more of a problem, depending on where the furnace is built. If mill or forge scale is available, it is quite satisfactory as ore when passed through about a 3 to 5 mm screen. Silica sand in a proportion of 15 to 20 per cent by weight is mixed with it to form slag, the mixture being the" ore" weighed. If iron ore is available, it should preferably contain more than 50 per cent iron, simply to avoid making too much slag which then has to be tapped. Different ores have quite different reducibilities, and to ensure that reduction is complete in the subject furnace, ore should be roasted and then crushed to a maximum piece size of about 3 mm - smaller if the ore is dense and of low porosity. A ratio of ore to charcoal of I to 2.5 by weight is suggested for the first run, which in the furnace described should produce a bloom containing about 0.6 to 1.0 per cent carbon. Lower carbon content will result from an increased ratio of ore to charcoal. Bar iron forged from such a bloom will contain a little more than half as much average carbon content, depending on the number of re-heatings necessary in forging. OPERATION
With air connections made and leaks checked and sealed, the furnace is preheated by lighting some wood with paper in the hearth and then running the blower or bleeding the air in such a way that about half the maximum air rate reaches the tuyere for the first half hour, and then the full air supply rate is used. The furnace is kept about half full of scrap wood and the preheating continued for at least an hour, letting the last wood added nearly burn out. The furnace is then immediately filled to the top with charcoal and the air supply set to full operating rate. During the preheating period, individual charges of ore and of charcoal must be weighed out and kept on sheets or in containers, since they will be consumed at a rate of one about every five to seven minutes. Each charcoal charge should be 500 grams, independent of the charcoal to ore ratio being used. The first charge of burden, i.e., charcoal and ore, is made as soon as the charcoal in the furnace has descended enough to accept it. The charcoal is to be spread evenly across the surface and the ore then spread evenly on the charcoal. It will become evident that the furnace burden is a little over 90 per cent charcoal by volume, and ore particles tend to disappear in the void spaces of the charcoal. This procedure is repeated as space becomes available below the
194
Appendix 4
furnace top, for as long a time as necessary for the size of bloom desired about two hours for a one kilogram bloom. When the desired number of charges have been added, full air supply is continued, and the burden is allowed to descend in the furnace until only about 100 to 200 mm is left above tuyere level, as determined by a measuring bar inserted from the top of the shaft. The air supply is then shut off and the furnace either allowed to cool naturally, or more rapidly, by sprinkling water from the top onto the burden. The latter does no harm and saves charcoal and time. When the two stack sections are cool enough to handle, they are lifted off and the bloom is then directly accessible. When cool enough it is removed with tongs, cleaned of adhering charcoal and slag with a hammer, and weighed. Troubles may occur. If the burden stops descending, the cause is probably a hang-up of burden, and the shaft body should be jarred with a fist or a piece of wood. Rodding from the top with an iron rod is a last resort, but downward flow of burden is normally smooth if the shaft lining is smooth. If the rate of descent simply decreases, then the air supply rate has been decreased, either by a leak in the system or partial plugging of the tuyere by slag. If the occurrence is early in the heat, the slag has come from above, and if later, may be rising from below. In either case the tuyere must be cleaned or replaced at once. The rate of air supply to the fuel is the heart of the operation, and continuous attention must be paid to it so that any unexpected leaks can be stopped and any tuyere blockage immediately dealt with. Molten cast iron will likely be made if the ratio of ore to charcoal becomes less than about 1 to 4 or 5 through errors in weighing charges. If it is desired to tap slag from the hearth for any reason, the clay outer plug in the slag hole is removed and the loose sand behind it teased out with a thin rod. As the interior wall is reached there is a thin layer of fused slag and sand, and the rod is then used to force an opening. To stop slag flow, a small blob of stiff mud on the end of a metal or wooden rod is forced down against the wall across the stream of slag and held in position until the slag in the hole has congealed. To reuse the furnace, the hearth is emptied and the walls cleaned if necessary, and the sand bottom is replaced to its former level. The tuyere and its setting are repaired if necessary, and erosion of the wall at the bottom of the shaft is repaired, particularly just above the tuyere location. The tops and bottoms of the refractory of the shaft sections are scraped clean so that a good reseal can be made, and the furnace is then reassembled with fresh joint grout.
Glossary
BLOOMERY FURNACE A bowl, hearth, or shaft made of or lined with refractory material, using lump charcoal as fuel and with bellows or, less frequently, natural draft for combustion air supply. When the furnace is in operation, iron ore is added with charcoal in an appropriate proportion by weight, and a solidstate forgeable mass or "bloom" of iron forms in the hearth of the furnace. Orcs of copper and tin can be smelted in the same kind of furnace but to a molten pool of metal and slag in the hearth.
BROWN COAL CHAR
When bituminous coal is heated out of contact with air
to a sufficient temperature, it decomposes to form coke. When the coal is the
softer variety called brown coal, the product of heating has different properties and is called a char rather than coke. BURDEN
The contents of an operating smelting or melting furnace.
CARBURIZED When iron is heated above a temperature of 730°C, a change in its crystal structure enables it to dissolve carbon, at a rate and to an extent that increases with further increase in temperature, to a maximum at 1,150oC. The surrounding atmosphere must contain a sufficient excess of carbon monoxide gas over carbon dioxide to act as the source of carbon. Carburization of iron cannot occur while it is being heated in a charcoal fire to be forged, since the combustion gases have a high ratio of carbon dioxide to carbon monoxide and remove carbon from the iron instead of adding to it.
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Mastery and Uses of Fire in Antiquity
COLD WORK This is the term for forging a metal at room temperature. Because of its higher strength, the metal requires much stronger blows with the hammer than when softened by high temperature; but the resulting deformation of the cold metal creates changes in its internal structure. These increase its hardness and decrease its ductility in proportion to the extent of deformation, up to a level where it can no longer be deformed without fracture.
COLD WORK HARDENING
The process of increasing the hardness of a metal
by cold work. DUCTILITY
The property of being plastic or deformable without fracture.
FLUX A material that is added with a furnace charge to combine with the gangue in an ore and so decrease its melting point sufficiently that it is fluid at furnace operating temperature. FORGING The act of deforming a metal by mechanical pressure from a hammer or a press, after it has been heated to a sufficient temperature to be plastic. FURNACE A container made of heat-resistant material within which heat is generated, in antiquity nearly always by the combustion of a biomass-related fuel, and transferred to objects to be heated. The purposes of the container are to decrease loss of heat to surroundings and to control the geometry of its contents. FURNACE CHARGES These constitute the burden of a furnace, added in small portions of materials such as ore, fuel, and flux in suitable proportions. GANGUE The barren rock component of an ore of a metal, composed of mixtures of oxides of a variety of metals. It often has a high melting point, and unless this is decreased by addition of a flux, a viscous mass can form which can lead to furnace "freeze-up."
The bottom part of the interior of a furnace, below tuyere entry level, where the products of furnace operation can accumulate.
HEARTH
I.D., O.D. The inside diameter and the outside diameter respectively, of a cylindrical hollow object. The I.D. of a furnace is the measure of its working capacity; the O.D. is the I.D. plus twice the furnace wall thickness. KILN A furnace that nearly always used biomass directly as fuel, with natural draft air supply.
197
Glossary
NATURAL DRAFT The supply of combustion air without physical effort. The products of combustion are at a temperature higher than ambient air, and so are lighter and rise within the furnace. An exit must be provided, as well as one or more entry points, and the rate of flow of air increases with the average temperature of the gas within the furnace and the height between inlet and exit. The rate can also be controlled by the size of the inlet opening.
pas Pascal-seconds, the quantitative measure of viscosity. QUENCHED If iron containing dissolved carbon is heated to a temperature above 830°c (a bright red colour) and then plunged into water, its hardness will be increased in proportion to its carbon content, up to about 0.70 per cent. At this composition the hardness of the quenched iron is 850 to 900 Vickers Hardness Number (VHN), about twice the hardness of glass, and ductility is nearly zero.
The chemical process of removing oxygen from a metal oxide so that the metal can appear as an element. The usual reducing agent is carbon monoxide gas, which is formed in quantity in the combustion particularly of charcoal, and which combines with the oxygen in the metal oxide to form carbon dioxide gas. Reduction starts at temperatures of a few hundred degrees celcius and increases in rate with increasing temperature. REDUCTION
RE-MELTING The melting of a metal in the forms of prills, ingots, or scrap, to convert it to a pool of molten metal that can be cast into an ingot for further processing, or a mould to shape it as it solidifies.
A workman's term for metal being brittle or lacking in ductility. Hotshort means that when being forged at high temperature, it does not deform smoothly, develops fissures, or even falls apart. Cold-short means that a metal lacks ductility at room temperature to the point of being brittle. SHORT
A material with approximately the composition of glass, which is the result of the interaction between the gangue of an ore and a flux that has been added to increase its fluidity at furnace temperature. It can separate from molten metal because of its much lower specific gravity and then can be tapped from the furnace by itself as a waste material. SLAG
SMELTING The whole process of extraction of a metal from its ore by the action of carbon monoxide gas at high temperature and the use of a flux to liquefy gangue. Lump charcoal is burned to supply both heat and gas, the gas combining with the oxygen in the metal oxide in the ore to leave solid metal,
198
Mastery and Uses of Fire in Antiquity
and the heat encouraging reactions and liquefaction. If the metal is not present as an oxide but as a sulphide, carbonate, or hydroxide, it must be converted to oxide by roasting in air before being charged to the furnace. SMOKE-HOLE Another name for the flue or hole in the top part of a kiln for the escape of the products of combustion. SOLID-STATE BLOOM In iron smelting the reduced iron normally is not melted, and very small reduced pieces accumulate as an irregular mass just beIowa furnace tuyere. The mass is called a "bloom" and is mostly iron, mixed with some trapped slag and porosity. To be made into a useful bar, it must be reheated to above the melting point of the slag (lIOO to I200°c) and forged extensively to squeeze out the slag and consolidate the porosity.
STOCK COLUMN
The name for the burden in a full furnace.
UNFORGEABLE Some metals can not be successfully forged because they either do not become very plastic even at high temperature, or are hot-short.
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Index
Page references in italics indicate the presence of a diagram or a table. Ababua tribe (Africa), 105 Abu Matar (site), 91 adiabatic flame temperature (AFT), 10- II, 15,27,28,75,170- 1 afforestation, 153 Africa: copper smelting, 163, 180; iron smelting, 70-2, 81, 104, 105-6, 128-9, 141,150, 163, 180, 183 air exit velocity, 69-70, 7 I air (reverberatory) furnaces, 42-4, 43, 52-4,75,106,131; use in antiquity, 53 air pipes, 7 alumina, 109 alumina clays, 22 Americas, the: North America, 59, 156; pre-Columbian New World, 76,78, 81; South America, 183. See also specific countries and sites amphorae, 44 Ancient China's Technology and Science,
143 Anfun (site, Nigeria), 92 annealin& 5~85, 89, 142 anorthite, 112 anthracite coal, 63 antimony, II3 arsenIC, 113, 119, 134
artifacts and archeological remains, 141, 160-3; of crucible furnaces, 91; misidentification of, 81-2; of natural draft furnaces,
128
ash, 3 I, 161 ash (tree), 31, 61 ash content, 28, 31-2, 36, 61 Asikli Huyuk (site), 47 Athens, IS 5 atmosphere composition control, 44 Attic pottery, 34 axes, 57 Ayia Triada (site), 40 balsawood, 30 bamboo, 31 basalts, 54 Batane Grande (site, Peru), 76, 77 batch operation, 14-15, 187 Baya-Kala tribe (Africa), 105 beech,61 Bellamy, C.Y., 71, 128, 136 bellows, 7, 68-9, 77-9; advantages oi, 79; artiiactual evidence, 162, 163; vs. blowpipes, 77, 78, 79; for copper smelting, 78, lIS; heat generation rate, 77; leakage, 79; mechanical power, 69, 72, 80,
208
Index
128,15 I; mechanism, 78-9; vs. natural draft, 82; piston, 78; power required, 178-9; pulsating air supply, 71, 72-3; tuyeres for, 77; use for iron smelting, 78, 124,128,13°-1; valves, 72, 78 bellows effect, 72-3 Bergbau Museum (Germany), 186 Bessemer process, 106, 136 biomass fuelled furnaces: products made in, 46-54; temperatures in antiquity, 54 biomass fuels, 6-7, 25-37; AFT of, 15; air dried, 26; ash content, 31; carbon content, 29; vs. charcoal, 160, 172-4; chemical analysis, 27-8; combustion, 25-6, 188; composition, 28, 188; decomposition, 26; definition, 25; features, 25-6; fossilized, 25; fuel beds, 17, 180, 187; furnace configurations for, 38-45, 187; geogra phie varia lion, 29; geometry of pieces, 25, 3 I; heat content, 29, 146; identification of species, }I, 161; maximum temperatures, 7; oxidizing effect, 172; proximate analysis, 27-8, 29, 161; relation of combustion air to heat generation, 34; relation of heat content to carbon content, 30; used for plaster-making, 146; use for carbothermic smelting, 104, ~07-8, ~15, 131; use for clay products, 144-5, 160; use for furnace preheating, 15,147, ISO, 193; use for glass-making, 7,42, 146-7; use for lime manufacture, 47-9,146,160; use for roasting ores, 146, 148; use for smelting and melting of metal, 52-4; use in crucible furnaces, 90; varieties of, 25,29; water content, 25, 188. See also wood fuel birch, 56 Biringuccio, 4, 51, 57, 84 Birkinbine,]., 168 bismuth, 113 bituminous coal, 30, 170, 195 black oak, 29 blast furnaces, 17, 127-8; charcoal fuelled, 59,65; coke fuelled, 59, 177; pulsating blasts, 73; vs. reverbatory furnaces, 54 bloom (of iron), 195, 198; carbon content, 125, 126; composition, 125; from fining operation, 134; formation, 123, 134; formation along with cast iron, 127; produced by carbothermic process, IF; produced from cast iron, 128; produced in Catalan forge, 88; reheating of, 150;
size, 80, 88-9, 134-5, 151; slag content, 135; solid state, 198; void spaces, 135; weight, 150. See also forging and forgewelding; iron bloomery furnaces, 127-8, 143, 151, 195; experimental (instructions for), 189-94 bloomery (direct) process, So, 123, 127, 12S, 130, 132, 139 blowpipes, 75-7, 76, II5; artifactual evidence, 162, 163; vs. bellows, 8, 10, 77, 78,79,124; heat generation rate, 76, 77; maximum temperatures with, 76; placement of, 85 Bohm, 1., 103 bomb calorimeter, 3 ° boron oxide, 50 boshes, 95 bowl furnaces, 84-92; archeological remains, 162; combustion air, 74; lateral gas flow, 9S-9; reduction power, 85; shape, 16; space velocity, 85; temperature distribution, 85, 86; temperature limitations, 22; use for forging, 89; use for smelting, 104, I I 5 Boydell, P.]., 119 Braga, R.N.B., 168 Braudel, E, 159 bronze, 113; annealing, 89; casting, 53, 138,143; cold work, 89; cold work hardening, 139; vs. iron, 122, 138-9; melting, 89; remelting, 42, 54; removal of iron from, 54; smelting, 76 bronze axes, 57 brown coal, 195 brown coal char, IS, 171, 195 bulk density: of charcoal, 58-9, 61,91, 100,173; of ore, 100; of wood, 30,173 burden, 59, 102, 195 Burma, 128 calciuIll carbonate, 47-8 calcium hydroxide, 47 calcium oxide, }I, 47 carbonates, 101 carbon content: in biomass fuels, 29, 30; control of, 132, 133; in iron, 122-3, 125,132-4,137-S carbon dioxide: in human breath, 75; reduction in charcoal combustion, 63-5, 67, 167-8, 169 carbon monoxide: in carbo thermic smeiting, 106; in charcoal combustion, 64-5,
209
Index
167-8,169,170; in reduction process, 35, 101, 102, 197 carbothermic smelting, 104-8, Il5, 131; reaction times, 106; reduction time, 107 carburization (of iron), 7,137-8,143,195 cassiterite sand, 104 Casson, L., 6 I cast iron, 140-1; in ancient China, 143; composition, 140; decarburization to make bloom, 128; ease of production, 127; fining, 131-2; formed simultaneously with bloom, 123, 127; gray, 14 I; large objects, 53; moulds for, 53, 141; oxidation in air furnace, 53; silicon content, 141, 142; white, 124, 140-I, 142,143. See also iron cast steel, 140 Catalan forge (smelting hearth), 70, 87-9 Catal I-Iuyuk (site, Near East), 9 cathode copper, 119 Cato, 48-9, 146 Cayony Tepesi (site), 3 cellulose, 28 cementite, 125, 140,142 ceramics. See clay products C-14 dating, 57, 119 chaff (as fuel), 25, 26, 36-7,45 charcoal, 6-7, 55-62; AFT of, 15, 170-I: archeological remains, 61, 161; ash content, 61; vs. biomass fuels, 16o, 172-4: bulk density, 58-9, 61, 91,100,173; C-14 dating, 57; charcoal-block furnace, 91: chemical reactivity, 56, 60, 61; vs. coke, 59, 158, 170: crushing strength, 59: for experimental furnace, 192-3; fixed carbon (F c) content, 55, 5 8, 60, 100,168: flame, 32: formation through pyrolysis, 28; furnace configurations for, 84-100,191; heap method of making, 56,57,61, l}0: heat content, 59, 60-I, 173: land required for perpetual supply, 156, 158; made from hardwood, 56, 57, 58,59,94,172: manufacture, 55-7: maximum temperatures attainable, 7, 170-I; measurement by bulk, 58, 59: ore-to-fuel ratio, 59,98: porosity, 58; properties, 57-6 I; quality, 57-8; transport of, 61; VM content, 60, 168: void fraction, 58, 177-8; wasteful aspects of, 60-I: wet, 58: wood used for, 56, 57, 61,94,100,143,161; yield from wood, 56,145,15 8
charcoal fuel beds, 65; combustion of, 6373,167-74: gas distribution, 66-9; natnral draft, 180, 181-3; plumes from tuyeres, 68: raceways in, 69 chert, 13 Chile, 1I9
chimney height, 34 China: copper smelting, 142-3: iron smelting, 104, 140, 141-3; reverberatory (air) furnaces, 42, 54; use of carbothermic process, 104, 105: use of coke, 62: use of piston bellows, 78 clay: alumina, 22: glacial, 23: heat content, "45-6: iron content, 35: for kiln construction, 23; use in iron manufacture, 137, I3 8
clay products (pottery, tiles, etc.), 3, 6, I}; batch operation manufacture, 14, 187: brick, 23; colour through oxidation, 35; fired with biomass fuel, 7, 46, 160; firing temperatures, 44,46: fuel consumption rates, 45,145-6,152,159; manufacture in antiquity, 44-5,159; porosity, 44; reducing atmosphere for, 34; softening temperatures, 23; uneven thermal expansion, 38,41; waterproofing of, 44, 45 Cline, W., 141 coal, 6, 25; anthracite, 63: bituminous, 30, 170,195: brown, 195: brown coal char, 18,171,195; formation from peat, 36; formation of coke from, 62, 170; heat content, 30; use in hearth furnaces, 170 cobalt oxide, 50 coke, 60, 62, 71, 195: AFT of, 15: vs. charcoal, 59, 158, 170; formation from coal, 62,170: fuel bed, 63, 69, 171; use for roasting ore, 183; use in blast furnaces, 177; use in shaft furnaces, 17,98 cold-short, 197 cold work, 89, 113, 196 cold work hardening, 133, 138-9,196 Colossos of Rhodos, 53 colours: of clay and glazes, 35: of flame, 88: of glass, 50: relation to temperature, 11-12
combustion air, 7-8, 32-4: artifactual evidence, 162: for biomass fuel, 188: in charcoal furnaces, 65, 74-83; entry through grate, 66-7, 167; entry through tuyeres, 67-72; as heat control, II, 34; human breath, 75-7; measurement of,
210
Index
99: movement, 74-5: power for, 75, 178-9: rates necessary in antiquity, 80: sources, 74; water content, 168-70 Comroe, J.H., 75 concrete, 47 Constantine, A., 168 continuous operation, 14-15: in iron smelting, 128; of lime kilns, 49, 146; of shaft furnaces, 14-15,92. copper, 113; iron in, I I 7-2.0 copper alloys, 113 copper carbonates, 114 copper hydroxides, 114 copper manufacture: annealing, 89: cold work, 89: iron removaL 54, 118: refining process, 1I8, 132, 148: use of biomass fuel, 42 copper ores, 113-14; concentration of, 148-9: copper content, 147-8: roasting of, 147; sulphide, 114, 115-17, 147-8, 157; weathered, 1I4-15, 147 copper oxide, 50, 114, 116 copper smelting, 102-3, 104, 1l}-21, 141-2: bloom formation, 103; carbothermic process, 104-8; fuel consumption rates, 147-9, "52, "57-9; iron as byproduct, II9-20; vs. iron smelting, 124: mythology of, 100: in natural draft furnaces, 81, 120-1, 182-3; in open charcoal fire, 84; relation to glassmaking, 49; in shaft furnaces, 115; slags, Ill.; of sulphide ores, 115-17, 147-8: use of bellows, 78, 115; use of blowpipes, 76, II5; use of crucibles, 89,114,115; use of iron oxide flux, 114,119,122; use of refractory basins, 98; of weathered ores, 114-15; in wind furnaces, 82-3, 186 copper sulphide, 1I4, Il6 cotton gin trash, 29 Craddock, P.T., lI8
craftsmen (of antiquity): development of mythology, 100; skill and experience, 88, 1}2; social position, 5 Crete, 40, 120 Crookes, E., 148 crucibles and crucible furnaces, 89-91; advantages of, 90; artifactual evidence, 9 I, 161-2; for melting glass, 5 I; use for carbothermic smelting, 104-8, 107-8; use for copper, 89, 114, 115: use for smelting, 91
cupolas, 17, 71, 106, 171 Cyprus, "57-9 Damascus swords, 136, 140 dampers and shutters, 34, 49, 90 Dande tribe (Africa), 106 Daniloff, B.M., 119 deadman, 182-3 deforestation (in antiquity), 57, IS 3-9: factors influencing, 154; land clearance, 154,15 8 ,159
diorite, 22 direct (bloomery) process, 80, 123, 127, 128,13 0 ,13 2 ,139
Dixey, F., 141 Domesday Book, 154 Douglas fir, 27, 2.9 Draper, A.B., 171 ductility, 119, 196 dung, 25, 36 Eglestone, E., 126 Egypt, 44-5, 7 8 elm, 31 England, 42,61; forests, 156, 158; glassmaking, 52; iron smelting, 15 I, "55, 158; land use, 154 equilibrium phase diagrams, I I I Ergun, S., 176 Ergun's equation, 177-8 Espelund, A., 108, 173-4 Evans, D.G., 171 excess air, 26-7; in biomass combustion, 188; in charcoal combustion, 60,63; effect on AFT, 27, 28 extracti yes, 28 faience, 50 £ayalite (iron silicate), Il7, II8 FC. See fixed carbon (FC) content ferrite, 140 ferrous oxide, Ill. fining, 53, 131-2, 143, 158; advantages of, 128; bloom formed by, 134. See also Iron fire, open: enclosure of, 9, 29-3 0 , 33, 38, 85; smelting in, 84 firebox, 27,33-4; analysis of ash layer, 31-2; fuel replenishment, 33-4; grate in, 28,34, 167; joined to work chamber, 39-44; perforated roof, 21, 39,90; permanent, 39; size, 40-1; variations of, 39
211
Index
fixed carbon (Fe) content, 26, 28; of charcoal, 55, 58, 60, 100, 168 flame, )2 flint, 13, 57 fluidization velocity, 169 f1ux(es), 42, 101, 109, 196; iron oxide, 114,116-17,119,122,151; lime, 158; role in carbothermic smelting, 104; role in reduction,
102.-3
forest growth rates, 153-5, 156, 159 forging and forge welding, 80, 89, 134-9, 149,196; carbon removal, 85, 132; furnaces for, 85; losses during, 136-7; oxide scale formation, 13 6; slag removal, 135; unforgeable metals, 198; welding temperatures, 103. See also iron forging and forge-welding. See bloom fossil biomass, 6, 154 frozen furnace, 96 fuel(s), 6-7; adiabatic flame temperature (AFT), IO-lI; artifactual evidence, 160I; cost and availability, 44; separation of work from, 39-44,187; shortages, 159; transport of, 61, 153, 154, 155. Seeaiso specific fuels (biomass, charcoal, etc.) fuel bed: height, 92-3, 94; lateral gas flow in, 98-9; penetration of tuyeres into, 71-2; pressure drop in, 175-9, 181, 182,186; void space in, 26; volume at temperature, 171-2 fuel consumption rates, 152; for clay products, 45,145-6,152, 159; for copper smeiring, 147-9, 152, 157-9; for glassmaking, 52, 146-7, T)2; for iron manufacture, 137, 149-52, 152. 158; for lead-silver mines, 156; for lime manufacture, 48,146,152,159; for pIaster-making, 146, T)2 fuel reactivity, 18 furnace(s), 9-10, 13-24, 196; atmosphere COIllposillon control, 44; configurations
for biomass fuel, 38-45,187; configurations for charcoal fuel, 84-100, 191; construction materials, 21-4; experimental (instructions for), 189-94; frozen, 96; inside and outside diameters, 196; maximum temperatures, 7, 10-11; vs. ovens, I}; preheating of, 14-15, 130, 14~ 193;shap~ I~95, 129, 130, 174, 184,185; size, 7, 15-16, 20, 24,91-2, 182-3; for specific projects, 53; surface area vs. volume, 15-16; wooden, 92. See
also specific types (bowl, shaft, natural draft, etc.) furnace charges, 196 Gale, N.H., I I 1 gangue, 101, 102, 109, 196 Germany, 158, 186 Gjers kiln, 183 glacial clays, 23 glass: link with metallurgical slag, 50, 109, llO; melting and working points, 50; viscosity-temperature curve, 50-1 glass blowing, 5 I glassiness, 50, 109, 110 glass-making, 49-)2; annealing, )2; fuel consumption, 52, 146-7, T)2; furnaces for, 42,51,52,147; melting and mixing of elements, 50-I; relation to copper smelting, 49; use of biomass fuel, 7, 42, 14 6-7 glazes (of pottery), 35 goats, 15 X, 159 gold, 113 Goltepe (site, Turkey), 91,104 gossan, I 14 Gowland, W., I I 5 grape pomace, 29 graphite, 141, 142 grasses (as fuel), 25 grates, 28, 34, 66-7, 167, 188 gray cast lron, 141 Greece, 153, 155, 15 6-7 Greg, J,L., 1I9 gypsum, 13, 47, 146 hardwoods, 26, IS8; charcoal made from, 5~ 5~ 58, 59,94, 17 2 Hausa tribe (Africa), 141 heap method (of charcoal manufacture), 56, 57,61, 130 hearth, 42, 7°,96-9, 196 hearth furnaces, 84-92; archeological remains, 162; coal fuelled, 170; disadvantages of, 85; lateral gas flow, 98-9; space velocity, 85; temperature distribution, 85, 86; tuyeres, 68; use for fining iron, 131; use for smelting, 87-9, 10 4 heat: of combustion, 30; containment, 9; properties, 9-10; quantity, II; rate of supply, 17 heat balance, 19-20
212
Index
heat content, 30-I; of biomass fuels, 29, }O, 146; of chaff, 45; of charcoal, 59, 60-1,173; of clay, 145-6; of coal, 30; of dung, 36; relation to carbon content, 30: of straw, 30,45; of wood, 30-1,59, 60-I, 172 heat flow, 9,19 heat generation, 10, I I, 19 heating value, 3 heat input rate, 16 heat loss, 10,20,23,85; in batch vs, continuous operation, 14; effect of furnace size and shape, II, 15-16; rates in antiq-
°
uity, 24; sources of, 13-14; as temperature control, 20 hematite, 35, 105, 107, I29, 136, 149 hemicelluloses, 28 Hetherington, R., 85 high-carbon steel, 123
higher heating values (HHV), 29, }O Hiles, J., 169, 170 History of Stones (Theophrastus), 6 Hittites, 5 hot-short, II9 human breath (as combustion air), 75-7 hydroxides, 101 igneous rock,
22
India, 104 ingots: bun-shaped, 90 inner face temperature, 23 Innes, l.A., 107 inorganic materials, 6 inside diameter (I.D.), 196 insulation, 21, 24 iron, 122-44; air cooling, 139; arsenic in, 119,134; vs. bronze, 122, 138-9: as by-product of copper smelting, 117, 119-20; carbon content, 103, 107, 117-18, 119, 122-3, 125, 131-4, I}7, "39-40 ; carburization, 7,137- 8, "43, 195; in copper, 113, 117-20; copper content, II9; ductility, II9; early records of, 5; hot-short, 119; lead in, II9; malleable, I}9, 141-2; melting temperature, 123; normalized, 13_9, 140; oxidation of, 35, 53-4, 89, II 8, 136; pig, 128, 131; plate or sheet, I37, 150; reduced from iron oxide slag, 114, II 6; slag content of, I} 5, 136; tools made from, 139,141; types of, 139; use for C-14 dating, 119; wrought, 139,
158. See also bloom; cast iron; fining; forging and forge welding iron bars, 80, 123, 136 iron carbide, 13 6, 140 iron manufacture: fuel consumption rates, 137,149-52, Lj'2, 158; geographical limitations, I4}-4; production rates, 99; technical complexity of, 122-3, 13 2, 143-4; as three-stage process, 123, 149 iron ores: composition, 127, 133-4, 149: for experimental furnace, 192-3 iron oxides, 122, 149: flux for copper smelting, 114, 119, 122; flux for silica, 126; in slags, 50, 112, 125-6 iron powder, I r 9 iron silicate, 117, I}6; slag, 54, II6, 1I8 iron smelting, 103, 104, 122-34; carbothermic process, 104-8, I} I; control in antiquity, 132: vs. copper smelting, 124: direct (bloomery) process, 80,123,127, 128,130, 1}2, 139; discovery of, 122, 124: fuel consumption rates, 149-52, 152; mythology of, 100; in natural draft furnaces, 70-2,81,104,120, I28-31, 180, I8}: in open fire, 84: ore-to-fuel ratio, 123, 124-5, I} 3; reduction process, 124-7; size limitations, 80; slags, 125, L26, 127, 151; temperatures for, 76, 77, 88,122-3: use of bellows, 78,124,128, 130-1; use of biomass fuel, 78, 173-4: in wind furnaces, 82-}, 186 iron sulphides, Il4, Il6, 149 ironwood, 30 Israel, 83
Japan, 95,115,123, I}6, 186 Jericho, 47 jewelry, furnaces for, 92 joule, II Juleff, G., 185, 186 kaolin, 2.2, 23 kiln(s), I}, 14,15,196; archeological remains, 161-2.; arrangement of work in, 16, sr, 180; biomass fuelled, 45, 48, 78, 145-6: charcoal fuelled, } 2., 173; classification of, 40; construction materials, 21-2,23; damage and deterioration, 21, 35-6; design variations, 39-44; firebox, 21: fuel consumption, 40,44-5; for glassmaking, )1: heat distribution, 412; insulation, 21; maximum tempera-
213
Index
tures, 22, 40-1,162; measurement, 162; natural draft, 24, 75,180; perforated floor, 21, 39, 90; roof arches, 21; similarity to crucible furnace, 90; size, 41-2; thermal efficiency, 146; use for carbothermic smelting, 104, 107-8; use for melting metal, 42, 52; use for smelting, 42, 52,78, LI4; work chamber, 39,
Meeks, N.D., 1I8 Merkel, J.E., 1I8 Mesopotamia: glassmaking in, 5 I metal(s): loss in slag, 71; melting with biomass fuel, 52-4; unforgeable, 198 molten lava, 54 molten metal: in shaft furnaces, 93-5, 96; superheat, 96; tap holes for, 97, 162
40 -
1110rtars, 6, 22
2
Kommos (Crete), 120 Kurth, E.F., 3 I land clearance, 154, 158, 159 Lapphattyn (Sweden), 128 laterites, 127 launder, 53 Lavrium (site, Greece), 156-7 lead, II3, II9; melting point, 84; smelting,
Matt, R.A., 169,170 moulds, 47, 53,14 1 muffle furnace, 44
lnanganese, 127, 134
natural draft, 32-4, 180-8, 197; in biomass fuel beds, 180, 187; in charcoal fuel beds, 180, 181-3; negative pressure, 80, 81; role in runaway condition, 35-6; use for carbothermic smelting, 108, I I 5; use in kilns, 24, 75, 180 natural draft furnaces, 80-2,180-8; air leakage, 181; archeological remains, 80, 81-2, 128, 162-3, 163, 180;vs. bellows, 82; for copper smelting, 81, 120-1, 182-3; crucible furnaces, 90, 91; deadman, 182-); effect of fuel lump size, 66; effect of wind, 81, 82; for iron smeiring, 70-2, 8 I, 104, 120, 128-3 I, 180, 183; for lime manufacture, 183; maximum temperatures, 81; shaft furnaces, 75; tuyeres, 70-1, 81, 182, 18 3 natural gas, 6, 25 Natural History (Pliny the Elder), 4, 138 Near East, 49, 143 Needham, J., 143 nickel, 127 Nigeria, 81, 92,129 nitrogen, 64-5, 75 N ok culture, 81 normalized iron, 139, 140 Nosek, E.l'v1., 128
manganese oxides, 50, 112 maple, 26, 188 matte, 114, 116, 148 maximum temperatures attainable, 7, 10II, "5; with biomass fuel, 7; with blowpipes, 76; with charcoal, 7, 170-1; factors affecting, 40; of kilns, 40-1, 162; of natural draft furnaces, 8 I measurement: accuracy of figures, 19; of charcoal, 58, 59; of combustion air, 99; of gases, 12,73; by volume, 99-100
oak, 61 obsidian, 50 oil, 6, 25 olive pits, 29 Ondulu (Angola), 105 open hearth process (for steel), I} 6 ore-carbon process, 104 ores: bulk density, 100; concentration of, 148-9; crushed or pulverized, 9 I , 94, 104; definition, 101; metal content, 100;
9 8 , "5 6-7
lignin, 28 lignite, 6, 25, 36 lime: in slags, 109, II2; use as flux, 158 lime kilns, 48-9, 146 lime manufacture, 13,47-9,146; in antiquity, 159; fuel consumption rates, 48, 146,152,159; in furnaces, 48-9,183; use of biomass fuel, 47-9,146,160 lime mortar, 6 lime plaster, 3, 47 limestone, 1}, 14 6; decomposition, 3, 47, 4 8 , 18 3 limonite, 105, 149 lower heating value (LHV), }O lump size (of fuel), 17, 56, 66; as archeological evidence, 161; limitations on, 170; relation to space velocity, 18,66 magnesia, 109 magnetite, 35, 13 6, 149 malleable iron, 139, 141-2
214
Index
powdered or granular, 119, 183; roasting of, II5, I46, 148, 183. See also specific metals (iron, copper, etc.) ore-to-fuel ratio, 20,91,94; for Catalan forge, 88; for charcoal, 59, 98; in iron smelting, 123, 124-5, I33; measurement by volume, 99-100; relation to temperature, 20-1, 96 organic materials, 6 orifice formula, 175-6 outside diameter (a.D.), 196 Ovambo tribe (Africa), 106 oven, 13 oxidation: of iron, 35, 53-4, 89, 118, 136; of mineral compounds, 101; of sulphur,
lIS oxides, 101; reduction rates, 94-5; in slags, 50, 109-10 oxide scale (on iron), 1}6 oxidizing atmosphere: in carbothermic smelting, 104, 107, 108; in charcoal combustion, 64, 67 Pascal-seconds, I I I, 197 pearlite, 125, 142 peat, 25, 36 peep-hole, 43 Percy,]., I02, 106, lOS, I26, I28 phosphorus, 134 phosphorus oxide, 50, 109, 1I2 pig iron, 128, 1}l pine, 26,56,61,188 Pirotechnia (Biringuccio), 4, 84 piston bellows, 78 pitch, 44, 4 S pit method (of charcoal manufacture), 56 plant residues, 30 plaster, 47 Plaster of Paris, 46-7,146,152 Pleiner, R., 120 Pliny the Elder, 4, 138 Plutarch, 5 Poland,128 population growth, 154, 155, 158 potash, 50 potassium oxide, 50 pottery. See clay products pozzolan, 47 primary cementite, "42 proximate analysis, 27-8,29, 161 pyroligneous liquid, 28 pyrol ysis, 28
pyrotechnology: ancient vs. modern practices, 7; development of, 3-5; forest consumption, 8, 153-9; use of term, 4 quench hardening, 138, 13 9, 197 quicklime, 3, 7, }1, 47, 146, 18 3 raceway, 69 reconstrnctions and replicas, 5; of bowl or hearth furnaces, 85; of crucible furnaces, 91; of shaft furnaces, 129,189-94; unreliability of, 73, 13 7 red alder, 29 reducing atmosphere, 34,35,44,89,1)8; in charcoal combustion, 60, 64, 67, 173 reduction, 101-9, 102, 197; mechanism of, 102-4 refractory basin, 98 refractory wall, 23-4 Rehder, J.E., 104-8 re-melting, 197 reverberatory (air) furnaces, 42-4, 43, 52-4,75,106,13 1 Reynolds number, 177 rice hulls, 29 Riden, P., 158 Rohrig, E., 148 Romans, 47, 138, 153: iron smelting, 80, 128,151; lime kilns, 146 runaway condition, 35-6 saggar furnace, 44 sandstone, 22 serpentine, 22 shaft furnaces, 92-100; advantages, 92; archeological remains, 162; brown coalchar fuelled, 18; charcoal fuelled, 18; coke fuelled, 17,98; combustion air, 74; continuous operation, 14-15,92; diameter, 95-6: fuel bed height, 92-3, 94; heat loss, 24; iron production rales, 99; measurements in, 73; mixing action, 98; modern replicas, 129, I89-94; natural draft, 75; residence time, 93-5; shape, 95; size and shape, 15-16; space velocity, 19,94, _95, 124i tuyeres, 67,71,95;
use for carbothermic smelting, 104, 108; use for copper smelting, I I 5; use for lime manufacture, 48-9; use for melting materials, 93; walls, 22, 24; wood fuelled, 173 Shockley, W.H., 105
215
Index
short, 197 shrubs, 25 siderite (iron carbonate), 149, 18} silica, 126, 1}6; in ash, 31; flux, 54, 116; in slags, 50, 109, 112 silicates, 101 silico-aluminate, 47 silicon, 141, 142 silver, II}, "56-7 slags, metallurgical, 108-12, 197; analysis of, Ill, 158, 163; as archeological remains, 151, 157, 163; fluidity temperature curve, 163; fluidity vs. viscosity, 109,110-13: formation, 109-10: link with glassmaking, 50, 109, 1I0; melting point, I l l , 163; properties, "09; ratio to metals, 9}; refractory basins for, 98; role of, 108-9; similarity to basalts, 54: tapholes for, 97,127,129,162; viscosity temperature curve, 110 smelting, 147,197-8; carbothermic process, 104-8; in crucible furnaces, 91; location of operations, 100, 155; mythology of, 100: time factors, 103; use of biomass fuel, 52-4; use of wood fuel, 173-4. See also specific metals (copper, iron, etc.) smelting hearth (Catalan forge), 70, 87-9 smoke-hole, 198 soda, 50 sodium oxide, 31, 112 softwoods, 26, 188; charcoal made from, 56 ,5 8 ,59
space velocity, 16,17-19,18, 127,186; in antiquity, 19, 64, 172; of charcoal combustion, 64, 65, 66; of hearth or bowl furnaces, 85; limitations on, 18-19,20, 169-70; role of fuel lump size, 18, 66; in shaft furnaces, 19,94,95,124; as temperature control, 20
Sweden, 128, 173-4 swords, I}6, "40, 141 tap-holes, 97,127,129, r<-;2 tar, 28 Taruga (site, Nigeria), 81 Tatara furnaces (Japan), 95,123,186 temperature: control, 20-1; maximum attainable, 7, TO-II, I5, 40-1, 76,81,
162; measurement, 11-12, 73 temper carbon, 142 Theophilus, sr, 1}8 Theophrastus, 6, 58 thermal efficiency: of batch vs. continuous operation, 14; of charcoal vs. wood, 173-4
Tillman, D.A., 27 Timna (site), 120 tin,
I
13
tin smelting, 104 titanium oxide, 112 transport of fuel, 61, 153, 154, 155 tuyeres: artifactual evidence, 162, 163; blockage by slag, 70,97,127; for blowpipes vs. bellows, 77; in Catalan forge, 87-9; in charcoal furnaces, 66-72: cone-shaped exterior end, 73; diameter, 69,71,77: effect of wind on, 81, 82-3, 184; fluxing of, 7 I, 183; joints with bellows, 79; length, 7°-2,176; multiple, 129; in natural draft furnaces, 701,81,182,183; placement, 71-2, 85; plumes from, 68; pressure drop in, 70, 175-9,182; shape, 70; in wind furnaces, 185, 186 tuyere velocity, 68, 69, 8r, 171,176; in hearth and bowl furnaces, 85, 86; in natural draft furnaces, I 8 2; in shaft furnaces, 95 Tylecote, R.E, 81, 113, 133; copper smelt-
specific air rate, 17,1"71"
ing experinleIlt, 119; experiulental
Sri Lanka, 82,95,185-6 steel, 53, 13 6, 139, 140, 142 stock column, 198; density, 19 straw (as fuel), 25, 29,36-7,45; ash, 36; heat content, 30, 45: water content, 26 Stuckofen furnaces (Germany), 12} sulphides, 101, "13-14 sulphur, 109, 115-16, 134, 158 sulphur dioxide, I I 5 Sumer,5 superheat, 96, Il7
bloomery furnace, 99, 125, 126, 129, 130-I, "49, 168, 178; reconstruction of crucible furnace, 9 I Varde (Denmark), 66 Varoufakis, G.J., 108 Venturi orifice, l } I Vickers Hardness Number (VHN), 139, 197 void fraction, I7: of biomass fueis, 25-6, 31, }2; of charcoal, 58, 177-8; of dung, 36
216
Index
volatile matter (VM), 26; of biomass fuels, 28-30; of charcoal, 60, 168; relation to flame size, 32 volcanoes, 54 wall effect, 16-17, 67, 170 walls: boshes, 95: flow rate close to, 98; heat loss through, 23, 24: material, 22: sealing of, 33, 181 walnut shells, 29 water content: of air, 168-70: of biomass fuel, 25, 26, 27, 28, 188; of human breath, 75 water power, 69, 80, 128,151 Wertime, Steven E, 4 Wertime, Theodore A., 4, 120 western hemlock, 29 white cast iron, 124, 140-1, 142, 143 white-heart malleable iron, 141-2 Whittaker, R.H., 154 ,vind, 81, 82, 18}-4 wind furnaces, 81, 82-3, 184-7
wood: charcoal yield from, 56, 145, 158: difficulty of felling trees, 57: ray structure, 57 wooden furnaces, 92 wood fuel, 25, 26: bulk density, 30, 173; carbonization to charcoal, 173-4; heat content, 30-1, 59, 60-1, 172: use in smelting furnaces, 173-4. See also biomass fuels
Woodwell, G.M., 154 Wootz steel, 136, 140, 141 work chamber, 39, 40-2 wrought iron, 139, 158 wustite, 35,136 Xenophon, 5 Yarim Tepe (site), 39 Yoruba tribe (Nigeria), 129 3 Zulu-Kafir tribe (Africa), 106
Zll1C, I I
