The Roman Iron Industry in Britain E-Book

David Sim

• Herausgeber: The History Press
• Kategorie: Geisteswissenschaft
• Sprache: Englisch

Veröffentlichungsjahr: 2011

Beschreibung

The invasion of AD 43 began the Romans' settlement of Britain. The Romans brought with them a level of expertise that raised iron production in Britain from small localised sites to an enormous industry. Rome thrived on war and iron was vital to the Roman military establishment as well as to the civil population. In this pioneering work, David Sim combines current ideas of iron-making in Roman times with experimental archaeology. The Roman Iron Industry in Britain stretches far beyond dry theory and metallurgy alone; it covers all the stages of this essential process, from prospecting to distribution, and describes the whole cycle of iron production. Photographs and line drawings illustrate the text well enough to allow keen readers to reproduce the artefacts for themselves. Fascinating to the general reader and all those with an interest in Roman history, this book is invaluable to students of archaeology and professional archaeologists alike. Dr David Sim is an archaeologist who has combined studies of the technology of the Roman Empire with his skills as a blacksmith.

Leseprobe

CONTENTS

Title Page

List of illustrations and tables

Acknowledgements

Foreword

Prologue

1 An overview of the Roman iron industry

2 Charcoal

3 Smelting

4 The blacksmith

5 Bloomsmithing and barsmithing

6 Artefact production

7 The use of steel

8 Mechanical processing

Epilogue

Glossary

Appendix: Museums containing Roman metalwork in the United Kingdom

Bibliography

Plate Section

Copyright

LIST OF ILLUSTRATIONSAND TABLES

Text figures

1 Overview of the Roman iron industry

2 Layout of the Roman iron making settlement at Bardown

3 Cleere’s classification of early iron smelting furnaces

4 Schematic diagram showing a section of a shaft furnace

5 Reconstruction of one of the furnaces found at Laxton

6 Roman relief sculpture of a blacksmith at his forge

7 Pottery fragment found at Corbridge showing a blacksmith at work

8 Hephaestos in his forge assisted by three strikers

9 A blacksmith’s tools

10 Anvil from Pompeii (a), anvil from Britain (b), bicked anvil (c)

11 Principles of upsetting

12 Bending a bar

13 Cutting iron using a set

14 Micrograph of a piece of bloom

15 Method of constraining bloom during fire welding

16 Billet found at Newstead fort

17 X-ray of billet found at Newstead fort

18 Micrograph of a consolidated bloom

19 Micrograph of a piece of Roman iron

20 Legionary of the first half of the first century AD

21 Legionary of the second half of the first century AD

22 Aquilifer of the mid-first century AD

23 Pole lathe

24 Stages in the manufacture of an arrowhead

25 Different types of plumbatae

26 Stages in the manufacture of a plumbata head

27 Wooden pattern for a plumbata mould

28 Plumbata in mould ready for lead casting

29 Finished plumbatae

30 Stages in the manufacture of a square pyramid-type bolt head

31 Modern copy of a plumbata

32 Development of the frustum of a cone

33 Development cut to shape from copper sheet

34 Copper sheet folded into a frustum of a cone

35 Two cones formed

36 Two cones soldered together

37 Two cones separated down the long axis

38 The finished mould

39 Casting produced from metal mould

40 The finished mould

41 Casting produced from metal mould

42 Two halves of a cylindrical copper mould

43 Cylindrical casting

44 Casting after hammering

45 The method of casting by burring in sand

46 Cast weight and mould

47 Stages in the manufacture of a pilum

48 Original Roman fire arrow

49 Stages in the manufacture of a fire arrow

50 Modern reproduction

51 Original Roman caltrops from Wroxeter

52 Schematic dimensional drawing of the Roman Caltrop

53 Replica caltrop made by the author

54 Stages in the manufacture of a caltrop

55 Stake before being driven into the ground

56 Only the barbed spike is visible above ground level

57 Seqence of operations to forge the spike

58 Stages in the manufacture of a gladius

59 Various arrangements of central core and edging strips of a pattern-welded sword

60 Stages in the manufacture of a pattern-welded sword

61 Basic construction of a legionary shield

62 Micrograph section of lorica segmentata

63 Stages in the manufacture of a shield boss

64 Basic designs of chain mail

65 Unit cell of chain mail

66 Production of punch and die set for internal holes in ‘solid’ rings

67 Production of punch and die set for punching out the ‘solid’ rings

68 Draw-plate from Vindolanda

69 Producing tapered holes in a draw-plate

70 Stages in the manufacture of an axe

71 Types of dolabrae

72 Roman knives

73 Roman hammer from Silchester

74 Cross-sections showing the hafting of modern and Roman hammers

75 Two cutlers at work from a mural of Vulcan’s forge

76 Stages in the manufacture of a hammer

77 Inchtuthil nail types as classified by Angus et al.

78 Stages in the manufacture of a nail

79 Micrograph of the head of an Inchtuthil nail

80 Micrograph of the point of an Inchtuthil nail

81 Micrographs of 0.1–1.3%C steels

82 Depth of carburisation of iron as a function of time and temperature

83 Types of water wheel

84 Vertical axis mill

85 Simple right-angled drive

86 Simple right-angled drives using pin gears

87 Cross-section of ore grinding mill

88 Water-powered ore mill

89 Roman edge runner mola olearia

90 Ore stamp from Agricola

91 Linear movement from a rotary power source

92 Hero’s windmill for pumping air

93 Forge hammer

94 Water-powered trip hammer

95 Roman stonemason’s lathe

96 Grinding wheel based on a lathe drive

Colour plates

1 Iron ore (haematite)

2 Reproduction of a Roman sword made using traditional methods

3 Cherub smiths attending a furnace, from a painting in the House of the Vettii, Pompeii

4 Cherub smiths forging, from a painting in the House of the Vettii, Pompeii

5 Fire welding in Ironstone forge

6 Section through a piece of bloom iron

7 Section through a piece of consolidated bloom iron

8 Iron billets from Newstead

9 Iron billets from Haithabu

10 Roman military parade

11 Roman commander, bodyguards and standard-bearer

12 Arrowheads from Vindolanda

13 Spearheads from Vindolanda

14 Socketed spearhead

15 Roman auxiliary archer

16 Manuballista

17 Three-span catapulta

18 Pyramid ballista bolts from Vindolanda

19 Reproduction javelin head, bent during field trials

20 Reproduction gladius

21 Auxiliary cavalryman of the mid-first century AD

22 Roman legionaries with shields

23 Bronze Roman chain mail

24 Iron Roman chain mail

25 Roman iron tyre on a cartwheel

26 Detail of an iron tyre

27 Roman hammer head made from iron billet

28 Reproduction Inchtuthil nails

29 Nails from the Inchtuthil hoard

30 Nail with poorly centred head from the Inchtuthil hoard

31 Steel masonry tie from Pompeii

Tables

1 Efficiency of the various processes involved in iron production

2 Colour of iron heated to different temperatures in a forge

3 Size and weight of iron billets found at Legionary fortresses

4 Duration of forging operations during production of stock sized bar from a billet

5 Forging times for sheets of iron from bar stock

6 Experiments with various media used to make moulds

7 Forging and finishing times for disposable weapons

8 Estimated production times for nails

9 Comparison of material weight loss in open hearth and enclosed hearths

ACKNOWLEDGEMENTS

Writing a book is a great undertaking, and one which could have only been achieved with the help and support of my family, friends and colleagues. I would like to extend my thanks to the following individuals and organisations for making this volume possible: Dr Henry Blythe and Dr Alan Williams; John Frew for his cheerful help with sample preparation, testing and photography over the years; Barry Winfield, Heather and Owen Hazelby, and Laura Dodd.

Alan Wilkins for his help with the classical texts; Cohort Secundo Augusta and David Richardson for their generous permission to reproduce colour plates 10, 11, 15, 21 and 22, The Roman Military Research Society for kindly allowing the use of colour plates 16 and 17; Alan Wilkins and Len Morgan for their expert advice on ballistas; the Trustees of the National Museums of Scotland for kindly allowing the use of figure 19; Robin and Pat Birley and the Vindolanda Trust for generously permitting the examination of various Roman iron artefacts over the years, and for the use of colour plates 12, 13 and 18 and figures 40 and 46; and Dr Mike Bishop for allowing the use of his collection of museum data published in the Appendix. I would also like to acknowledge the financial support for the some of the research reported here received from the Leverhulme Trust.

There are many others who have been involved in differing ways with the preparation of this book and while it has not been possible to name them all and list their kindnesses individually, their support is also gratefully recognised.

Special acknowledgement and thanks are made to the following: Dr Jamie Kaminski for his expert knowledge of the Roman charcoal industry, who has contributed the chapter on charcoal production, and additionally assisted with the preparation of the manuscript; Professor Michael Fulford for his support and guidance; and Professor Richard Chaplin for allowing the use of the facilities in the Department of Engineering.

FOREWORD

The inspiration for this book was the desire to produce a single volume, which covered the whole Roman-iron making process, from the first stage of finding the ore right through to the completion of the artefact. Other books have been written focusing in great detail upon some aspect or another of iron production in antiquity, and these have provided valuable sources of information. In order to keep to a manageable size, I have chosen to focus on the British iron industry, although due reference is made to the influence of the rest of the Roman empire where appropriate. Similarly, I have adopted as a theme the production of the familiar arms and armour of a Roman soldier. The basic techniques used in the production of these items will however be the same for all iron artefacts.

This book does not claim to be the final word on its topic; many aspects of the Roman iron industry are the subject of current study by experimental archaeologists, archaeologists, archeometallurgists and classicists, and new information is becoming available all the time. This study probably raises as many questions as it answers. I have highlighted the huge amount that is not known, and discuss some alternative procedures to the traditionally held views of production. It is hoped that, as well as giving information and pleasure, this book may stimulate some readers to join the group who work on this fascinating subject, and so help provide answers to the many problems that remain unsolved.

It has been difficult to strike a balance between saturating the text with references, making it unreadable in the process, and giving due credit to the many other researchers. I have aimed to be thorough and comprehensive in my acknowledgement of both early and recent work in the field, but inevitably some errors or omissions may remain. For these I accept full responsibility.

David Sim

Reading

November 2011

PROLOGUE

On a warm summer afternoon in the year AD 128, just south of the great wall built by Hadrian, a Roman blacksmith is standing outside his workshop. He has just completed a full set of arms and armour, and is making a tally to check that everything he has been asked to make is accounted for. He is a legionary and a superb fighting man, but he is also proud of his skills as a blacksmith, and the sight of the equipment that he has made is a boost to his self-esteem. He has often thought that people never seem to understand either how much work goes into making things, or how important it is to be well-organised. For anything to be made, he has to make sure the iron, charcoal, tools and the other smiths are all in the same place at the same time. Nothing is left to chance; no one will ever say he forgot part of an order. On his wax tablet he marks off the following items: one helmet, one set of body armour, one shield boss, one sword, one dagger, two pila heads, one pick axe, total weight 20kg of iron. His gaze wanders over the camp and his eyes come to rest on the temple where the standard of the Legion is kept, and he remembers that day all those years ago when as a young man full of pride he swore his oath to the eagle of his Legion. His reverie is disturbed by the sight of several civilian men unloading from the mules the billets of iron he ordered. The billets are heavy, and most of the group can only manage to carry two or three at the most. One man calls out to him,

‘Blacksmith, what’s this lot for?’

And he replies: ‘It’s iron for the Eagles’

In this fictional account we meet the finished product of the blacksmith’s art, namely the weapons and armour that will be used by the soldiers. These items represent the end of a long story that involves large amounts of both natural resources and manpower expended to transform iron ore into the arms and armour of a Roman legionary. It is the purpose of this book to reveal the complicated and fascinating processes that have to be conducted to make such iron artefacts.

Our Roman blacksmith stands at the end of a long tradition of craftsmen, whose skills go back to the very start of man’s use of metal. The first metalworkers hammered the metal to change its shape. This is the process known as forging, and copper and later bronze were fashioned in the same way. For more than a thousand years before iron was discovered, the men who forged copper and bronze were highly skilled in their art; and it was these craftsmen who were the first to forge iron, once it became available in large enough quantities. The discovery of iron, and later steel, changed everything. Steel could be made harder than any other known metal, and thus could be used to cut almost every material; even very hard stone could not resist an edge made of hard steel.

By the time of the Romans, the blacksmith’s tools had reached their terminal stage of development; indeed they were in every respect almost identical to those used by blacksmiths today. Even the advent of electricity in the modern era has only meant that electric rotary fans have replaced traditional hand pumped bellows.

Quite often, iron artefacts from the archaeological record are no more than a fragile mass of corrosion products (rust), and in this condition they are neither attractive nor stimulating to those who are not enthusiasts. It is likely that this unappealing appearance has led to the importance of iron being underestimated. The lack of interest has also meant that little attention has been given to how iron artefacts came into being in the first place. Little thought has gone into considering the immense technical problems that had been overcome, and the huge amount of resources in terms of materials and man power needed to obtain a piece of iron ore (colour plate 1) and transform it into an item such as a sword (colour plate 2).

This book aims to take you back into that forgotten world through the different processes involved in the vast iron industry that, as we shall see, must have employed thousands of people. Although considering examples and drawing on information from throughout the Roman Empire, this book will deal mainly with the production of iron in Britain. In later chapters discussing the manufacture of artefacts I will consider a range of items that our Roman legionary would have created. However, the basic techniques that he would have employed would be the same for military and civilian blacksmith alike.

1

AN OVERVIEW OF THE ROMAN IRON INDUSTRY

I start with a tale from blacksmithing mythology. When the temple at Jerusalem was completed, King Solomon was so pleased with the work he decided to confer the title Father of craftsmen on one of the artisans who had helped to build it. All the craftsmen were summoned to appear before the King and give an account of why the honour should be theirs; this meeting was to be followed by a banquet with the chosen craftsman as the guest of honour.

Accordingly each man stated his case. The stonemason said, ‘If I had not cut the stone there would be no building’. The carpenter said, ‘If I had not carved the wood there would be no doors or fittings or furniture’. The tiler said, ‘Without me there would be no beautiful floors’, and the weaver said, ‘Without me there would be no wall hangings.’ So each in turn spoke, until last of all they came to the blacksmith, who said, ‘I am the Father of craftsmen, because I make all the tools for the others and without me none of them would be able to perform their skill’.

Solomon saw the truth of this and conferred the title Father of craftsmen on the blacksmith. (The King’s tailor was so outraged that he crawled under the banqueting table and snipped pieces out of the blacksmith’s apron with his scissors, which is why to this day all blacksmiths have a fringe at the bottom of their aprons.)

This tale helps us to understand how essential iron was to ancient societies. We will see later in this chapter both the huge variety of iron items which existed in Roman times, and which relied on the skill of the iron workers for their production, and the vast scale of the industry.

The Roman world

While it is not the place of this study to detail what life was like in the Roman world – and there are other excellent books on this very topic – it is important that the reader should have at least a general picture. In the past romantic ideals, or perhaps a wish not to cause offence, has restrained authors from portraying in lurid detail some aspects of Roman ‘culture’. However, in order to gain a correct appreciation of the way in which Roman life operated, it is necessary to make some fundamental observations. Rome existed for war; indeed it may be said that to the Romans, as to the Greeks, peace interfered with war (Hanson Davis 1989). Rome conquered with its military might and controlled its conquests with the same military machine. It was a society that was ruled by a small ruling elite, and their world was divided into two: those who lived within the empire, and the barbarians who were outside it, and therefore of no consequence.

The role of iron in the Roman world

The importance of iron in the Roman world cannot be over-emphasised. It can be equated with the dependence of the modern world on steel, and one would be hard-pressed to find any manufacturing activity in the ancient world that did not rely in some way on iron. Manning (1985), in the contents of his catalogue, classified the following iron artefacts:

Tradesmen’s tools embracing:

metalworking, (blacksmithing, copper and bronze-smithing, also jewellery making (goldsmithing and silversmithing))

the timber industry: woodland management, carpentry, fine cabinet making

stone working: quarrying, stone dressing, carving and sculpting

plasterers’ tools;

tools for processing wool and cloth, leather working (tanning, cobling);

agricultural tools

farriers’ tools

mining

Other equipment for daily activities such as:

transport: land, river and sea

surgical instruments

domestic equipment: knives, razors and cleavers, locks and keys, styli, toilet implements

structural fittings for both domestic and civil buildings

armour and military equipment, weapons, shackles

This is not a complete list, but it serves to show how vital iron was to the functioning of society in the Roman world.

Overview of the technical aspects of iron making

The production of iron artefacts is a complex task, dependent upon not only the raw materials, but also the skills of the craftsmen involved in all aspects of the iron industry. Figure 1 shows schematically an overview of the iron production process. Initially the iron ore must be found (prospected) and then mined, before being prepared for smelting by roasting. The activities of mining, roasting and smelting require considerable input from other resources: notably wood (for both fuel and structural purposes) but also manpower, with the attendant organisation. Once the ore has been mined, roasted and smelted to produce the bloom, it again requires considerable processing and support in terms of fuel and labour to refine the bloom to bar ready for the blacksmith to manufacture the final product. The various stages that are covered in detail in the following chapters in this book are identified here.

1Overview of the Roman iron industry

Prospecting

The first stage in the production of an iron artefact was to find a source of iron ore. The prospector relied on the surface appearance of rocks and soil to indicate the presence of mineral ores. He had to be able to recognise the visual signs, such as the type of vegetation, which could be an indicator of the kinds of minerals present. With little in the way of scientific method available, experience was the prospector’s most valuable asset.

Mining

Mining, or the extraction of the ore, was a major undertaking. The Romans established a system of control on many mines to ensure that they were run for the benefit of the Empire. The simplest form of mine, open cast, was essentially a pit that was open to the elements. This type has many advantages over the more traditional image evoked (a shaft leading below ground from which various galleries and workings follow the ore seam), the most direct being the immediacy of return upon effort: the material could be dug from the ground straight away, with little or no preparation. There was also no need for lighting, ventilation, drainage and shoring (of the galleries and shafts), and it would have been easier to remove the ore from a pit.

Having said all this, the Romans did create mines accessed by shafts; but these tended to be reserved for high value (e.g. silver or gold) or high quality (yield) ores. An intermediate technique of mining was bell pitting, in which an access shaft was dug, and the seam excavated in a cylinder around the shaft until the roof became unsafe, at which point it was backfilled and another shaft dug nearby to repeat the process.

Preparation of the ore – washing and roasting

Once the ore had been removed from the ground it was first washed to remove any excess material such as clay. It was then roasted, which served several functions: it dried the ore and made it more porous, and also made it easier to break the ores into smaller pieces needed for smelting.

Fuel

The single most important fuel in the ancient world was charcoal, the residue obtained as a result of the incomplete combustion of either vegetable or animal materials, but most usually of wood. At the end of the process the material has lost both volume and weight and is almost pure carbon: for example if wood is used, the volume is reduced by one third and the weight is approximately one quarter of the original. Charcoal is capable of burning at a temperature of 900°C, but the use of an air blast will enable temperatures of 1600°C to be reached in a furnace. It was the principal fuel for the smelting of iron.

It should be noted that although the use of coal is recorded on many Roman sites, and was readily available in some locations, it is not suitable for smelting iron because the sulphur content makes the metal too brittle for forging. Coal was utilised however in other industrial processes, and certainly can be used for the forging of iron; as a superior fuel, there is no doubt the smiths exploited this. In fact, in most places where mineral coal outcrops it was mined by the Romans.

Smelting

Ancient iron production differs in many ways from its modern counterpart, and so the two basic processes are outlined here in order to avoid confusion between ancient and modern methods. The processes are called the direct method, or bloomery process, which was used in antiquity, and the modern indirect method employed today. The latter is a relatively modern process in which liquid iron is produced from the ore in a blast furnace at a high temperature, and then further processed into other ferrous products such as cast iron and steel. In the direct method, the ore is heated at a temperature below that at which iron melts, but high enough that the unwanted mineral content of the ore (termed slag) can become liquid and run out. The ore undergoes a chemical reaction with the gas produced by burning the charcoal, and this reduces the ore to iron. Thus the iron is extracted both physically and chemically, and a sponge-like bloom of iron is produced for further refining. This process was used in Europe from the time iron was first smelted until the fifteenth century.

Refining the bloom – bloomsmithing

The product of the smelting process is called a bloom, which is a mass of pure iron separated by slag. In this condition the iron is unworkable, and has to be refined to produce the material which can be forged into artefacts. The objective of the next stage in the cycle is therefore the removal of slag and the welding of the iron into a solid mass or billet.

For welding to occur, the surfaces to be joined must be clean and have as little oxide on them as possible; iron oxide has a higher melting point than iron and acts as a barrier between the pieces to be welded. The presence of slag between the particles of iron also prevents the iron from welding. However, the slag also binds the bloom together and keeps it as a solid mass, so needs to be removed in a way that prevents the bloom collapsing as a result. This process is called bloomsmithing – heating and hammering the bloom to expel the slag and consolidate the iron by welding the particles together.

Smithing a billet to a bar – barsmithing

A billet is often too large to be of any use for artefact production save when large objects such as hammers are to be made. For smaller items the iron billet would have been forged down to bars of various sizes and cross-sections (square, round, rectangular, etc.) depending on the type of work the smith was conducting. Semi-skilled workers or apprentices would have carried out this task, as it requires only basic skills.

The bars would have been of what is called in smithing jargon ‘a handling length’ – long enough to be held comfortably while working on the heated end. A smith only uses tongs when there is no alternative and it is always preferable to work on the end of a bar.

Artefact production

The bar is now ready to be turned into an artefact and, as we have already seen, the range of iron items from the Roman era is astounding. There was hardly any activity that did not make use of iron in some part of its operation. Many artefacts could be produced by semi-skilled labourers, who might be employed in making large numbers of the same item such as nails but many other items needed the skills of an experienced blacksmith to bring them to realisation. Iron was a material that was in constant demand, not only for new objects such as weapons and tools but also to replace existing items, which will either have been lost, broken, stolen or simply worn out.

Chapter 7 describes the production of a variety of weapons and tools in common use in the Roman world. The function of these items would have been specific, but the techniques employed to produce the various forms would have been used for the full range of items discussed earlier.

The production and heat treatments of steel (the iron-carbon alloy) are discussed in chapter 8. It will be seen that the Romans made full use of the alloy and appreciated its enhanced strength and hardness.

The final chapter discusses the possibilities of the use of mechanical processing to speed up the many processes which will be described. It will be seen throughout that the Roman iron industry must have been of considerable importance and size to produce enough for everyday needs.

The size of the British iron industry

Although Britain did export some iron, most was mined for the manufacture of artefacts to be used in Britain. Davies (1935: 140) notes that there was a very good supply of iron ore in Gaul that would have been much easier to export to the rest of continental Europe should it be required. If we accept this fact, it is a great help in sizing the British iron industry as a whole. Aiano (1975: 40–1) places a ‘conservative estimate’ of the annual iron consumption in Roman Britain at 1.5 kg/head. This figure is based on the assumption that the need would be rather lower than the 4.5 kg/head that he quotes for the seventeenth century; and from this he goes on to project an annual output of 2250 tonnes (with an assumed population of 1.5 million).

More recent work by Millet (1990: 185) puts the population of Britain at that time at nearer 3.6 million, which would equate to 5400 tonnes per year: whichever figure is more correct, it can be seen that the volume of ferrous products used was considerable. Healy (1978: 196) estimates that the consumption of the Empire as a whole was 82,500 tonnes per annum.

The iron industry required a great deal of support and organisation. Using the figure of 5400 tonnes, we can work backwards to obtain an estimate of the mass of raw materials needed to produce this final output. It will be shown in later chapters that the extraction of iron from the ore and each stage of the subsequent processing entails loss of material, as summarised in Table 1.

Table 1Efficiency of the various processes involved in iron production

Process

Yield (%)

Extraction of bloom from ore*

20

Consolidate bloom

50

Smith billet

80

Forge bar

80

Forge artefact

90

* This assumes a high-grade ore with 50% iron content

Hence a total of 5400 tonnes of finished product would require 93,750 tonnes of ore to be mined and processed, which would need 112,500 tonnes of charcoal just to smelt it (Crew 1998: 51), the equivalent of 787,500 tonnes of wood (Cleere 1976: 240). Further processing would require even more charcoal – not a trivial matter, and one discussed in chapter 3.

The volume of labour needed to support this level of production would have been high, and is very difficult to estimate accurately. Based on his experimental work, Sim (1994: 393) calculates a figure equivalent to about 50,000 men just for the smelting and bloomsmithing operations. If we treble this figure to allow for the barsmithing and artefact production, as well as mining, charcoal production and overall administration of these activities, the industry as a whole would have required at least 150,000 men, which is equivalent to 4.2% of the total population (based on Millet’s figures). In addition to this direct involvement, the support industries required to feed, clothe and shelter this workforce indicate the level of organisation and importance which must have been attached to the iron industry.

2

CHARCOAL

Of all the impacts associated with iron production in pre-Industrial Revolution society, those related to the generation of the fuel have achieved the widest attention. Prior to the discovery of the conversion of coal to coke by Abraham Derby in 1709, charcoal was the only major fuel available for industrial operations such as smelting. This is because the fuel needs for the iron smelting process are highly specific. The impurities found in coal, such as sulphur, can contaminate iron smelted with it, and dry wood alone could not attain the high temperatures required for smelting. To compensate for this, wood needed to be converted to charcoal, the carbon residue created when wood is heated without sufficient air for complete combustion. Charcoalification results in the removal first of water, then the volatile compounds. Wood converted to charcoal has two functions in iron production.

Firstly, it provides an excellent source of heat for smelting. The absence of water in charcoal compared to ‘dry’ wood results in a hotter, more easily controlled heat; ‘dry’ wood produces an inconsistent temperature during combustion because of the vaporisation of internal moisture. Secondly, in the context of bloomery iron production, charcoal represents more than just a source of heat energy; it is a source of almost pure carbon, that is converted first to carbon monoxide, then to carbon dioxide. This allows the chemical reduction of the ore during smelting.

Archaeological evidence for charcoal production

There is little evidence from the archaeological record for the method of charcoal burning used in the Roman period, but classical authors writing about the Mediterranean region suggest that both charcoal kilns and pits were used. Theophrastus in his History of Plants (V.9.4) records the progress of a charcoal burn:

They cut and require for the charcoal heap straight smooth billets: for they must be laid as close as possible for the smouldering process. When they have covered the kiln, they kindle the heap by degrees … such is the wood required for the charcoal heap.

This can be supplemented with Pliny’s account in his Natural History (XVI.8.23) of a clay structure used as a charcoal kiln. Although evidence for pit structures is also recorded, this is predominately by Greek authors such as Theophrastus in his History of Plants (IX.3.1–3), and Aelian (NA 1.8) (Olson 1991: 414).

However, archaeological evidence for charcoal production sites from any period is rare. Tylecote (1986: 225) suggests that charring pits, or ‘pit-steads’, have been recovered from an Early Bronze Age context in Mildenhall, East Anglia. He also tentatively suggests that trenches found on the Roman iron production site at Wakerley could be examples of pit-steads. There is currently no archaeological evidence from the Weald or the Forest of Dean for pit kilns but a possible example of the remnants of an above-ground charcoal kiln, or clamp, has been recovered from the Weald, at the Romano-British industrial complex at Bardown (Cleere 1970: 15). Here a 3m diameter area of Ashdown Sand had been baked to a depth of 1–2cm. A little charcoal was found in association with the area, and the feature was sealed by the construction of a slag-metalled road. It is possible that the burning horizon could be the archaeological manifestation of a charcoal clamp that was originally at the periphery of the Bardown site, but later became incorporated into the industrial area. It is likely that the major form of charcoal production would have been through the utilisation of heaps and kilns.

The two methods of charcoal production each have their own advantages and disadvantages. The creation of a pit requires a significant input of labour, unless a minepit or natural hollow could be modified; however, the same pit can be reused and, if several episodes of such reuse are undertaken on a single site, the resultant baking of the pit walls would serve to prevent contamination of the charcoal. In contrast, the above-ground clamp would be destroyed after every episode of charring and the major advantage of this method derives instead from its mobility: the simplicity of clamp construction allows it to be constructed near, or at, the site of wood cutting.

The rarity of evidence for charcoal production in the archaeological record is not surprising, given that the process would have occurred in woodlands and forests often at some distance from the sites of iron production which tend to be the focus of excavation. Charcoal production is also a mobile activity, that follows the available woodland resources and is dependent on the demand from iron producers; and this mobility, in conjunction with the low weight of charcoal, is not conducive to the development of significant infrastructure, as it does not require (for example) the metalled trackways associated with other off-site activities such as iron ore mining.

Similarly, the primary components of a kiln would leave little trace in the archaeological record: when the charcoal is removed, the kiln wall is destroyed. In an ideal context the only remaining evidence for charcoal production would be the area of intensely fired earth or clay, carbon-rich soil in the immediate vicinity of the kiln and, in exceptional circumstances, burnt kiln lining and material culture. However, the absence of above-ground features means that sites only tend to be revealed when the carbon-rich soil is exposed by ploughing, which itself almost inevitably results in the degradation or destruction of the site.

It is also quite difficult to distinguish between the archaeological remains of a charcoal clamp and a bonfire; both have a similar signature of carbon rich soil. By their very nature, charcoal production sites are not associated with much diagnostic material culture due to the short periods of occupation and the mobile nature of charcoal burners, who are unlikely to travel with little more than perishable belongings. Bonfire sites are also unlikely to have much material culture, and in the absence of such evidence, the dating and interpretation of these charcoal scatters is highly problematical because the ambiguous stratigraphy of these sites, in addition to the cost, deters the use of radiocarbon determination. The major difficulties associated with the recovery of these sites therefore include the low archaeological visibility, the ambiguity of interpretation of charcoal production and bonfire sites and the problem of dating.