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| ISBN-13: | 9780752478593 |
|---|---|
| Publisher: | The History Press |
| Publication date: | 11/30/2011 |
| Sold by: | Barnes & Noble |
| Format: | eBook |
| Pages: | 160 |
| File size: | 7 MB |
| Age Range: | 18 Years |
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The Roman Iron Industry in Britain
By David Sim
The History Press
Copyright © 2012 David SimAll rights reserved.
ISBN: 978-0-7524-7859-3
CHAPTER 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.
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 1 Efficiency 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.
CHAPTER 2Charcoal
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.
(Continues...)
Excerpted from The Roman Iron Industry in Britain by David Sim. Copyright © 2012 David Sim. Excerpted by permission of The History Press.
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Table of Contents
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,