This multimedia textbook was originally intended for Chinese students of archaeology, but I hope it will also prove useful for Western students, especially those who wish to know more about the Chinese scene. It introduces the necessary background for reading and understanding archaeometallurgical publications, and provides a broad foundation for students who wish to go further in archaeometallurgy. Readers will profit from some knowledge of chemistry and physics, but I have in general tried to avoid too much technical detail. Students preparing for original research in archaeometallurgy will need more advanced training, especially in chemistry and the science of metallurgy.
I have written this textbook in collaboration with the Department of Archaeology of Sichuan University, where it is being translated into Chinese.
This is hardly a complete textbook of the archaeometallurgy of iron. There are several important subjects that I have barely touched, including meteoritic iron, mining, and ore preparation. The most important lack is the subject of slag – the slag found by archaeologists on production sites and the microscopic slag inclusions found in iron artefacts. Slag has in the last decade or so become a very important material for understanding ancient production methods. But I know too little about these subjects. After working on the textbook on and off for almost ten years, I shy away from spending more years learning enough to write authoritatively about them. I hope others will take up the challenge.
I suggest that Chinese students of archaeometallurgy should read these four publications:
An interesting book by one of the most important British archaeometallurgists is:
泰利柯特，世界冶金发展史，华觉明编、译，科学技术文献出版社，1985年. (Original: R. F. Tylecote, A history of metallurgy, 1976).
This major compilation will be consulted by everyone concerned with the archaeology of iron in China:
李映福 、 马春燕. 中国古代物质文化史。铁器。北京， 开明出版社， 2019.
These publications of mine have been translated into Chinese:
华道安， 中国古代钢铁技术史，李玉牛译， 四川人民出版社，2008年。
Original: Wagner, Donald B., Iron and steel in ancient China, 1993.
Original: Wagner, Donald B. 2001. The state and the iron industry in Han China, 2001.
Readers who are comfortable with English may find it useful to read my volume for Joseph Needham’s series on science and technology in China:
Wagner, Donald B. Science and civilisation in China. Vol. 5, part 11: Ferrous metallurgy, Cambridge University Press, 2008.
Some useful preparatory readings for Chinese students are listed in the box on the right. These include publications by the pioneers of archaeometallurgy in China, and will give important background for serious students of the subject.
In this textbook I have included a good deal of material from the Internet, especially from Youtube, Internet Archive, and Facebook. Since these services are blocked in China it has often been necessary to copy material to the server, and some content providers may find this inappropriate. When taking material from printed books I have made an effort to obtain permission from the copyright owners, but these have often been difficult to find. If anyone feels that their intellectual property has been infringed here, I ask them to contact me so that we can come to a mutually satisfactory arangement. If required I will remove the offending material.
We have some problems of terminology here. How ‘indirect’ is indirect smelting? In ancient China, cast iron was probably the most important form of iron, and blast furnaces produced cast iron directly. The process is called ‘indirect’ because the most important form of iron in the West has been wrought iron, which was produced in two steps rather than the one step of bloomery iron production. Should we find other terms for the two processes? While ‘blast furnace’ is a well-defined term, ‘bloomery’ is not. Some of the furnaces we call bloomeries do not produce blooms, and some direct smelting processes, for example that of the Khasi Hills in India, operate without anything that can really be called a furnace. I have suggested calling the processes ‘continuous’ and ‘discontinuous’, but colleagues object that some 19th-century bloomeries in Sweden and North America produced bloom after bloom continuously for long periods. Two correct descriptive terms would be ‘solid-state smelting’ and ‘liquid-state smelting’, but no one uses these terms, and introducing them here would lead to confusion.
In this textbook we will primarily refer to ‘direct’ and ‘indirect’ smelting, and often refer to the direct-smelting furnaces as ‘bloomeries’.
The two historically and archaeologically important means of producing iron from ore are the bloomery and the blast furnace. In a bloomery furnace, lumps of wrought iron, called blooms, are produced in a discontinuous process. Commonly 10–50 kg of iron can be produced in a day. This iron can be used directly by a blacksmith. A blast furnace operates continuously for long periods, producing tons of cast iron per day. Cast iron has a high carbon content, and this fact has two implications: it is brittle, and cannot be used directly by a blacksmith; and its melting point is low, so that it can be cast into practical objects. If a blacksmith is to use this iron, most of its carbon content must be removed; in pre-modern production this was done in a finery, which will be discussed in Section 4 below. Because bloomery smelting produces wrought iron in a single step, it is called the direct process. The blast furnace and finery produce wrought iron in two steps, so this is called the indirect process.
Direct iron smelting
Blast furnace iron smelting
Geographic and economic factors have important implications for the organization of iron production. Iron ores, and the fuel to smelt them, are available almost everywhere in the world in quantities that are adequate for pre-modern needs. This fact and the fact that iron is cheap by weight mean that pre-modern iron industries have often been divided into sectors: small-scale production in the most isolated regions, and large-scale production in regions with access to cheap freight transport, in China typically by river. Examples of this two-sector split can be found in several periods of Chinese history, and also in parts of Europe in recent centuries.
The economics of direct and indirect iron production
Blast furnace iron production was dominant in China from very early times, perhaps as early as the 5th century BCE. In Europe, blast-furnace iron production was a late development. Here iron was being produced by direct smelting from the earliest times until well into the 19th century. The earliest known blast furnaces in Europe are from the 13th or 14th century, and it was not until the 15th century that the greater part of iron production used blast furnaces. Much has been written by historians about this development. My own work suggests that blast furnaces began to be used in Europe when the economic conditions were right for large-scale iron production, and that they then came into use almost simultaneously in many places. In Germany it seems to have been an independent invention, while in Sweden and Italy it may have been learned from Iran, where it almost certainly had been learned from China (Wagner 2007: 347–356).
No bloomeries in China?
A quite different iron-smelting technology has been used in Shanxi and to some extent in Xinjiang, Shandong, and Liaoning. It was also adopted by the Höganäs company in Sweden, and it may still be in use there.
A mixture of iron ore and mineral coal is packed in crucibles and heated with coal in a stall furnace. The coal reduces the ore, and the result is a cake of iron. This technology gives at least two advantages over blast-furnace smelting: it requires very little capital investment, and its fuel can be coal of low quality.
Crucible smelting in Shanxi
Crucible smelting in Xinjiang
Crucible smelting in Liaoning
References on crucible smelting
Fe–C equilibrium diagram
Here on the right is the iron–carbon equilibrium diagram, which gives the atomic state of iron–carbon alloys in relation to temperature. We shall discuss this diagram in considerable detail elsewhere in this textbook, but what is important here is the liquidus line. This can also be called, somewhat imprecisely, the “melting point” of an alloy. Practical casting temperatures lie 50–100°C higher than this.
We can see that casting steel, with up to about 1.5% carbon, requires temperatures well over 1500°C. Such temperatures were often reached in early times in various parts of the world, but the refractory materials necessary to work with molten steel and cast it into useful artefacts were not developed until the late 18th century, in Britain.
With a higher carbon content, 3–4%, practical casting is much easier, requiring temperatures around 1300°C. Iron with this carbon content is called cast iron because it is easy to form by casting. It lends itself well to large-scale production, which was often important in ancient China. It was undoubtedly the use of cast iron in ancient China that made it possible for every peasant to have iron implements.
Danish cupola furnace, 19th century.
Molten iron flowing from the blast furnace, containing typically 3–4% carbon, can be cast directly into moulds for end-products, and this has sometimes been done in pre-modern practice, for example at an ironworks in Yangcheng, Shanxi, and in some early European cannon-foundries. But most often the output of the blast furnace has been cast as simple ingots (‘pigs’) which are later charged into a cupola furnace. A cupola furnace is a shaft furnace, and resembles to some extent a blast furnace. It is charged with iron and fuel at the top, air is blown in near the bottom, and molten iron is tapped periodically at the bottom. Cupola furnaces have taken many different forms, and many different types of mould have been used for casting iron products.
In some places in Shanxi, iron was melted in crucibles for casting (Liu Peifeng 2014, e.g. p. 41).
Here are some films and photographs of two iron foundries in Shanxi, by a German visitor, Sergey Ershov, in April 2007:
There is more on iron casting in these web articles of mine:
Cast iron from the blast furnace has a high carbon content, generally about 4%. To make wrought iron, shutie 熟铁, the raw material used by blacksmiths, most or all of this carbon must be removed. The most common processes used in Europe until the mid-19th century were fining and puddling. Two rather similar processes have been used in China from ancient times until recent centuries. The modern words are jinglian 精炼 for fining and jiaolian 搅炼 for puddling, but traditionally both have been called chao 炒.
Both fining and puddling involve melting the cast iron and stirring it under a blast of air. In fining, the fuel, charcoal, is mixed with the iron, while in puddling the iron and the fuel are separated, so that mineral coal can be used without adding too much sulphur to the iron. If the worker is good at his job, carbon is burned out of the iron and not too much of the iron is burned.
Fining and puddling in Europe
Fining and puddling in China
A process similar to fining, also called chao 炒, is described in the book Tian gong kai wu 天工开物 from 1637. It is different enough from what seems to be the usual Chinese fining process that it deserves a separate discussion.
Conversion to wrought iron according to Tian gong kai wu 天工开物
Instead of fining the pig iron it is also possible to cast it into a plate or rod and anneal this in an oxidizing atmosphere, decarburizing it in the solid state. This is essentially the same process as that used in making whiteheart malleable cast iron objects, except that the product is intended as raw material for a smith rather than being cast in its intended final form. There seem to be some advantages to using this process instead of fining: there are few if any slag inclusions in the product and, perhaps more important, the quality of the product is less dependent on the skill of an individual artisan (the finer), so that quality control in a large ironworks is simplified. On the other hand, fuel consumption would surely be much higher than in fining. Solid-state decarburization of pig iron to produce raw material for further working was tried occasionally in Europe in the nineteenth century, but has never had much importance here (Wagner 1993: 291).
Iron plate from the Han-period ironworks site at Guxingzhen in Zhengzhou, Henan, and its metallographic structure (Kaogu xuebao 1978.1: 21, pl. 2.4).
The microstructure: etched, scale bar 125 μm. Ferrite with some scattered ceentite. The black spots are not discussed in the text; they may be temper graphite.
The evidence for the use of this technique in Han China is clear enough, though details have not yet been published. Cast-iron plates and bars, some of white cast iron and some decarburized, have been found at several Han-period ironworks sites, along with iron moulds for casting them. One trapezoidal plate, found at the Han ironworks site at Guxingzhen in Zhengzhou, Henan, has dimensions 19 × 7–10 cm, thickness 0.4 cm. From its external form there is no doubt that it was cast, but its carbon content is only 0.1%. If this plate originally had 4.3% carbon, and was annealed at 1000°C, the annealing time would perhaps have been 2–3 days. A similar plate from the same site is shown here on the right, together with its microstructure. It appears to be 100% ferrite, i.e. its carbon content is essentially zero.
There is more on this subject in Wagner 1993: 291–292; 294–295; 317 (Chinese translation, Wagner 2018: 197–198; 202–204).
(This section is largely copied from Wagner 2008: 65–73, with minor editing and some additions. I am grateful to the Needham Research Institute, Cambridge, for permission to use this material here.)
Chinese fining processes produce shu tie 熟铁, iron with carbon content generally in the range 0.1–0.3%. In modern technical terminology this is ‘mild steel’ (ruan gang 软钢), but shu tie was used in the same applications as wrought iron (with close to zero carbon) in the pre-modern West, so I follow standard English practice and translate shu tie as ‘wrought iron’.
Edged weapons and tools require a higher carbon content than this, and therefore various processes have been used to make gang 钢, ‘steel’, generally with carbon content in the range 0.5–1 per cent (in modern terminology, ‘tool steel’, gongju gang 工具钢). Chinese traditional steelmaking processes have either added carbon to wrought iron by cementation or mixed wrought iron and cast iron to obtain a product with intermediate carbon content.
Zhang Xiaoquan scissors, purchased in San Francisco in 1975. The blade has an edge of steel, about 1 mm thick, forge-welded onto a wrought-iron base.
Diagram of a section through a Chinese razor, reproduced from Middleton (1913). A hard steel edge was forge-welded onto a soft iron back.
In traditional China, iron was made either by cementation (men 焖、tiantan 添碳) or by co-fusion (guan’gang 灌钢). In cementation, low-carbon iron is heated together with carbon (normally charcoal) for a period of hours or days. Carbon diffuses into the iron from the surface. In co-fusion, cast iron (with high carbon content) and low-carbon iron are heated together, producing a product of intermediate carbon content.
Chinese smiths in recent centuries seem not to have much used ‘case-hardening’ (biaomian yinghua 表面硬化) in which a finished weapon or tool of wrought iron is cemented to produce a hard layer of steel (the ‘case’, biaoceng 表层) on a soft tough base. Instead, steel was produced separately, most often by specialised steelmakers, and the smith forge-welded this onto a wrought-iron base. This method no doubt saves fuel, and if the smith is competent the product is as good as or better than a case-hardened weapon or tool. Two examples are shown here on the right.
Lee Sauder and his friends have made a film demonstrating four traditional European steelmaking techniques, and they have provided this short preview. They have also recently placed the whole DVD on Youtube, and I have copied it here.
The work of the blacksmith is and always has been something to be learned from a master through a long apprenticeship. What is important for a student of archaeometallurgy is to understand the basic science that underlies this craft and to have some idea of what blacksmiths do. Further below are some films showing blacksmiths at work. Here are some important technical points to notice while watching the films.
From Tharwa Valley Forge.
Austenite. To be shaped by hammering, the iron must be heated into its austenite range of temperatures, the area GSEJN in the iron-carbon equilibrium diagram. This will usually be 950–1100°C. Traditional blacksmiths speak of colours rather than temperatures: this is a bright-red to bright-yellow heat. At such forging colours the iron is much more plastic than at lower temperatures, and easier to form into the desired shape.
Solid-state welding. Two pieces of iron can be ‘forge-welded’, that is, combined into a single piece, by heating to a very high temperature and hammering them together. The temperature used is typically in the range 1300–1400°C, a white heat. This is a much higher temperature than blacksmiths use for any other purpose.
The blacksmith’s name is Yan 闫。 He was 55 years old in 1987 and had six children.
Quench-hardening. If a piece of steel at a temperature in its austenite range is cooled very quickly, it transforms to a non-equilibrium form called martensite, which is very hard and brittle. The hardness can be reduced by heating to a lower temperature, a few hundred degrees, depending on how much hardness is needed for the intended purpose.
Work-hardening. An iron piece can be hardened to some extent by hammering while it is cold. This technique is seldom used by European blacksmiths, but I have seen it used by a blacksmith in Kaifeng in 1987. He and his assistant cold-hammered two hoe-heads that they had made the previous day. They used a sledge-hammer until the sound made by the blows changed, indicating that the hoe-head had become significantly harder and more wear-resistant; then he continued for some time with a smaller hammer.
The seven basic skills of a blacksmith. From Youtube.
|A French blacksmith makes a fibula using charcoal and ancient techniques. From Youtube.||A Russian blacksmith makes part of a decorative wrought-iron gate. From Youtube.||An American blacksmith makes a pair of tongs. From Youtube.||A British blacksmith makes a pattern-welded knife. From Youtube.||A Danish blacksmith in 1955, shoeing a horse and "ringing" a wagon wheel. More on this video, with translations, here.||A British blacksmith folds and welds bloomery iron.|
To understand how an iron artefact was made, it is necessary to look at its microstructure.
To be very brief: a small sample is cut from the artefact to be investigated, using whatever tool is appropriate (hacksaw, emery wheel, diamond wheel, etc.). One surface of this sample is polished to a plane and mirror-like surface. The first practical step is usually to encapsulate the sample in a polymeric material (e.g. epoxy resin) in order to have a somewhat larger object, of a more regular shape that is easier to deal with in the following steps. The chosen surface is then ground with successively finer grades of emery paper and finished off with a polishing agent: in the case of iron artefacts this is often aluminium oxide, which is cheap and fairly effective; but microscopic industrial diamonds (with diameter e.g. 5, 3, or 1 µm) give better results and are not prohibitively expensive. As late as the 1950’s grinding and polishing were still sometimes done entirely by hand, the sample being rubbed against a glass plate covered with emery paper or the polishing agent. Today various technical improvements ease the task considerably, but skilled hand-work is still essential for the best results. The reason for the many stages in the preparation of the sample, typically using four different grades of emery paper followed by one or more grades of polishing agent, is that the surface to be examined in the microscope must be undamaged and also extremely flat.
University metallurgy departments often teach students to prepare metal samples using films showing sample preparation in their particular laboratories. Here are a few such films – your department may also have one.
After this grinding and polishing some aspects of the microstructure of the artefact can be seen directly in the microscope, especially the form and distribution of non-metallic inclusions in wrought iron and steel. The metallic components of the microstructure of an iron object, however, all look alike after polishing. Normally, therefore, unless one’s only interest is in non-metallic components, such as graphite or slag, it is necessary to etch the polished surface with an etchant. The most important etchant is nital, which is a mixture of 1–5% concentrated nitric acid in ethyl alcohol. Picral is similarly a mixture of picric acid in ethyl alcohol. Both of these make the microstructure visible by etching its different components in different ways. There are many other more specialized etchants.
The microscope that we use for this work is different from ordinary microscopes. Light must shine directly down onto the sample and reflect directly up to the eye. The diagram on the right shows how this is done, with light reflecting downward from a glass reflector, going down to the sample, and then up through the reflector to the eye.
To interpret what one sees in the microscope it is necessary to know something of the iron–carbon phase diagram, shown here on the left. Diagrams like this are a fundamental theoretical tool of metallurgists. Here are three films to explain the most important aspects of the interpretation of microstructures of steel and cast iron.
A version with better resolution is here (343 MB).
A version with better resolution is here (278 MB)
A version with better resolution is here (314 MB)
These pages tell about metallography in more detail:
Cast iron metallography
Garney, Joh. Carl (1816). Garney’s handledning uti svenska masmästeriet, omarbetad af Carl Johan Lidbeck. 2 vols, Stockholm: Fr. Cederborgh. In Swedish. Vol. 1.
Gordon, R. B. 1996. American iron 1607–1900. (Johns Hopkins studies in the history of technology). Baltimore and London: Johns Hopkins University Press.
Gordon, Robert B., and David J. Killick. 1993. ‘Adaptation of Technology to Culture and Environment: Bloomery Iron Smelting in America and Africa’. Technology and Culture 34.2: 243–270. www.jstor.org/stable/3106536
Huang Quansheng 黄全胜 and Li Yanxiang 李延祥. 2008. ‘Guangxi Guigang diqu zaoqi yetie yizhi chubu kaocha’ 广西贵港地区早期冶铁遗址初步考察. Youse jinshu 有色金属 (Non-ferrous metallurgy) 60.1: 137–142.
———. 2012. ‘Guangxi Pingnan xian Tieshitang yelian yizhi chubu yanjiu’ 广西平南县铁屎塘冶炼遗址初步研究 (Preliminary studies of the smelting site at Tieshitang in Pingnan County, Guangxi). Sichuan wenwu 四川文物 2012.1: 92–96.
Kaogu xuebao 考古学报. 1978.1. ‘Henan Handai yetie jishu chutan’ 河南汉代冶铁技术初探 (‘The iron and steel making techniques of the Han dynasty in Henan’). Kaogu xuebao 考古学报 1978.1: 1–24 + pl. 1–2.
Li Yingfu 李映福. 2014. ‘Guangxi Pingnan “wanshi” lianlu yu woguo “wanshi” lianlu de qiyuan’ 广西平南“碗式” 炼炉与我国“碗式”炼炉的起源 (“Bowl-shaped” smelting furnaces excavated in Pingnan County, Guangxi, and the origin of “bowl-shaped” smelting furnaces in China). Kaogu 考古 2014.6: 64–77.
Liu Peifeng 刘培峰. 2014. Shanxi chuantong ganguo liantie jishu yanjiu 山西传统坩埚炼铁技术研究 (‘The Study of Crucible Smelting in Shanxi Province’). Unpublished dissertation, Beijing Keji Daxue 北京科技大学.
Middleton, Albert B. 1913. ‘Native iron and steel practice in China’, Iron and coal trades review, May 23, 1913: 853.
Pleiner, Radomír. 2000. Iron in archaeology: The European bloomery smelters. Praha: Archeologický Ústav Avčr. www.academia.edu/34485002
Schultze, Lars T. 1732. Kort berättelse, om myr-ugnar eller såkallade bläster-wärk, uti Östra och Wästra Dahle-orterne brukelige. Repr. Jernkontorets annaler, 1845, 29 (no. 1), 1–32 + pl. 1–3.
Schüz, E., and R. Stotz. 1930. Der Temperguss: Ein Handbuch für den Praktiker und Studierenden. Berlin: Springer.
Wagner, Donald B. 1993. Iron and steel in ancient China. (Handbuch der Orientalistik, vierte Abteilung: China, 9). Leiden: Brill. Translation by 李玉牛, 中国古代钢铁技术史, 2018.
Wagner, Donald B. 2001. The state and the iron industry in Han China. (NIAS report series, 44). Copenhagen: Nordic Institute of Asian Studies.
Wagner, Donald B. (2008). Science and civilisation in China. vol. 5.11: Ferrous metallurgy, Cambridge University Press.
Wagner, Donald B. (华道安). 2018. Zhongguo gudai gangtie jishu shi 中国古代钢铁技术史. Chengdu: Sichuan Renmin Chubanshe. Translation of Wagner 1993 by Li Yuniu 李玉牛.
Wayman, M. L., and C. Michaelson. ‘Early Chinese ferrous swords from the British Museum collections’. In Metals and mines: Studies in archaeometallurgy, edited by S. La Niece, D. Hook and P. Craddock. London: Archetype Publications / British Museum, pp. 226–232.
Zheng Chaoxiong 郑超雄. 1990. ‘Pingnan xian Liuchen Handai yetie yizhi’ 平南县六陈汉代冶铁遗址 (Han-period ironworks sites at Liuchen in Pingnan County, Guangxi). Zhongguo Kaoguxue nianjian 中国考古学年鉴 1989: 236–237.
Zou Heng (chief ed.) (2000) 鄒衡. Tianma–Qucun 1980–1989 天馬–曲村 １９８０–１９８９ (Excavations 1980–89 at Tianma and Qucun in Quwo and Yicheng Counties, Shanxi). By the Shang–Zhou Group, Archaeology Department, Peking University 北京大學考古學系商周組 and Shanxi Provincial Institute of Archaeology 山西省考古研究所. 4 vols., Kexue Chubanshe, Beijing, 2000. English summary, tr. by K. C. Chang, vol. 3, pp. 1181–3.
Last edited by DBW, 4 April 2023.