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Introduction to the archaeometallurgy of iron
Donald B. Wagner

Traditional Chinese fining and puddling

The most common pre-modern means of decarburizing cast iron from the blast furnace to produce wrought iron involve heating to a molten or semi-molten state and stirring under oxidizing conditions. All of the different Chinese methods of doing this have been called chao 炒, ‘stir frying’. Various English words have been used to translate this word, including ‘roasting’, ‘refining’, ‘converting’, and ‘puddling’. The methods generally resemble the European fining and puddling processes, and I use these terms to translate chao: fining for those in which the iron and the fuel are mixed together, and puddling for those in which the iron and the fuel are separated. We shall see, however, that the Chinese processes are rather different from the corresponding European processes. The Chinese processes are smaller-scale, and appear to be more fuel-efficient and less labour-efficient than the European processes.

Fining in the Dabieshan region

These drawings show how fining furnaces were traditionally constructed in the Dabieshan region in the 20th century.

Diagram of a fining hearth used in Shangcheng 商城, Henan, in 1958. (Reproduced from Yang Kuan 1982: 225; orig. Tufa diwen lian’gang 1958: 23.) 1. Hearth. 2. Tamped fireclay. 3. Hearth opening. 4. Cover. 5. Blast pipe. 6. Hearth opening. 7. Iron reinforcements. 8. Nest. 9. Ground level.

Drawing by E. T. Nyström of a pair of fining hearths in southern Henan, ca. 1917. In the background a traditional Chinese ‘windbox’, in the foreground a plate of pig iron and a wrought-iron bar. Reproduced by courtesy of Tom Nyström and the Museum of Far Eastern Antiquities, Stockholm.

 
Wood, charcoal, and broken pieces of cast iron were charged into the hearth and ignited; air was pumped in, and when the iron was semi-molten, with the consistency of bean-curd dregs (豆腐渣), it was stirred about with an iron rod under a blast of air. If the worker was skilled, carbon was burned out of the iron and not too much of the iron was burned. When the carbon content of the iron was sufficiently reduced it was removed in small balls which were hammered to remove slag and form them into bars.

Operation of a furnace in Shanxi for converting cast iron to wrought iron. Photograph reproduced from Alley 1961a.

I have not seen a photograph of a fining hearth in use in the Dabieshan region, but the figure on the right, which shows a similar hearth in use in Shanxi in 1958, will give a general impression of the operation.

According to a description of the fining operation as practised in Shangcheng 商城, Henan, about 70 kg of wrought iron were produced in one fining cycle, and there were eight or nine cycles in a 12-hour shift. To produce one tonne of wrought iron the inputs were about 1.2 tonnes of cast iron, 85 kg of wood, and 100 kg of charcoal; the labour used was about 170 worker-hours. The carbon content of wrought iron produced in the Dabieshan region seems generally to have been between 0.1 and 0.3 per cent.

More detailed descriptions are in Tufa diwen lian’gang 1958; Yejin bao 1958.41: 31–41; Tegengren 1923–24: 335–336, 327–329; donwagner.dk/MS-English/Guide.html; and donwagner.dk/MS-Zhongwen/Chaogang.html.

Temperatures of at least 1400° C must have been reached in this hearth. In the fining operation carbon in the iron was oxidised both by the oxidising atmosphere in the hearth and by an oxidising slag. The pasty lumps of wrought iron removed from the hearth were heavily intermixed with this slag; hammering the iron on an anvil ‘squeezed’ the slag out like water from a sponge.

Puddling in Sichuan

The furnace and its operation were described in detail by Luo Mian 羅冕 in 1936. He gives the diagram and photographs reproduced here.

Diagram of a puddling hearth in Sichuan. a. Firebox, height 2 chi 尺 8 cun 寸 (93 cm). b. Puddling bed, diameter 2 chi (67 cm). c. Blast pipe. d. Flame channel. Photograph of the front of a puddling hearth in Sichuan. Photograph of the back of a puddling hearth in Sichuan.

The photographs are unfortunately not very clear because of poor reproduction. The furnace is built entirely of a type of refractory sandstone which is common in Sichuan. Charcoal is burned in the closed firebox. Blast is blown into the firebox, and the flame proceeds downward into the puddling bed.

Operation. First semi-charred charcoal [chaitan 柴炭] is burned in the stone firebox a to heat the puddling bed. Then broken pieces of pig iron are charged into the puddling bed b and the blast is increased, blowing through c to a. The flame passes through the stone aperture d and heats the cast iron thoroughly. The flame gradually becomes the colour of mung beans; [the iron] is stirred [puddled] with a wooden pole; then the colour [of the iron] turns from red to white, the temperature being about 1000° C. It is puddled again with a wooden pole until the iron breaks up into a granular form. Then a small amount of ironsand (iron oxide, commonly called hongzi 紅子, [‘red stuff’]) is added while the blast is worked and stirring continues. [The iron] gradually melts, and as puddling continues it goes from free-flowing [xi 稀] to ‘dry’ [gan 乾, i.e. viscous] and transforms to a plastic state. At this time the blast is reduced and [the iron] is formed into a ball; this is wrought iron [shu tie 熟鐵], commonly called mao tie 毛鐵 [‘semi-finished iron’]. The entire operation takes about 20 minutes.

The semi-finished iron is then heated in an ordinary smithy hearth and hammered to force out the slag which it contains. From 100 jin 斤 of semi-finished iron about 75 jin of wrought iron can be obtained. This is then hammered into plates, bars, rods, and the like and sold in the markets.

. . . It is customary to pay the puddler a combined price for his labour and the charcoal which he uses. For the puddling of 100 jin [60 kg] of pig iron to semi-finished iron the fixed price for labour and charcoal is 3 jiao 角 [0.3 yuan 元; the pig iron itself cost 5 yuan]. The labourer is not permitted an allowance for lost metal [huohao 火耗], but he is permitted to add as much ironsand as he wishes; therefore for every 100 jin of pig iron given to the puddler the return is about 100 jin of [semi-finished] wrought iron. However, when this is hammered in the smithy hearth, although the price of labour and charcoal is the same, 3 jiao, so much slag is forced out of the iron that the allowance for lost metal is about 25 per cent: if the smith is given 100 jin of semi-finished iron the return is only 75 jin. (Luo Mian 1936: 23–26).

As the iron is melted the carbon in it is oxidised by the oxidising flame together with a slag which is rich in FeO. The slag is formed partly by the addition of red ironsand (presumably a mixture of quartz and hematite, SiO2 and Fe2O3), partly by the combustion of some 25 per cent of the pig iron charged. Unfortunately Luo Mian gives very little quantitative data on this process, and I have not found any elsewhere, but the small size of the puddling bed and the short time required for the puddling operation suggest that the amount of pig iron charged was perhaps 10–20 kg. The amount of fuel needed probably varied greatly with the skill of the puddler: this would be the reason for requiring him to supply the fuel himself.

In the puddling process, as the carbon content of the iron decreases, its melting point (liquidus) rises. At the same time the temperature in the furnace also rises, probably coming close to 1500° C. Therefore the iron in the puddling bed is, through most of the process, in the form of a paste composed of microscopic crystals of lower-carbon iron in a higher-carbon liquid. Consulting the iron-carbon equilibrium diagram, in the neighbourhood of 1500° C the liquid has about 0.5 per cent carbon and the solid about 0.1 per cent. As the carbon content continues to decrease, more low-carbon iron precipitates from the melt, the proportion of liquid decreases, and finally the plastic state mentioned in the description above is reached, at which time the iron has been nearly fully decarburised. English puddlers spoke of the iron ‘coming to nature’, and in the large puddling furnaces of the +19th century this was quite a spectacular phenomenon.

In the ‘fining’ process, as it was practised in the Dabieshan region, the fuel was mixed directly with the iron. There the temperature appears to have been lower, and the decarburisation presumably took place with much of the iron in the solid state. Decarburisation of iron in the solid state is much slower than in the liquid state, and that process, though efficient compared with early Western fining processes, was probably less fuel-efficient than the Sichuan puddling methods. Its great advantage was no doubt the use of a lower temperature, which meant that local materials, less refractory than the sandstone available in Sichuan, could be used for the furnace.

The European puddling process was in principle very like the Sichuan process. Mineral coal was burned in a firebox and its flame was used to heat cast iron in a separate ‘puddling bed’. The puddler performed essentially the same operations as described here. There were two essential differences: the British process used mineral coal as the fuel, and it was a much larger-scale operation than the Sichuan process. The puddling bed was many times larger, tons of iron per day were converted to wrought iron, and puddling was ‘probably the severest kind of labour in the world’ (Percy 1864, pp. 656–7). It is unlikely that the work of the Sichuan puddlers was equally severe, but it cannot have been as effortless as Luo Mian’s description might suggest.

It is a surprise to see that mineral coal was rarely used in puddling in Sichuan. The great benefit of puddling in Britain was that it separated the fuel from the iron and therefore made possible the use of coal in converting cast iron to wrought iron. European fining processes consumed prodigious amounts of charcoal (See e.g. Percy 1864:596, 601, 607, 615), and mineral coal could not be substituted directly because its high sulphur content was in large part taken up by the iron. Chinese fining processes were much more fuel-efficient, and the separation of the fuel from the iron in the Sichuan puddling process no doubt increased this efficiency to some extent; perhaps, then, a need for a cheaper fuel was rarely felt.

A type of puddling furnace used in Sichuan in 1958, redrawn from Yang Kuan (1960: 187, fig. 69). The dimensions are not given. Mineral coal or charcoal is burned in the large chamber; blast is blown in at the left. The flame travels through the long passage to the puddling bed at the right.

A type of puddling furnace used in Sichuan in 1958, redrawn from Yang Kuan (1960: 69). The dimensions are not given. Anthracite coal is burned in the firebox at the upper right. Ashes fall into the ash-box at the lower right. Blast is blown into both the firebox and the ash-box. The flame travels through the large empty chamber and then into the puddling bed at the lower left.

Two types of puddling furnace in use in Sichuan that did use mineral coal are described briefly by Yang Kuan (1960: 187–8; see also Yejin bao 1958.41: 42–46). They were used in the Great Leap Forward period, and presumably also earlier. He gives the diagrams shown here on the right. It can be seen that these furnace designs provide much greater separation between fuel and iron than the one discussed above.


References

Alley, Rewi. 1961a. China’s hinterland—in the leap forward. Peking: New World Press. 

Luo Mian 1936 羅冕. Gang tie 鋼鐵 (Iron and steel). Report no. 15 in Zhongguo Gongchengshi Xuehui Sichuan Kaochatuan baogao 中國工程師學會四川考查團報告 (Reports of the Sichuan Investigation Group, Chinese Engineering Society). N.p., n.d., preface dated 1936.

Tegengren, F. R. 1923–24. The iron ores and iron industry of China: Including a summary of the iron situation of the circum-Pacific region. 2 vols.  (Memoirs of the Geological Survey of China, series A, no. 2 Peking: Geological Survey of China, Ministry of Agriculture and Commerce. Part I 1921–23, part II 1923–24. English text and abridged Chinese translation by Xie Jiarong 謝家榮. Chinese title: Zhongguo tiekuang zhi 中國鐵鑛誌, by Ding Gelan 丁格蘭 (Dizhi zhuanbao, A.2 地質專報甲種第二號).

Tufa diwen lian’gang 土法低溫鍊鋼 1958 (Regional methods of low-temperature steel-refining). Jiangsu Office of Metallurgical Industry, Nanjing, 1958. Cf. Wagner (1985, pp. 60–66).

Wagner, Donald B. 1985. Dabieshan: Traditional Chinese iron-production techniques practised in southern Henan in the twentieth century.  (Scandinavian Institute of Asian Studies monograph series, no. 52 London & Malmö: Curzon Press. 

Yang Kuan 楊寬. 1982. Zhongguo gudai yetie jishu fazhan shi 中国古代冶铁技术发展史 (The history of the development of siderurgical technology in ancient China). Shanghai: Renmin Chubanshe. 

Yejin bao 1958.41: 31–41. ‘You erbainian lishi de Shangcheng diwen lian’gang fa 有二百年歷史的商城低溫煉鋼法’ (The low-temperature steelmaking [i.e. fining] process of Shangcheng County, Henan, with 200 years of history). By the Central-South Work Team, Ministry of Metallurgical Industry 冶金部中南工作組.

Yejin bao 1958.41: 42–46. ‘Tufa fanshelu lian’gang 土法反射爐煉鋼’ (Steelmaking [i.e. puddling, chao 炒] in a traditional reverberatory furnace). By the Central-South Work Team, Ministry of Metallurgical Industry 冶金部中南工作組.

Last edited by DBW 25 February 2023