Traditional Chinese iron-production technology

Process diagram for iron production in a traditional Chinese ironworks.

Around 1600 the Chinese iron industry was the world’s largest and most efficient, and had been so for 2000 years. The great difference from the rest of the world lay in the use of the blast furnace, which was invented in China by the 3rd century BC. In Europe the first blast furnaces were introduced (possibly from China via Iran) in the 12th century AD, and it was not   

until the 16th or 17th century that the blast furnace largely replaced the bloomery and became the dominant iron-smelting technology.

The diagram above shows the most important processes in a blast-furnace ironworks. This is called indirect iron smelting, because the blast furnace produces cast iron (also called ‘pig iron’) with 4 percent carbon, and this carbon is then removed to make wrought iron. Iron production in a bloomery, on the other hand, is called direct because wrought iron with a low carbon content is produced in a single step.

A technical description of the operation of a blast furnace can be seen here. Briefly, a blast furnace is charged with iron ore, fuel (charcoal or coke), and normally a flux (usually limestone), and produces molten iron with ca. 4 percent carbon in a process which continues day and night for long periods: in modern times, years, in older times, weeks or months.

Iron from the blast furnace, with high carbon content and relatively low melting point (as low as 1150°C), is cast iron. This can be used directly by an iron foundry, but it is brittle and cannot be worked by a smith. To obtain wrought iron, with low carbon content (and therefore high melting point, over 1500°C), the carbon must be removed, entirely or in part. This has been done in numerous ways: in Europe the earliest process was fining; in the 18th century puddling was introduced, in the 19th century the Bessemer process, and in modern practice numerous other processes.

Fining of cast iron to wrought iron in Shanxi, 1958. (Rewi Alley, China’s hinterland—in the leap forward, Peking 1961).

Common to all the traditional Chinese processes for conversion of cast iron to wrought iron is that pieces of cast iron are heated to a very high temperature (1200–1500°C) and stirred about in an oxidizing atmosphere. If the worker is skilled at his job the carbon is burned out of the iron without too much of the iron being burned. All of the variants of this process used in China are called in Chinese chao 炒, a word whose basic meaning is ‘stir-frying’; numerous Chinese metallurgical terms come from the kitchen. In English I call the processes either ‘fining’ or ‘puddling’, depending on which of the two European processes they most resemble.

For technical reasons which are sketched here, the blast furnace gives enormous economies of scale: the greater the production the cheaper the iron produced. A general rule is that for maximum efficiency a blast furnace should be as large as the supply of its raw materials and labour, and the market for its production, will reliably allow. No larger than that, for efficiency also requires that the furnace operate continually for years at a time; the cost of interruption of operation caused by labour or raw materials shortages, or by a failure of the demand for pig iron, will quickly eat up the intended economies of scale.

Iron ores and the fuel needed for smelting are found nearly everywhere in the world in quantities sufficient for pre-modern production levels, and iron is relatively cheap by weight. These two facts mean that transportation costs are an extremely important factor in the geography of pre-modern iron production. In Europe, until late medieval times, iron was most often a local product, and only especially high-quality grades were traded over long distances. The situation was rather different in China. The higher efficiency of large-scale blast-furnace iron production meant that transport costs played a smaller role, and it was profitable to produce for larger markets. Larger markes meant, however, that competition between producers was intense and profits correspondingly low. Iron production therefore most often took place where the resources required for more profitable activities were lacking – where the comparative advantage was in iron production. This was generally in the poorest regions of China.

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