Iron 铁

Iron is the most common metal. Besides, like many ore deposits present mixtures of different ores, iron ore deposits with traces of other metals are relatively frequent. While abundant, however, iron metallurgy is demanding, and the product may appear useless. Pure iron is a soft metal, and it rusts. Its high melting point (1538° C) makes smelting difficult. Moreover, in the reduction process, iron has the tendency to absorb carbon, becoming increasingly brittle.

Relatively pure wrought iron (with under 0.5% carbon in terms of mass) is a soft metal that can be shaped by forging. Steel is iron with a slightly higher carbon content (0.5-2%) that is superior to other metals in hardness and tensile strength, but tends to rust. Cast iron has a high carbon content (2-5%), can be cast due to its lower melting point, does not rust much, but is brittle. Although all three different types are called iron, they are in fact iron alloys, commonly of iron and carbon, at times of iron and phosphorous, and often containing other trace elements.

Two main processes were employed to produce iron. Smelting in simple furnaces did not reach the melting point of the metal and produced a bloom, a sponge-like lump of largely reduced iron and slags. This product had to be freed from slags and homogenised by repeated forging. High furnaces with efficient ventilation reached temperatures above the melting point of the metal and produced pig iron or high-carbon cast iron. This product could be used as cast iron or de-carburised by forging, by puddling, or by the heating of the iron under oxidising conditions.

History

The beginnings of iron metallurgy in the investigated region are in the dark. Iron metallurgy similar to central China is evidenced by archaeological records at many sites dating from the period 200 BCE-400 CE (Yang Shouchuan 2014, 33).

There are some indications of an active iron industry in the Nanzhao and Dali periods, but hardly any specific records. A cast iron pillar that is dated to 872 CE by an inscription at its base survived in Midu county 弥渡县 in the south of Dali, demonstrating proficiency in cast-iron technologies. Tang military records provide another intriguing information. According to several sources, another pillar was cast in 707 by a Tang general after destroying two iron-chain bridges installed by Tibetan armies (Yang Shouchuan 2014, 38-39). At the time, the Tibetan empire achieved short-lived overlordship over Nanzhao and not quite identified tribes further south. By aligning the recorded rivers with known later names, the bridges and the pillar have been located just to the west of Dali. It thus appears possible that Tibetan or Nanzhao engineers were capable of building iron chain bridges, a stunning achievement! In the absence of other evidence, however, there is also the possibility that these were pontoon bridges held together by iron chains, still involving the casting of chains, probably on site, but considerably less demanding.

12th century Chinese records provide descriptions of famously sharp and deadly Dali swords [Fan Chengda 范成大 (1126－1193）《桂海虞衡志》and 《续博物志》]. Explicit reference to “forging raw iron” (duanshengtie 锻生铁) reflect a high-carbon raw product, which presumably was de-carburised to steel by repeated forging.

The Yuan History records the presence of iron mines throughout Yunnan 云南 and western Sichuan 四川.

Only in materials of late imperial China, mining sites and centres of iron metallurgy begin to become visible. A few names of iron mines and production centres of cast-iron woks appear in Ming records.

And bridges built with cast-iron chains are firmly documented for western Sichuan and Yunnan, as well as for southern Nepal from the early 15th century. Many aspects remain unknown, including the metallurgical technologies required for casting the massive chains in corrosion-resistant iron and their anchoring in stone abutments, as well as possible lines of transmission (For iron chain bridges in China, see Needham 1971, 193–208).
Qing official records include 7 registered mines, while many more existed. Yi 彝 documents and traditions reflect the presence and cultural valuation of iron metallurgy, with specific emphasis on wok-making. Chinese populations valued thin-walled and large woks that required special casting technologies. Historic iron manufacturing centres are commonly known as “pot factories” (guochang 锅厂).

Descriptions in some detail become available in the 19th and early 20th centuries. These reflect small-scale but high-quality iron metallurgy to most ethnic groups of the region. Han-Chinese 汉, Yi and Bai 白 employed high furnaces for puddling, and a special smaller furnace for the production of steel. Thai iron tools were valued for their quality, and may have employed bloomery furnaces.

Modern exploitation of the numerous rich iron deposits in the region has mostly employed open-pit mining, erasing most remains.

Research questions

This project does not aim at systematically covering recorded and probable sites of iron mining and manufacturing. This decision has been taken less by choice than due to the fact that few remains survived. Our investigation focuses on technologies recorded by Emile Rocher (1879) and the sites connected with his account. In addition, we will undertake fieldwork as work economy permits and pursue indications found in early descriptions.