Early and traditional copper metallurgy in western China - a podcast by UCL

Copper underpins the technology and economy of most societies of the last four thousand years, and ancient China is no exception. It relies heavily on copper for the production of bronze objects, such as weapons, tools and vessels, but also for its coinage and other monetary instruments. The artistic expression preserved in highly decorated and intricately cast bronze objects is rightly admired, and has attracted much scientific and art historical attention. Little, however, is known about the primary production and geological origin of the copper used to manufacture these objects.

A long-term collaboration between the two institutes involved concerns the identification and reconstruction of past copper smelting activity. The long-term aim is to identify and understand the technological advances within a given region and period, and how much of that technology is a reflection of geological constrains, and how much is due to human ingenuity and problem-solving. An important aspect is the identification of cultural difference between different societies and regions, sine these may shed light on the transmission routes and mechanisms of early technology across Central and Eastern Asia.

The paper will look at two case studies of copper smelting technology in Xinjiang, to illustrate the wide range of variables to be considered in this seemingly simple process. The earlier smelting activity is based on the processing of chalcopyrite, a copper-iron sulphide ore, and resulted in the production of a particular type of thin, platy slag fragments. These dark, almost black plattenschlacken are well-known from several Bronze Age to medieval copper smelting sites in Europe and indicate a well-controlled use of ore selection and potentially added fluxes, in order to produce a low-viscosity and low-melting slag. They are fayalitic, often with magnetite inclusions and act as the main outlet for the iron initially present in the chalcopyrite ore.

In contrast, the later glassy slags are based on the processing of a predominantly pure copper sulphide ore, such as chalcosin, are much more colourful and chemically much richer in calcium than iron oxide. They resemble typical blast furnace slags in their appearance and composition. This is underlined by the presence of high amounts of metallic copper alloyed to the copper prills preserved in the slag, indicating strongly reducing conditions. The resulting copper metal would have required a further refining step to make it useable, and the use of a blast furnace indicates availability of a strong, probably mechanically-operated wind supply.

In the long term, we expect to see pattern emerging, relating different technologies to different ore types and cultural traditions, potentially revealing knowledge transmission paths and hubs of independent developments of metallurgy.

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