Coal-fuelled crucible lead-silver smelting in 12th-13th century China: A technological innovation in the age of deforestation

Silver was an important metal in the economy of imperial China. However, until now, research on silver production technology in its social-economic and environmental contexts has been limited. Here we present a unique silver-lead production site in Hebei province, north China, dated between the 12th and 13th century AD, yielding vast numbers of slag-filled tubular crucibles and coal-ash slag chunks. Microstructural and chemical analysis reveals the crucibles were manufactured from refractory clays and that the slag inside contains lead silver particles, un-reacted ore and numerous fragments of metallic iron. These finds indicate that the crucibles were used for smelting argentiferous sulphidic lead ores, which were reduced to metal by desulphurization using metallic iron. Mineral coal was employed to fuel this process from outside the crucibles. The use of mineral coal and externally-fired crucibles for smelting was an important technological innovation, but not one that could be adopted by all industries. We argue that it was most likely associated with rampant deforestation and the fuel crisis historically documented for the early second millennium in northern China. Contrary to received wisdom, this study demonstrates that the early adoption of coal was not as widespread as typically assumed, as it required a range of technological innovations. Crucible smelting, as one of the solutions, was embraced by lead-silver smelters, while most iron smelters in this period still persisted with the charcoal-fired furnace smelting tradition.