Zinc from Zawar

01The problem and the answer

Zinc ore must be heated to about 1,000°C to free the metal, but zinc boils at 907°C, so in an open furnace it simply flies off and burns. The smiths of Zawar packed roasted ore with charcoal into long clay retorts shaped like aubergines, set them mouth-down in a perforated plate above a cooler chamber, and heated the furnace above. The vapour ran down the neck, condensed in the cool chamber and collected as metal. Excavated furnaces each held 36 retorts in a square grid.

02The excavation

In the 1980s Paul Craddock of the British Museum worked with K. T. M. Hegde and Hindustan Zinc at Zawarmala. They found ancient mine galleries with timber props radiocarbon-dated to around 2,000 years ago, and slopes covered by heaps of spent retorts, estimated in the tens of thousands of tonnes. The furnace rows they excavated date mainly to the 16th century, and production went on until the early 19th century. The Rasaratnasamuccaya describes the same downward distillation in a closed vessel.

03Bristol and China

William Champion patented a zinc distillation process at Bristol in 1738, and some historians have suggested he learned of the Indian method, but no document proves it. China also distilled zinc from at least the Ming period, by a different upward method. Exactly when distillation began at Zawar, before the 16th-century furnaces, is still being worked out; claims of a firm 9th-century date should be treated with caution. What is sure is that Zawar is the oldest zinc-distilling site yet excavated anywhere.

In the texts

• Rasaratnasamuccaya

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