Minerals and Metals in Ancient India v1.c26.s7: Summary of the Ancient Process

Minerals and Metals in Ancient India v1.c26.s7

Language: English

Summary of the Ancient Process

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The possible salient features of the ancient zinc distillation industry at Zawar may now be re-constructed and summarized as follows:

(1) It appears that the zinc ore mining and smelting industry at Zawar began in the mid-centuries of the first millennium BC and became sophisticated step by step till it assumed a large scale proportion in the thriteenth century AD. The preceding experimental stages of the industry are topics for future investigations.

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(2) Sphalerite with impure dolomite and quartz was mined, crushed, beneficiated by selective picking and roasted to convert the starting materials to oxides. The residual traces of sulphide and sulphate of zinc show that roasting was not done to dead heat.

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(3) The smelting charge was rolled into pellets of 1 cm³; it included a small quantity of common salt and an adequately large quantity of carbonaceous matter.

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(4) The charge (about 1.5 kg per retort) was loaded into clay retorts fitted with funnel-like condenser tubes and heated externally at 1250°C for six hours in specially designed furnaces. The entire set-up has been described before.

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(5) The reduced zinc vapour was condensed and collected. A part of the zinc oxide was converted to well-identified silicate phases and thus could not be recovered as reduced metal.

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(6) It is likely that 200-500 gms of zinc were extracted per retort, or 7-18 kg per smelt of 36 retorts.

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(7) The fourteenth century AD text *Rasaratnasamuccaya* (RRS) described the above zinc distillation process in detail. (Biswas, 1987) There are references in the said text to the brinjal-like crucible (*RRS*, 2.157), the charge containing carbonaceous matter, salt etc. (2.157-164), the *tiryakapātana yantra* or the downward distillation set-up (9.48-50) and the actual process of distilled and condensed zinc being collected in water (2.165-166).

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(8) Freestone *et al.* (1985a: 239) estimated that 8 lakh tons of debris at Zawar contains 6 lakh tons of spent retorts. Each retort weighs about 3 kg and might have produced 500 g of zinc. On this basis one lakh tons of zinc might have been produced during the thirteenth-seventeenth century AD. Our estimate is more conservative. Possibly 40 to 50 thousand tons of metallic zinc were produced. 6 lakh tons of spent retorts contain 2 lakh tons of residues within, assaying about 3 per cent zinc. Therefore some six thousand tons of zinc metal remain within the retort residue and another thousand tons in the lead slag. We have shown that most of this zinc can be recovered from the siliceous slag by a special ‘fast leaching’ process. (Biswas *et al.*, 1986)

Linked texts (1)

Rasaratnasamuccaya — Vāgbhaṭa (alchemist), Sanskrit