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Minerals and Metals in Ancient India v1.c25.s4.t2: Pre-Mauryan era Lead Slag at Zawar Mines

Minerals and Metals in Ancient India v1.c25.s4.t2

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Pre-Mauryan era Lead Slag at Zawar Mines

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The third mine-system investigated by Willies *et al.* is the Zawar Mines, 30 km south-west of Udaipur (*fig.* 17.3 a). The host rock at Zawar is a metamorphosed sheared dolomite, hard and compact, which, except within the weathered zone, had mainly been worked by firesetting. There was evidence of retreat working of the pillars. Dumping of waste was done systematically. The C-14 dates of the timber samples show that the Zawar Mala and Zawar & Mochia mines were active during 430-380 BC period and again during 200 BC-AD 60 period. There are also evidences for the mine being active in the seventh century AD and later for medieval period thirteenth-eighteenth century AD.

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Zawar Mine area exhibits wide-ranging evidences for ancient mining and smelting of zinc ore, which we would present and discuss in the next chapter. Zawar has also yielded considerable evidence for the smelting of lead, in the form of at least sixteen deposits of slag, distributed along 13 km of the valley of the River Tiri, the main water course in the area. These slag deposits, containing several pieces of lead metal as smelting residues, are of two types: (a) glassy and (b) crystalline. Analyses are provided in Table 17.5.

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Analysis of Crystalline and Glassy Lead Slags at Zawar Mine, Rajasthan

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| Sample No. | Pb (%) | Zn (%) | Fe (%) | Ag (ppm) | Site | Morphology |
| 1 | 5.25 | 0.72 | 32.0 | 31 | 5 | Crystalline |
| 2 | 2.21 | 1.80 | 18.5 | 10 | 5 | Crystalline |
| 3 | 2.57 | 2.41 | 21.0 | 12 | 5 | Crystalline |
| 4 | 2.30 | 2.72 | 25.1 | 10 | 5 | Crystalline |
| 5 | 3.50 | 10.30 | 8.1 | 9 | 1 | Glassy |
| 6 | 4.73 | 10.56 | 9.4 | 8 | 1 | Glassy |
| 7 | 7.20 | 8.71 | 9.0 | 11 | 4 | Fine crystalline (stoney) |
| 8 | 7.02 | 8.26 | 8.1 | 8 | 4 | Glassy |
| 9 | 6.30 | 7.09 | 8.7 | 16 | 4 | Glassy |

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The reserve of glassy slag is estimated to be 30,000-35,000 tons. (Straczek et al. 1967) The glass slag, not associated with zinc smelting debris, assays high in zinc (7-10 per cent) and contains sparsely crystalline phases such as zincian melilite Ca₂ZnSi₂O₇, Zincian olivine, magnesium-rich clino-pyroxene CaMgSi₂O₆ etc. The crystalline slags assay lower zinc per cent but much higher iron content. Scanning electron microscopy revealed the existence of fayalite, Fe₂SiO₄ and iron rich clinopyroxene CaFe Si₂O₆ (Freestone et al., 1985, 1987). The author of this monograph and his colleagues (Gangopadhyay et al., 1984) detected hemimorphite Zn₄(OH)₂ Si₂O₇, H₂O, willemite, Zn₂SiO₄, hydrozincite, Zn₅(CO₃)₂(OH)₆, esperite Ca₃Pb(ZnSiO₄)₄, larsenite, PbZnSiO₄ etc. as some of the major phases in the slag.

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It may be inferred that the glassy slags were obtained in the early stages to recover lead (and silver) but not zinc. At a later stage, the value of zinc was realised, sphalerite removed carefully from the lead ores, and the resulting reduction of the flux content was compensated by adding more iron as pyrite which precipitated in the crystalline slag as fayalite (Freestone et al., 1987).

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Chatterjee et al. (1981: B.78) recorded that in the modern Zawar mine, the run-of-mine ore assays 0.7-1.4 per cent lead, 3.5-4.4 per cent zinc and about 40 ppm silver yielding on beneficiation, zinc concentrate (assay 52 per cent zinc, 1.3 per cent lead, 140 ppm silver) and lead concentrate (assay 63.7 per cent lead and nearly 0.1 per cent or 980 ppm silver). Thus most of the silver is associated with the galena or lead sulphide fraction. The ancient zinc distillation retort residue assays 120-260 ppm of immobile silver (Freestone et al., 1985: 9) whereas the Table 17.5 shows that the lead slags assay 10 ppm or even less in silver. These data coupled with the fact that pieces of metallic lead were found in the slag, strongly suggest that lead-smelting was followed by recovery of silver from molten lead by a process such as cupellation.

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The above trend was also exhibited in Rajpura-Dariba as well as Rampura-Agucha. The extensive mining traditions in the fourth-third century BC. India are recorded in Kauṭilya’s Arthaśāstra which would be discussed in our Vol. II chapter 5. The Arthaśāstra described the ores of lead (2.12.13), argentiferous galena (2.12.6) and silver (2.13.10-14), and also the uses of lead in purifying gold (2.13.47) and silver by the cupellation process (2.13.49). It is quite likely therefore, that the silver in the punch-marked coins, bent-bars and jewelleries found at Taxila, was obtained through the contemporary smelting practices at Ajmer and Zawar as described in the Arthaśāstra of the Maurya Period.

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