History of Technology in India, Vol. I: From Antiquity to c. 1200 A.D. v1.c17.s4.t2: Some Observations on the Metallic Slags in Ancient India
History of Technology in India, Vol. I: From Antiquity to c. 1200 A.D. v1.c17.s4.t2
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Some Observations on the Metallic Slags in Ancient India
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There are heaps of ancient slag in the excavated sites which conclusively prove indigenous smelting in India. A detailed examination of the slags and slag-ore correlation as suggested above (c) should help us in ascertaining the kind of ores and fluxes which might have been used during the process of smelting.
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During the excavation of Period I levels at Ahar, heaps of slag-like material were recovered from more than one trench at three different depths. Evidently the copper smelting industry at Ahar was extensive. The analysis of the slag is:
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| SiO2 | 35.2-38.2 p.c | MnO2 | 1.2-2.3 |
| Fe2O3 + FeO | 43.9-48.3 | CuO | 0.7-0.9 |
| MgO | 2.4-3.6 | SO3 | 3.6-6.1 |
| Al2O3 | 5.3-7.8 | | |
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Evidently, complete removal of sulphur and recovery of copper were not possible, and the ore was sulphidic. Copper ore from the neighbouring area contains less than 20 p.c. silica and therefore it may be presumed that the copper ore was fluxed with silica during smelting (Hegde, 1969).
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It is unfortunate that phase studies were not conducted on the Ahar slag. The modern Khetri Copper Plant produces slag which contains phases such as magnetite Fe3O4 and copper ferrites such as CuFeO2, Cu6Fe3O7 and some free copper oxide (Sharma and Shekhawat, 1984). Similar phase studies on the ancient slag would have shown whether (a) such a slag could have been derived from the neighbouring ore containing specific gangue meterials and (b) if so, what temperature and other physico-chemical conditions might have been employed.
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Atranjikhera showed not only iron objects but also furnace and iron slags assaying 64-67 p.c. Fe2O3, 25-30 p.c. SiO2, 3-5 p.c. Al2O3, 1-3 p.c. CaO, 0.6-0.9 p.c. MgO and 0.7 p.c. P2O5 (Gaur, 1983: 496).
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It has been proposed that iron could have been made from the iron-bearing shales occurring in the neighbouring Agra-Gwalior range which assays 21.7 p.c. Fe, 41.9 p.c. SiO2, 14 p.c. Al2O3, 6.3 CaO. 2.3 MgO, 1.4 Na2O and 6.8 p.c K2O. This suggestion (Gaur, 1983: 487-500) is plainly unacceptable, since the proportions of SiO2, Al2O3, CaO and MgO are lower in the slags, and iron percentage much higher, than in the iron shale. The raw material must have been rich iron ore (from eastern Uttar Pradesh or Bihar) out of which the iron recovery was incomplete and inefficient on account of solid-state reduction.
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Excavation by Willies et al (1987, 1989) has revealed ancient mining of the Rampura-Agucha deposit (25° 50' N, 74° 44'15" E) in Rajasthan. There is the clear base of a gossan at about 15 m. depth from which marmatite, pyrite and sphalerite (ZnS) were selectively oxidised and leached by the natural agencies leaving galena-cerussite-anglesite ore-body enriched in terms of silver.
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The ancient mining timbers have been C-14 dated as 370 and 250 B.C. Tiwari et al (1984:85) have proved that here mining done at the 80-125m. depth was selective following lead mineralisation and pure galena veins in preference to the zinc ore veins.
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There are huge slag dumps in the Rampura-Agucha area. The petrographic thin sections of slag sample indicate the presence of quartz and feldspar grains which have not undergone much change. Evidently, low-temperature smelting was done which could have yielded argentiferous metallic lead; the main target was possibly silver.
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The slag dump area also showed several cylindrical retort-like pieces with inside coating of dirty white ZnSO4 and slag assaying 0.01 p.c. zinc and as high as 9.30 p.c. lead. Considering the fact that the ore-body assays 12p.c. Zn and 2-3 p.c.Pb, Paliwal et al (1986) rightly inferred that an appreciable amount of zinc must have been recovered by retort distillation process which was perfected in the Zawar Mines.
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At Agnigundala, Andhra Pradesh, extensive mining, mineral processing (crushing, grinding and beneficiation) and smelting (of copper and lead) took place in ancient India which is evident from the rock dumps, slag heaps, cupolas and crucibles (Ziauddin, 1961: 120). The ore must have been complex and multi-metallic. Similarly, selective mining of the Ambamata deposit might have resulted in recoveries of copper, lead and silver, and zinc which was strikingly rich in cadmium (Shekar, 1983).
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One of the most spectacular discoveries during the recent eighties related to mining archaeology and archae-metallurgy has been the reconstruction of the glory of zinc ore mining and smelting in ancient India (Biswas et al, 1984; Craddock et al, 1985; Willies, 1987 & 1989; Hegde, 1989 etc.). Limited space does not permit us to present the full report which is available elsewhere (Biswas, A.K., 1991). Only a few brief comments may be made here with reference to the 13th century A.D. ancient slags in Zawar mine, Rajasthan.
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There are at least sixteen deposits of ancient slag around the Zawar Mine, which are basically of two types; (a) glassy and (b) crystalline. The first variety resulted on account of smelting and recovery of lead and silver but not zinc.
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At a later stage, the value of zinc was realised, sphalerite removed carefully from the lead ores and reduced/distilled in retorts leaving some residual slag. Pyrite used as flux precipitated in the crystalline slag as fayalite (Freestone et al, 1987).
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Detailed X-ray and electron diffraction studies were conducted on the Zawar retort residue (content), wall and (lead) slag (Biswas, et, al, 1984). The phases identified are reported in Table 6.
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It is evident from the data that a small part of sphalerite was not converted into oxide and remained in the retort as ZnS and ZnSO4. The presence of goslarite or ZnSO4, 7H2O (hydrated at later stage) was confirmed by the endothermic DTA peaks apart from X-ray and electron diffraction studies. Some ZnO was converted to complex silicates during roasting and some underwent atmospheric conversion at a later stage to the basic carbonates (vide Table 6). Trace element detection in the zinc residue/slag by the Energy Dispersive X-ray Micro-Analysis or EDAX in the Scanning Electron Microscope showed the existence of elements such as Na, K, S, Ti etc., which must have originated from the ore-body charged into the retort.
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The above-mentioned work on zinc-lead slags illustrates that valuable information may be obtained by sophisticated techniques regarding the nature of
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Table 6 — Phases identified in Zawar Retort Residue (Content), Wall and (Lead) Slag by X-Ray and Electron Diffraction Studies (Biswas et al 1984: 236).
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| | Phases identified | |
| *Sample* | *X-ray diffraction* | *Electron diffraction* |
| Retort content | Zn2P2O7. 5H2O goslarite ZnSO4, 7H2O hemimorphite, Zn4 (OH)2 Si2O7, H2O esperite Ca3Pb (ZnSiO4)4 Sphalerite ZnS, chalcopyrite CuFeS2 larsenite, Pb ZnSiO4 | Zn2P2O7. 5H2O goslarite hemimorphite, esperite, quartz, Mg2 Al4Si5O18, hardistonite Ca2ZnSi2O7, hydrozincite or aurichalcite, Zn5(CO3)2 (OH)6 |
| Retort wall | Goslarite, hardistonite, hemimorphite Zn2P2O7. 5H2O, aurichalcite, quartz, sphalerite | Goslarite, hemimorphite, Willemite Zn2SiO4, hardistonite |
| Slag | Quartz, goslarite, Zn2P2O7. 5H2O hemimorphite, aurichalcite, sphalerite, chalcopyrite, hardistonite. | Hemimorphite, goslarite, hydrozincite, quartz, sphalerite |
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the ore and reduction/smelting techniques, the details of which are outside the purview of this paper.
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Electron diffraction studies have further shown that the complex Zawar ore contains not only sphalerite (α- and β ZnS) and galena (PbS) as the major phases but also red silver, pyrite, chalcopyrite, anglesite (PbSO₄), PbCO₃, calamine or Zn₄(OH)₂Si₂O₇, H₂O etc. (Biswas, A.K., 1981). The gangue impurities are dolomite, quartz and white mica.