Minerals and Metals in Ancient India v1.c28.s7.t5: Elemental Analysis and Metallography of Coins
Minerals and Metals in Ancient India v1.c28.s7.t5
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Elemental Analysis and Metallography of Coins
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In our chapter 17 we have discussed and provided chemical analysis of Mohenjo-Daro silver pieces, Taxila punch-marked silver coins (Table 17.3), Devnimori silver coins, Nagara lead coins and their trace element impurities (Tables 17.6 and 17.7). Earlier, silver-copper alloy coins were discussed in chapter 10 and copper-nickel coin mentioned in chapter 14 (Table 14.8).
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Elemental analysis provided considerable insight to the contemporary raw material situation. The issue of progressive debasement of silver coins with increasing proportion of copper was discussed in chapter 10 as an index of dwindling silver reserve in the economy and the political situation. The nickel content of the ‘white copper’ coin in Taxila (Table 14.8) reflected the contact of Taxila with the nickel-rich areas of Central Asia and China.
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The trace element impurities in coins (several examples provided in Tables 17.3, 17.6 and 17.7) suggest the possible sources of mines from which the ores had been reduced to the respective metals. The Ahicchatra coins have been reported to contain appreciable quantities of antimony and some contain lead as well. (Lal, 1953: 202) Bismuth and arsenic were detected in the Indo-Greek silver, and there are iron globules in the Kuṣāṇa copper coins, rendering them to be responsive to the magnet. (Bivar, 1987) We would discuss later in this chapter the scientific and archaeological interpretation of the entry of iron globules in ancient copper artefacts. In ancient India, shortage of copper led to the use of lead coins as in Nagara (vide Tables 17.6 and 17.7) on the one hand, and on the other to the debasement of copper coins through alloying with iron as in Paunar, Maharasthra. (Hegde, 1975) Some 310 Paunar coins dating from seventh century AD assayed 71.9-77.3 per cent copper, 21.8-23.2 per cent iron, zero to 4.4 per cent tin, 0.3-0.4 per cent lead and trace of Ni, Co, Bi and Zn.
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The Gupta Period gold coins with dates between AD 375 to 467 assayed 74-77.5 per cent gold alloyed with 22.5-26 per cent silver, and contained trace of iron. (Bhardwaj 1979: 127) A silver coin belonging to AD 900. Rājghāt assayed 61.5 per cent Ag, 32.1 Cu, 0.57 Sn, 0.54 Fe, 0.5 Pb, 0.5 Ni, 0.2 S and some CaO and insoluble component. The coin also contained trace of bismuth. (Bhardwaj, 1979: 109)
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Fig. 20.1: The sites for the manufacture of coins in ancient India.
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Metallographic examination provides further insight to the steps which might have been taken to make these coins. Bharadwaj (1979: 107-22) studied the Taxilian silver coins (reported in our Table 17.3) and the AD 900 Rājghāt silver coin just now reported. The presence of lead globules (and sulphur) in the coins indicated that argentiferous galena must have been the principal ore from which the metal was extracted. In the Rājghāt coin, bismuth was present; this being insoluble in copper and silver, spread as a thin film along the grain boundaries and caused embrittlement and crack in the micro-structure. The punch-marked coins from Taxila showed dendrites of silver and eutectic mixture of silver and copper; this implies an essentially cast structure, slowly cooled before being die-punched. The grains appeared to be equiaxed. On the edges, grains were elongated and distorted, the micro-constituents were curved around the punch-marks. (Bhardwaj, 1979: 110 and 113-22)
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Bhardwaj (1979: 127-28) also examined the Gupta Period gold coins from Rājghāt analysing about 75 per cent gold and 25 per cent silver. The silver-content results in increased solubility of oxygen in molten gold; some of the pits and cavities revealed in the metallographic examination might be of this origin. The micro-structure shows equiaxed fine grains of dimensions around 20 µm. This indicates that the die-struck coins were either hot-worked or cold-worked followed by annealing above the recrystallisation temperature. The excellent details of embossing on both sides indicate that the die-striking techniques were well-developed in the Gupta Period; the die alignment was however not perfect, and this resulted in slight eccentricity of the striking on the reverse side.
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The gold coins of the Gupta Period (fourth-fifth century AD) and of the Colas, Cālukyas and Kadambas of South India (after 10th century AD) were like commemorative medals depicting the rulers and deities in many modes. In the words of B.N. Mukherjee (1988: 50), “the transformation of the pieces of metal called coins into the objects of art was facilitated by the artistry of the obverse and reverse devices — dies or moulds — purity and/or suitability of the required metal and efficiency in the technique of minting”. The excellence of this transformation was matched and possibly surpassed by the artistic and ageless icons which we would like to discuss next.