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Minerals and Metals in Ancient India v1.c28.s7: Metals and Alloys in Ancient Indian Coins

Minerals and Metals in Ancient India v1.c28.s7

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Metals and Alloys in Ancient Indian Coins

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Numismatics is also outside the scope of our discussion for which the specialised treatises written by Cunningham (1891), V.A. Smith (1906, reprinted in 1974), Rapson (1922), Allan (1936), Sahni (1945), Prakash and Singh (1968), Thakur (1972), Basu (1977), Bhardwaj (1987) and the series of excellent articles and books by B.N. Mukherjee (1978, 1982, 1985, 1986, 1988) may be consulted. We would merely highlight some of the points for our discussion.

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Mukherjee (1986: 47 and 1988: 11) divided the ancient Indian coins into four classes: (A) archaic die-struck including the so-called ‘punch-marked’, (B) cast, (C) repoussé and (D) sophisticated die-struck on both sides.

Punch-Marked Coins

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Small sheets or strips receiving one punch or a few successive punches on one side may be defined as ‘punch-marked’ coins. The sophisticated die-struck coins received punches from both sides simultaneously.

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Dikshit (1981) reported on the overlooked silver pieces, some flat, circular, square and rectangular, found in a silver vase from the DK area in Mohenjo-Daro. He submitted that these pieces were ‘the precursors of punch-marked coins of later times’. Comparing the weights, Mukherjee (1988: 54) came to the same conclusion that the pieces could be related to standard cubical weights found at the Indus sites and therefore belonged to a metallic currency system. There was alphabetic cuneiform script on one of the pieces which must have been imported from Ur, Mesopotamia around 2000-1700 BC. The expression *manā hiraṇyayā* in *Ṛgveda*, 8.78.2 could have an allusion to the Babylonian weight called *minā*. The *Ṛgveda* (1.56.2; 1.126.2; 4.24.9-10; 5.19.3; 8.47.15) contains many references to *niṣka* or monetary currency of noble metals. Post-Vedic literatures refer to gold pieces, *śatamāna* etc. Pāṇini’s *Aṣṭādhyāyī* refers to *rūpya-rūpa* and Kauṭilya’s *Arthaśāstra* to *tāmra-rūpa* meaning punch-marked silver and copper coins respectively. Archeological data corroborate the literary evidences.

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The hoard of coins unearthed at Bhir Mound, Taxila contained 1055 punch-marked pieces of the older type, 33 long bent-bar coins, 79 minute coins, 2 pieces of Alexander the Great, an Achaemeninan siglos, but not a single die-struck coin. On this basis Mukherjee (1988: 12 and 67) has argued that around fourth century BC the coins in vogue were punch-marked and die-struck ones. The Chaman-i-Hazuri hoard in the Kabul area suggests that the punch-marked and saucer-shaped pieces were in circulation even in the fifth century BC.

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The punch-marked pieces bearing 5 symbols and struck on the standard of 32 *raktikās* (or 57.2 grains) have been unearthed from different parts of the subcontinent and must have been issued by the Nandas and the Mauryas. The *signati argenti* presented by the king of Taxila to Alexander the Great were stamped or punch-marked silver coins.

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The tradition of die-struck coins must have been initiated around third century BC under the foreign influence specially at Taxila. During this time we find the obverse die-struck copper coin of Dharmapāla at Eran, Madhya Pradesh.

Cast Coins

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Cast Coins are those which used to be made from hot metal cast in moulds bearing the required impressions in negative. Sahni (1945) has made a classic study on the technique of casting coins as practised in Rohtak, Eran, Taxila, Mathurā etc. (*fig.* 20.1). His findings have been summarised by Mukherjee (1986, 1988).

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The negative of the intended coin-type was engraved or etched on a blank mould made of heat-resisting plastic clay or on a mould in cast metal. If a coin was destined to carry devices on both faces, a second mould had to be prepared and an opening made through the rim of each of the moulds. Some cast coins found still joined together by a bar of metal indicate that these were cast in pairs.

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Molten metal could be poured into different moulds separately, or which was more popular as in Sunet, it would be made to flow down through a vertical column communicating with the channels leading to the coin sockets of the moulds placed vertically.

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The most elaborate of coin-casting has been revealed by the fragments of early Yaudheya coin moulds found at Khokrakot, near Rohtak (site dated around 100 BC). Each disc seems to have a hole in the centre, from which radiating channels run into eight coin-sockets arranged in a circle. The practice of producing cast coins in ancient India was gradually abandoned after third-fourth century AD. (Mukherjee 1988: 33)

Repoussé Coins

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Repousé coins were thin metal samples put over stone or metal displaying some positive impression in high relief and then carefully hammered to receive in relief the impression of the matrix on which it had been placed. Mukherjee (1988) has given the illustration of a repoussé gold coin of king Mahendrāditya.

Die-Struck on Both Sides

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During the beginning of the third century BC, the metal-smiths of Taxila, who had developed the die-striking technique indigenously, were convinced of the superiority of the circulating Greek coins, die-struck on both sides, and switched over to the new technology. This technology spread to the other parts of the sub-continent. The early gold coins of the Imperial Guptas, who issued fine die-struck pieces, were made following the earlier Kuṣāṇa tradition. (Mukherjee, 1988: 68)

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The coins of the Indo-Greeks, Scytho-Pārthians, Kuṣāṇas and the Imperial Guptas were struck from two separate dies used probably simultaneously. Around the Gupta period the metals used for coinage were: gold, silver, copper, lead, billon (alloy of silver with copper or tin), brass and potin (an alloy of copper, lead, zinc and tin which was used for making icons also).

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The molten metal or alloy was first poured into circular shaped moulds of regulated sizes to produce blanks of specific weights. Negative impressions of the obverse reverse types (and inscriptions) were probably engraved on two separate dies made of stone or soft steel or bronze. Each metal blank was heated or slightly softened by annealing and put in between the anvil die and the punch die, properly aligned and then hammered.

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No die-striking assembly has been discovered so far from any of the ancient sites. Several coins, scrutinised under microscope, show homogeneity or near-homogeneity in size of the grains of their metal. This indicates that either the blank or the die-struck coin might have been heat-treated to promote a good amount of recrystallisation and homogenization of grain-sizes. (Mukherjee, 1988: 21)

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.

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