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Minerals and Metals in Ancient India v1.c28.s9: On Lead and Iron in Indian Coins and Icons

Minerals and Metals in Ancient India v1.c28.s9

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On Lead and Iron in Indian Coins and Icons

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These two elements have been frequently the common constituents in the alloys used in India. Lead and silver were often extracted together, the former being a massive by-product of the latter and utilised as cheap constituents in alloys etc. Sātavāhana and Kṣatrapa empires introduced lead coins to meet the deficiencies of copper metal in the country.

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Later, copper used to be alloyed with lead particularly in icons. We have mentioned about the lead globules in the Gujarat icons. Craddock (1983: 61-64) has discussed in great detail that lead was used to alloy copper in the ancient world for no good metallurgical reason. Lead is low-melting and insoluble in copper. The mobility of the alloy is improved only up to 2 per cent lead. At higher percentages, the insoluble globules link up to produce ‘lakes’ and seriously weaken lead-bronze interfaces. Hammering of such an alloy leads to cracking.

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High lead-content copper alloy, known in Europe as ‘Caldarium’, is brittle and suitable only for castings; this has been used in Gujarat icons and some South Indian icons (vide Nos. 11-21 in Table 20.10). Item No. 12 contains nearly one-third by weight of the lead metal and 60 per cent copper; such copper-lead alloys have been defined as Śulvanāga in the Rasaratnasamuccaya (RRS, 8.18-19). The RRS has also defined two grades of brass: the malleable ritikā and the kākatuṇḍi. The latter when dipped in acidic sour gruel turns black; evidently this is leaded brass, the acid liberating the black globules and the elongated grains of lead which are insoluble in the copper-zinc alloy or brass phase.

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While lead was deliberately alloyed with copper in the ancient world, this was probably not the case in most instances for iron. The copper lumps in Mohenjo-Daro assayed 0.03-1.5 per cent iron. The iron percentages of the copper-based alloys were 0.07-1.4 in Harappa, zero to 4.0 in Lothal and 0.8-6.8 in Atrañjikhera. Metallographic and phase analysis have not been done for most of these samples to indicate whether iron exists as metallic dendrites or globules of oxide/sulphide inclusions. The Kuṣāṇa copper coins contain small amounts of iron globules. (Paramasivan, 1942; Bivar, 1987) Whereas the Sultanganj/ Birmingham Buddha (Craddock, 1981: 2), Nepalese copper and Tibetan icons show small quantities of iron, the Gujarat and South Indian icons (vide items 24 and 30 in Table 20.10) contain appreciable amounts. Some of the Nālandā icons (Table 20.8) assay higher proportions (1.3-5.8 %) of iron. Table 20.11 shows the compositions of five iron-rich copper coins from Paunar, Maharashtra state, India, dated seventh century AD. (Hegde, 1975)

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Composition of seventh century AD Paunar Copper Coins in Percentages of Elements

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| Sample No. | Cu | Fe | Sn | Pb | Impurities |
| 1. | 74.60 | 21.76 | 3.20 | 0.30 | Ni, Co, Bi, Zn |
| 2. | 73.77 | 22.00 | 3.35 | 0.37 | - do - |
| 3. | 71.91 | 23.21 | 4.40 | 0.24 | - do - |
| 4. | 77.31 | 22.10 | — | 0.42 | Ni, Co, Bi, Zn and Mn |
| 5. | 77.21 | 22.31 | — | 0.26 | - do - |

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Probably these copper coins were made when there was some scarcity of the principal metal in the country. The presence of tin indicates deliberate alloying. But regarding the high proportion of iron, we are not sure if this was externally added.

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The possibility that high melting point copper and iron metals were melted together in special (wootz-making) refractory crucibles withstanding high temperature (1400°C or above) may not be altogether ruled out; but this is unlikely in view of the fact that inclusions in the copper matrix often corresponded to the copper-iron sulphides. Craddock and Meeks (1987: 193-201) have suggested a series of alternative propositions.

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The earliest or primitive smelting method for copper involved the use of high grade copper ore, usually oxide or carbonate, not too high a temperature and only moderate reducing condition. These parameters resulted in inefficient metal recovery but purer copper uncontaminated with slags or iron. With progress of time, low-grade iron-bearing sulphidic copper ores and iron fluxes were used. Better refractories and more charcoal resulted in high temperatures inside the furnace and better reducing conditions which resulted in reduction of even the iron-bearing phases to the metal. It has been shown (Peters, 1907; Merkel, 1982) that under these conditions the temperature of the copper-reducing furnace may reach or even exceed 1400°C, and the reduced iron, in presence of carbon, sulphur and metallic sulphides, becomes soluble in copper to a large extent. On cooling however, the solubility of iron in copper is much reduced and what we have ultimately in the solid phase are alpha-iron dendrites (1.4 % copper in solid solution) in a matrix of copper-rich epsilon phase, and dispersed globules of fayalite slag and copper-iron sulphides (low melting point 950°C). Merkel (1982, 1983) has established that this is indeed the end result of his copper-smelting simulation experiments. The phase analysis of his end-products and the sixth-third century BC ramo secco high-iron (20-40 % Fe) copper bars from Italy are indeed similar. (Craddock and Meeks, 1987: 196-99)

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Thus it is possible that the items like the seventh century AD. Paunar coins, the eighth-ninth century AD Nālandā bronzes assaying 4-6 per cent iron and all other Indian artefacts produced through three to four millennia and assaying more than 1 per cent iron have been the products of co-smelting under the special physico-chemical conditions which arose only with the somewhat advanced level of technology. Under less advanced conditions the chalcolithic objects contained much less iron.

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Craddock (1981: 16) has suggested that if iron rod were used to stir and mix the multi-metallic melt before casting, some iron could have been dissolved. This topic definitely needs further investigation.

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