Minerals and Metals in Ancient India v1.c28.s9.9

v1.c28.s9.9

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)