History of Technology in India, Vol. I: From Antiquity to c. 1200 A.D. v1.c13.s2.t3.21

v1.c13.s2.t3.21

Silver metal was obtained from the alloy of Pb-Ag by reheating it in a shallow hearth furnace having the bottom prepared with bone charcoal. During remelting of the alloy it is subjected to oxidizing condition by blowing excess air when lead gets preferentially oxidized to PbO (litharge) which has a very low melting point. This molten litharge is skimmed off and partially absorbed by the furnace hearth. This process is known as ‘Cupellation’. At the end of the refining process purified liquid silver is tapped out and cast into ingots. Although no clear evidence has been found at Zawar of any such furnace or the remains of Litharge but it has been known in ancient times for the production of lead and silver. Litharge might have been reduced in another ore hearth furnace to produce metallic lead. Kautilya’s Arthasastra states the following details of the process of cupellation for the purification of silver.