History of Chemistry in Ancient and Medieval India, Incorporating the History of Hindu Chemistry by Acharya Prafulla Chandra Ray c37.60

c37.60

During heating, the surface of the article is blackened due to oxidation and formation of cupric oxide. This is intensified by hammering in the intervals between repeated heating Mention may be made in this connection of a particular artifice, which is sometimes employed by the goldsmiths to cleanse the surface of a gold-copper alloy that has been blackened by fire. It is found that if a blackened piece of gold copper alloy be heated in a charcoal fire to redness, and then water be sprinkled on the fire, the alloy at once acquires the bright colour of gold. The explanation of the phenomenon seems to be that, when placed in a charcoal fire, the cupric oxide is reduced to metallic copper But if it is taken out when hot, it comes into contact with the air again and is oxidized What the goldsmith does, is to suddenly cool down the metal when in the reduced state, and not to allow it to come into contact with the open air unless it is quite cold. This prevents reoxidization and hence blackening