A History of Hindu Chemistry from the Earliest Times to the Middle of the Sixteenth Century A.D. v1.c18.s3.6

v1.c18.s3.6

We may, perhaps, in this connection mention a particular artifice which is sometimes employed by our goldsmiths to cleanse the surface of a gold and copper alloy, which has been blackened by fire. This method is employed whenever it is desirable in the course of working to expose the bright surface, but where it would be inconvenient, impossible or unnecessary to apply the long and laborious mode of cleansing or colouring which we shall hereafter describe, It is found that if a blackened piece of gold and 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. This phenomenon, though often practised by our goldsmiths, is a puzzle to the most intelligent among them. * * * * * The true explanation of the operation 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 oxidised. 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, until it has cooled sufficiently down, so as not to be liable to re-oxidisation.