History of Chemistry in Ancient and Medieval India, Incorporating the History of Hindu Chemistry by Acharya Prafulla Chandra Ray c37.63

c37.63

The articles, after being removed from the boiling *jamak* solution, are thoroughly washed and rubbed with a brush. They are then ready for treatment in another bath. This consists of a solution of tamarind pulp, nitre and common salt in water, which is placed in an earthen pot on fire and heated to ebullition. A little sulphur is then added after which the articles are placed in the liquid. The sulphur is added in a finely divided state obtained by rubbing a stick of sulphur with a little water on a piece of slate. Care is always taken to add an insufficient amount of sulphur at first, for it is found that the shade of colour that is produced on the gold depends solely on the quantity of this ingredient, which accordingly requires exceedingly careful regulation. The colour of the articles is now examined from time to time, adding more sulphur gradually for increasing the depth of colour. When the requisite shade of colour is reached, further addition of sulphur is stopped. The boiling is continued for some time more, after which the articles are taken off the solution and given a final wash and polish. Addition of too much sulphur renders the colour deep violet and finally black The chemical reactions, that occur during this stage of colouring, are rather obscure. P C. Ray, in his book, has suggested a tentative explanation to account for the formation of violet colour on the surface of the metal by the treatment described. According to this view the solution of nitre and common salt, when boiled along with the acids from the tamarind pulp, is likely to liberate chlorine. This attacks the gold, giving rise to auric chloride in solution: the addition of sulphur serves to remove the excess of chlorine, due to the formation of sulphur chloride. The auric chloride slowly breaks up into aurous chloride by heat, and the liberated chlorine is taken up by sulphur. The aurous chloride is then changed into aurous oxide, which is deposited in the form of a thin violet layer on the surface of the articles The explanation appears to be a very probable one, as aurous oxide possesses a dark violet colour and is produced from auric chloride, when a solution of the latter is boiled with salts of organic acids. A considerable amount of gold is also lost in this process.