History of Chemistry in Ancient and Medieval India, Incorporating the History of Hindu Chemistry by Acharya Prafulla Chandra Ray c37.61

c37.61

The gold ornaments are usually coloured bright red or orange, and sometimes reddish violet. The process of colouring, as practised by the goldsmiths or by the *rungwalas*—a class of people whose sole business is to colour ornaments or other articles of gold, involves rather cumbrous and complicated chemical operations. The process may be briefly described as follows. The articles are first cleansed of all external dirt, as well as of the blackness, due to the formation of cupric oxide on the surface when heated in a charcoal fire. They are then placed in a bath of boiling concentrated decoction of unripe fruits of tamarind (*Tamarindus indica*). This liquid contains considerable amount of tartaric and other acids in the free state. These acids dissolve away the copper oxide from the articles, leaving a yellow surface of pure gold. This is indicated by the fact that the solution of the tamarind pulp becomes distinctly blue in virtue of the formation of copper salts This treatment also removes the oxide of zinc from the soldered surface. The articles, thus cleansed, are then covered with a thin layer of a paste, made of equal parts of common salt and potash alum, and afterwards placed on the fire. After the paste is dried up, it is washed off with water. The colour of the gold is thereby somewhat improved, as this treatment removes any particles of oxides of copper still remaining on the surface.