Gold alloys of not less quality than 15 carat may be made to assume the colour of fine gold by carefully boiling them in a mixture of nitrate of potash 15 oz., table salt 7 oz., alum 7 oz., and spirit of salts 1 oz. The work must be previously annealed and boiled out in aquafortis pickle, and wired with platinum wire. It must only be exposed to the colouring mixture for five minutes at a time, and well rinsed in boiling water between each operation. If 18-carat gold alloys are employed, the colouring mixture may consist of 1 oz. more of each of the above ingredients, omitting entirely the spirit of salts, and making the other powders into a paste with hot water. In all cases it is advisable to thin the colouring mixture with hot water as the process of colouring progresses, so as to avoid overdoing the work.