Electro - Gilding. The operation of gilding, or covering other metals with a coating of gold by the battery, is performed in the same manner as electroplating, with the exception of a few practical modifications.

3725. To Prepare Chloride of Gold

3725.    To Prepare Chloride of Gold. Dissolve 1 part gold in 3 parts nitro - hydro-chloric acid (aqua regia); evaporate until vapors of chlorine begin' to be disengaged, then set the solution aside to crystallize. Aqua regia consists of 1 part nitric acid and 2 parts (both by measure) muriatic (hydrochloric) acid.

If aqua ammonia be added to a solution of gold in aqua regia, it precipitates a reddish-yellow deposit, which may be collected, washed, and dried. This is the ammoniuret of gold, and must be handled and prepared with great caution, it being the fulminate of gold.

3726. To Prepare a Solution of Gold

3726.    To Prepare a Solution of Gold. Add a solution of cyanide of potassium to a solution of chloride of gold (see No. 3725 (To Prepare Chloride of Gold)) until all the precipitate is redissolved; but this gives chloride of potassium in the solution, which is not good. In the preparation of the solution by this means there are some interesting reactions. As the chloride of gold has always an excess of acid, the addition of cyanide of potassium causes violent effervescence, and no precipitate of gold takes place until all the free acid is neutralized, which causes a considerable loss to the cyanide of potassium. There is always formed in this deposition a quantity of ammonia and carbonic acid, from the deposition of the cyanate of potash ; and if the chloride of gold be recently prepared, and hot, there is often formed some aurate of ammonia (fulminate of gold), which precipitates with the cyanide of gold. Were this precipitate to be collected and dried, it would explode when slightly heated. By previously diluting the chloride of gold, or using it cold, this compound is not formed. After the free acid is neutralized by the potash, further addition of the cyanide of potassium precipitates the gold as cyanide of gold, having a light yellow color; but as this is slightly soluble in ammonia and some of the alkaline salts, it is not advisable to wash the precipitate, lest there be a loss of gold. Cyanide of potassium is generally added until the precipitate is redis-solved; consequently much impurity is formed in the solution, namely, nitrate and carbonate of potash with chloride of potassium and ammonia. Notwithstanding, this solution works very well for a short time, and it is very good for operations on a small scale.

3727. To Prepare Cyanide of Gold

3727.    To Prepare Cyanide of Gold. Dissolve 1 ounce of fine gold in 28 pennyweights nitric acid and 2 ounces muriatic acid, and add 1 quart hot water. Precipitate with the second preparation used for cyanide of silver (see No. 3697 (To Prepare Cyanide of Silver)), and proceed in the same manner.