3192. Assay of Gold by Chemical Analysis

3192.     Assay of Gold by Chemical Analysis. The richness of gold in any substance, whether liquid or solid, especially where the quantity is small, is most easily obtained by chemical analysis. The gold is thrown down from its solution by adding a solution of protosulphate of iron; the precipitate, after being washed, dried and gently heated, may be weighed as pure gold.

If 100 grains of the substance or liquid under test be taken for examination, the weight in grains of the dried precipitate will give the percentage of gold contained in the sample.

3193. To Obtain Gold Chemically Pure

3193.     To Obtain Gold Chemically Pure. Dissolve gold in nitromuriatic acid (a mixture of 1 part nitric acid with 2 parts muriatic acid, and called aqua regia); by adding to the gold solution a solution of protosulphate of iron, the pure gold is precipitated in the form of a brown powder, which should be thoroughly washed to free it from acid, and then dried. In this form it is ready to mix by fusion with other metals ; or the powder can be reduced to solid metallic form by melting in a crucible, with a charcoal fire, sprinkling occasionally into the crucible a little saltpetre and potash as a flux. The gold will form a button at the bottom.

3194. Grain Gold

3194.      Grain Gold. Cupelled gold, 1 part; silver, 3 parts; melt and pour in a small stream into water; dissolve out the silver with nitric acid, and heat the grains to redness. Used to make preparations of gold.

3195. Liquid Gold

3195.       Liquid Gold. Agitate ether with a solution of terchloride of gold for some time, allow it to repose, and decant the supernatant portion. ■ Naphtha and essential oils possess the same property as ether, of taking gold from its solutions. This liquid was formerly held in great esteem as a cordial medicine. It is now only employed for writing on steel, gilding, etc.. As it dries, it leaves a coating of pure gold. (See No. 3585 (Ethereal Solution of Gold for Gilding on Steel).)

3196. To Make Watch Hands Red

3196.    To Make Watch Hands Red. Mix to a paste over a lamp, 1 ounce carmine, 1 ounce chloride of silver, and 1/2 ounce tinners' japan. Put some of the paste on the hands, and lay them face upwards on a sheet of copper, holding it over a spirit lamp until the desired color appears on them.

3197. French Method for Coloring Gold

3197.     French Method for Coloring Gold. A solution is made of 2 parts nitre, 1 part Roman alum, and 1 of sea salt. The jewels or articles of gold are kept in the solution at a boiling point for from 15 to 25 minutes ; and then washed in water. The surface of the gold is dull, but perfectly uniform, and ready for burnishing.

3198. To Color Gold

3198.    To Color Gold. Take 1 part salt, 1 part alum, and 2 parts saltpetre; each material to be well pounded separately in a -mortar; put them into an iron pot with £■ pint water, and heat slowly over a fire: boil gently and stir with an iron rod until it rises. It is then ready for the reception of the articles to be colored, which must be not less than 18 carat fine. They are suspended in the color by 18 carat wire, and kept in motion till the liquid begins to sink, then taken out and dipped in aquafortis pickle. The color liquid will rise again, and then another dip, and sometimes two, may be necessary to give the articles the proper color. This process of coloring is no more than taking from the surface the inferior metals, leaving a thin coating of pure gold; its application should not be too long continued, as it also dissolves a small portion of the gold.