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To Color Gold

Take 1 part salt, 1 part alum, and 2 parts saltpetre; each material to be well pounded separately in a Weighing. The pure gold is next accurately weighed. This weight doubled (if 12 grains are under assay), or quadrupled (if 6 grains), gives the number of carats fine of the mortar; put them into an iron pot with 1 of carats fine of the mortar; put them into an iron pot with alloy examined, without calculation. The pint water, and heat slowly over a fire: boil loss of weight by cupellation gives the gently and stir with an iron rod until it rises. amount of copper in the sample; that after It is then ready for the reception of the arparting, the amount of silver, deducting, of ticles to be colored, which must be not less course, the weight of silver used in the pro- than 18 carat fine. They are suspended in cess, which is called the witness. When the sample contains but very little gold, the dry method of assaying cannot be depended on, and chemical analysis must be had recourse to. (Cooley.)

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. 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.