This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
(See also Gems, Gold, and Watchmakers' Recipes.)
Following are several recipes for coloring: Saltpeter, 40 parts; alum, 30 parts; sea salt, 30 parts; or, liquid ammonia, 100 parts; sea salt, 3 parts; water, 100 parts. Heat without allowing to boil and plunge the objects into it for 2 or 3 minutes, stirring constantly; rinse in alum water and then in clean water. Another recipe: Calcium bromide, 100 parts; bromine, 5 parts. Place the articles in this solution, with stirring, for 2 to 3 minutes; next wash in a solution of hyposulphite of sodium and rinse in clean water. Another: Verdigris, 30 parts; sea salt, 30 parts; blood stone, 30 parts; sal ammoniac, 30 parts; alum, 5 parts. Grind all and stir with strong vinegar; or, verdigris, 100 parts; hydro-chlorate of ammonia, 100 parts; saltpeter, 65 parts; copper filings, 40 parts. Bray all and mix with strong vinegar.
Chuck the hole in a lathe with cement. Place a spirit lamp underneath to prevent the cement from hardening. Hold the pointed bit against the hole, while the lathe is running, until the hole is true, when the lamp should be removed. The broach to widen the hole should be made of copper, of the required size and shape, and the point, after being oiled, should be rolled in diamond dust until it is entirely covered. The diamond dust should then be beaten in with a burnisher, using very light blows so as not to bruise the broach. After the hole is widened as desired, it requires polishing with a broach made of ivory and used with oil and the finest diamond dust, loose, not driven into the broach.
Reduce bread crumbs into small particles, and introduce into all the curves and hollows of the jewelry, while rubbing with a flannel.
In coloring gold below 18 carat, the following mixture may be used with success, and if carefully employed, even 12 carat gold may be colored by it: Take nitrate of potassa (saltpeter), 4 parts, by weight; alum, 2 parts; and common salt, 2 parts. Add sufficient warm water to mix the ingredients into a thin paste; place the mixture in a small pipkin or crucible and allow to boil. The article to be colored should be suspended by a wire and dipped into the mixture, where it should remain from 10 to 20 minutes. The article should then be removed and well rinsed in hot water, when it must be scratch brushed, again rinsed and returned to the coloring salts for a few minutes; it is then to be again rinsed in hot water, scratch brushed, and finally brushed with soap and hot water, rinsed in hot water, and placed in boxwood sawdust. The object being merely to
remove the alloy, as soon as the article has acquired the proper color of fine gold it may be considered sufficiently acted upon by the above mixture. The coloring salts should not be used for gold of a lower standard than 12 carat, and, even for this quality of gold, some care must be taken when the articles are of a very slight make.