The pieces to be operated on must first be slightly corroded by placing them for a minute or two in dilute sulphuric acid. They are next rubbed with sand and water, washed and dried. Brown of any shade is produced by dipping the pieces in some solution of a nitrate or in iron per-chloride. The shade depends on the concentration of the solution. A chocolate color is obtained by roasting with moist red iron oxide and polishing with a small quantity of galena. Black brass for optical instruments is obtained by dipping the brass objects in a mixture consisting of solutions of gold or platinum and stannic nitrate.
To very handsomely color brass black, mix 180 grams carbonate of copper, 400 grams aqua ammonia, and 400 grams water. The cleansed brass articles can be dipped into this mixture, frequently withdrawn to inspect them, rinsed in water, and dried in sawdust, and the process is repeated twice; the articles are then freely rubbed with a little linseed oil; the color will then be that of ebony. The oil process of silver is somewhat dearer, and another of dipping, hot, into nitrate of copper, is ruinous to delicately soldered articles, wherefore the first mentioned method is preferable.
A steel color is developed on brass by using a boiling solution of arsenic chloride, while a careful application of a concentrated solution of sodium sulphide causes a blue coloration. Black, being generally used for optical instruments, is obtained from a solution of platinum chloride, to which tin nitrate has been added. In Japan the brass is bronzed by using a boiling solution of copper sulphate, alum and verdigris.