The following process of irisation is due to Puscher. It allows of covering the metals with a thick layer of metallic sulphide, similar to that met with in nature—in galena, for example.

These compounds are quite solid and are not attacked by concentrated acids and alkalies, while dilute reagents are without action. In 5 minutes thousands of objects of brass can be colored with the brightest hues. If they have been previously cleaned chemically, the colors deposited on the surface adhere with such strength that they can be worked with the burnisher.

Forty-five parts of sodium hyposulphite are dissolved in 500 parts of water; a solution of 15 parts of neutral acetate of lead in 500 parts of water is poured in. The clear mixture, which is composed of a double salt of hyposulphite of lead and of sodium, possesses, when heated to 212° F., the property of decomposing slowly and of depositing brown flakes of lead sulphide. If an article of gold, silver, copper, brass, tombac, iron, or zinc is put into this bath while the precipitation is taking place, the object will be covered with a film of lead sulphide, which will give varied and brilliant colors, according to its thickness. For a uniform coloration, it is necessary that the pieces should be heated quite uniformly. However, iron assumes under this treatment only a blue color, and zinc a bronze color. On articles of copper the first gold color which appears is defective. Lead and tin are not colored.

By substituting for the neutral acetate of lead an equal quantity of cupric sulphate and proceeding in a similar way, brass or imitation gold is covered with a very beautiful red, succeeded by an imperfect green, and finally a magnificent brown, with iridescent points of greenish red. The latter coating is fairly permanent.

Zinc is not colored in this solution, and precipitates in it a quantity of flakes of greenish brown (cupric sulphide), but if about one-third of the preceding solution of lead acetate is added, a solid black color is developed, which, when covered with a light coating of wax, gains much in intensity and solidity.

It is also useful to apply a slight coating of wax to the other colors.

II

Beautiful designs may be obtained, imitating marble, with sheets of copper plunged into a solution of lead, thickened by the addition of gum tragacanth, and heated to 212° F. Afterwards they are treated with the ordinary lead solution. The compounds of antimony, for example the tartrate of antimony and potash, afford similar colorations, but require a longer time for their development. The solutions mentioned do not change., even after a long period, and may be employed several times.

III

By mixing a solution of cupric sulphate with a solution of sodium hyposulphite, a double hyposulphite of sodium and of copper is obtained.

If in the solution of this double salt an article of nickel or of copper, cleaned with nitric acid, then with soda, is immersed, the following colors will appear in a few seconds: Brilliant red, green, rose, blue, and violet. To isolate a color, it is sufficient to take out the object and wash it with water. The colors obtained on nickel present a moiré appearance, similar to that of silk fabrics.

IV

Tin sulphate affords with sodium hyposulphite a double salt, which is reduced by heat, with production of tin sulphide. The action of this double salt on metallic surfaces is the same as that of the double salts of copper and lead. Mixed with a solution of cupric sulphate, all the colors of the spectrum will be readily obtained.

V

Coloration of Silver.—The objects of copper or brass are first covered with a layer of silver, when they are dipped in the following solution at the temperature of 205° to 212° F.: Water, 3,000 parts; sodium hyposulphite, 800 parts; lead acetate, 100 parts.

VI

Iron precipitates bismuth from its chlorhydric solution. On heating this deposit, the colors of the rainbow are obtained.