A beautiful and permanent dark or light green coating, resembling enamel, can be applied to all kinds of zinc articles, especially those made of sheet zinc, in the following manner: Fifty parts of hyposulphite of soda are dissolved in 500 of boiling water, and the solution poured at once, in a fine stream, into 25 parts of strong sulphuric acid. The milk of sulphur that separates will soon ball together in lumps and settle. The hot liquid containing sulphate of soda and sulphurous acid is decanted, and the cleansed zinc put in it. In a short time it will acquire a very brilliant, light green coating of sulphide, and only needs to be washed and dried. By exposing it repeatedly and for a longer time to this hot bath, the coating grows thicker and the color darker and more brilliant. The temperature must not fall below 145 degrees Fahr.; when it does it should be heated up to 190 degrees Fahr., to obtain a fine and brilliant deposit.
By dipping these articles in dilute hydrochloric acid, 1 of acid to 3 of water, sulphuretted hydrogen is evolved, and this enamel-like coating loses its lustre and gets lighter in color. Aqueous solutions of aniline colors have little effect upon this dull surface, and none on the gray brilliant coating.
The effect of marbling can be obtained by moistening the gray zinc and applying hydrochloric acid in spots with a sponge, then rinsing it off, and while still wet flowing over it an acidified solution of sulphate of copper, which produces the appearance of black marble. As the zinc has generally a dull surface it must receive a coat of copal varnish. If 15 grammes of chrome alum and 15 more of hyposulphite of soda be added to the above solution, the article will have a brownish color. The above can all be applied to articles made of cost-iron.