Direct Coloration Of Iron And Steel By Cupric Selenite.
The selenite of copper is a greenish salt insoluble in water, and but slightly soluble in water acidulated with nitric or sulphuric acid. It is preferable to mix a solution of cupric sulphate with a solution of selenious acid, and to acidulate with nitric acid, in order to prevent the precipitation of the selenite of copper.
This process, originated by Paul Malherbe, is quite convenient for blackening or bluing small objects of iron or steel, such as metallic pens or other small pieces. It does not succeed so well for objects of cast iron; and the selenious acid is costly, which is an obstacle to its employment on large metallic surfaces.
The baths are quickly impoverished, for insoluble yellow selenite of iron is deposited.
Brilliant Black Coloration.—Selenious acid, 6 parts; cupric sulphate, 10 parts; water, 1,000 parts; nitric acid, 4 to 6 parts.
Blue-Black Coloration.—Selenious acid, 10 parts; cupric sulphate, 10 parts; water, 1,000 parts; nitric acid, 4 to 6 parts.
By immersing the object for a short time the surface of the metal can be colored in succession yellow, rose, purple, violet and blue.
Coloration Of Copper And Brass With Cupric Selenite.
| a. | b. | |
|---|---|---|
| Selenious acid | 6.5 | 2.9 parts |
| Sulphate of copper | 12.5 | 20.0 parts |
| Nitric acid | 2.0 | 2.5 parts |
| Water | 1,000.0 | 1,000.0 parts |