Prussian black is calcined Prussian blue. It is not of a good colour, nor economical.
Prussiate black is the carbonaceous residue from making yellow prussiate of potash. Used chiefly for decolorising syrups, &c.
Spanish black is cork black.
Tannin black is proposed to be made by exhausting the tannin from refuse leather and tanning agents, and adding alum and sulphate of iron. The colour is blue-black, weak, and unstable.
CHAPTER III.
BLUES.
Cobalt Blues.—Some of the compounds of cobalt with alumina, phosphoric acid, silica, and tin, are remarkable for possessing a fine blue colour of great permanency and indestructibility, and still find a limited application. They are chiefly as follows:—
| Cœruleum—a mixture of the oxides of cobalt and tin. |
| Cobalt blue—a mixture of the oxides of cobalt and alumina. |
| Smalts—a double silicate of cobalt and potash. |
Cœruleum.—This is a light-blue slightly greenish colour, with no purple tendency in artificial light. It is non-granular, covers well, mixes with water or oil, and is a good artists’ colour for sky effects. It is permanent in strong sunlight and impure atmosphere, and resists acids and alkalies at normal temperatures. Hot hydrochloric acid dissolves it, and addition of water to the pale blue solution produces a violet red; evaporation to dryness restores the original pigment. The green tint of a nitric acid solution is due to iron and nickel impurites. Dilute sulphuric acid causes partial decomposition of cœruleum, but it is proof against caustic potash, acetic acid, and concentrated sulphuric acid. Its composition is given as
| Cobalt oxide | 18·66 |
| Tin oxide | 49·66 |
| Silica and sulphate of lime | 31·68 |
| 100·00 |