In another method ammonium chromate is very gradually heated. At a certain temperature the salt suddenly becomes incandescent and is converted into a dark green, almost black, mass which has a very similar appearance to dry tea-leaves. The green obtained after washing and powdering is handsomer the lower the temperature of the decomposition.
Chromium oxide may also be made in the wet way, but the product does not compare in beauty with that obtained in the dry way. When soda solution is added to a solution of chrome alum, a greyish green precipitate of chromium hydroxide is formed; on ignition of the washed precipitate pure chromium oxide remains.
In a similar manner chromium oxide is produced by precipitating with soda solution the solution of chromium chloride obtained by adding hydrochloric acid to a solution of potassium bichromate, and then alcohol in small quantities, so long as a reaction occurs and the green liquid becomes deeper in colour.
A handsome and bright chrome green is only obtained when the potassium bichromate is quite free from iron, a very small quantity of which has a very harmful effect upon the brightness of the pigment. The author has found that there is no great difficulty in rendering commercial potassium bichromate tolerably free from iron by recrystallisation. As much of the salt is dissolved in boiling water as it will take up, the boiling solution is quickly filtered and rapidly cooled with continual stirring; the fine crystalline meal is left on a strainer until no more liquid filters through, and then washed with a little cold water to remove the mother liquor. The salt obtained by this simple operation is of great purity and always produces chrome green of a good shade.
When chrome green contains ferric oxide it shows a blackish colour. From such chrome green, which has little value, a product of more pure colour can be obtained with a little care. By treatment with dilute hydrochloric acid the ferric oxide dissolves, even when it has been somewhat strongly ignited; chromium oxide loses almost all solubility in dilute hydrochloric acid after heating at a relatively low temperature. To facilitate the solution of the ferric oxide, the green to be treated is finely powdered; it is then covered with a mixture of equal parts of hydrochloric acid and water. After several days the mass is strained and washed with pure water until the acid reaction disappears. Comparative analyses have shown that almost the whole of the ferric oxide can be removed from chrome green in this manner; the brightness of the colour is considerably increased. It is, however, advisable to employ materials free from iron in the manufacture of chrome green.
Chromium oxide can be obtained as a colour of particular brightness, according to the process of Leune, by precipitating it very slowly from a solution in which it is contained in the green modification. For this purpose a solution of chrome alum is boiled until the violet colour has changed to green; the green modification of chromium oxide is now contained in the solution. It is cooled to about 10° C., and either freshly precipitated alumina or zinc carbonate added, very gradually and in small quantities. A fine green shade of chromium oxide separates; after washing and drying it is obtained in a condition in which it can be used as a pigment.
Chromium oxide can indeed be obtained by this process, but it is not distinguished by particular beauty; the author has never been able to obtain, even by using the precipitant in very small quantities, a chromium oxide surpassing in shade that obtained by the direct precipitation of a chromium salt with an alkali.
The emerald green described in the following chapter is obtained in a similar manner by precipitation with zinc oxide.
CHAPTER XXXII.
OTHER GREEN CHROMIUM PIGMENTS.
Guignet’s Green is a chromium oxide made in the dry way; it is obtained by grinding 1 part of potassium bichromate with 3 parts of pure boric acid and water, drying and heating to a dark red heat with access of air. The hot mass is brought into water; by boiling repeatedly with water it is freed from the boric acid which it obstinately retains. In order to remove the last portions of boric acid it is necessary to boil with sulphuric acid and then with caustic soda; this treatment may be omitted for technical purposes, since the small quantities of boric acid contained in the pigment are not harmful.