Chrome Green.—Although very different substances are commercially known as chrome green, this name is usually applied to the very valuable pigment which consists of pure chromium oxide. There is hardly another pigment for the preparation of which such varied directions have been given and whose shade varies so much according to the manner of production.

The cheapest process is to ignite potassium bichromate with sulphur, extract the mass with very dilute sulphuric acid and wash the residue. The chromic acid is reduced by the sulphur; when the mass is treated with sulphuric acid sulphur dioxide is evolved, potassium sulphide and sulphate are dissolved, whilst pure chromium oxide remains. The larger the quantity of sulphur used the paler is the chromium oxide. The purity of the potassium bichromate has considerable influence on the shade of the pigment; if iron is present in any quantity a discoloured product always results. Convenient proportions are: 19 parts of potassium bichromate and 4 parts of sulphur, which produce 9·33 parts of chromium oxide. If potassium bichromate free from iron cannot be obtained, the shade of the product is somewhat improved by treatment with dilute hydrochloric acid, in which ferric oxide dissolves more easily than chromium oxide. When the latter has been strongly ignited it is soluble with very great difficulty.

There are many formulæ for the preparation of chromium oxide by means of sulphur: in all the above statement holds good, that the more sulphur the paler the product.

According to A. Casali, a chrome green which satisfies every requirement is obtained by strongly igniting 1 part of potassium bichromate with 3 parts of burnt gypsum. After ignition the mass is boiled with very dilute hydrochloric acid. In this process the chromic oxide is produced according to the following equation:—

2K₂C₂O₇ + CaSO₄ = 2Cr₂O₃ + 2K₂SO₄ + 2CaO + 3O₂.

On boiling with hydrochloric acid the lime is dissolved; when the liquid remains distinctly acid after long boiling it is poured off and the chromium oxide washed with hot water and dried.

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.