Green Oxide Of Chromium

Synonyms:

Chromium Sesquioxide, True Chrome Green, Opaque Oxide Of Chromium, Vert De Chrome, Grimes Chromoxyd

The so-called native oxide of chromium, or chrome ochre, is a mere greenish clay, containing not more than 10 percent of chromium oxide. But another mineral (from Okhansk, in Siberia) is of a deeper green, and sometimes contains one-third of its weight of this oxide. We are not aware that any attempt to employ these minerals as pigments has ever been made.

The green oxide of chromium, Cr₂O₃, varies in hue, in depth of colour, and in opacity according to the process of its preparation. It may be made either in the 'dry' way or the 'wet' way. Perhaps the finest quality is that produced by heating mercurous chromate in a retort till the whole of the mercury has been distilled off. This is an expensive process, but the product is excellent in colour and body. Ammonium bichromate gently heated, in small portions at a time, yields a dull-coloured but a useful variety of this pigment; but there is a similar, though more economical process, yielding a better-coloured product. This consists in gently heating together and then calcining a mixture of 3 parts of neutral potassium chromate with 2 parts of ammonium chloride. The mass is to be thoroughly washed with hot water, dried, and again ignited. Two other processes for preparing this pigment in the dry way may be commended. In both of them potassium bichromate (free from iron) is used, but in the one case sulphur, in the other starch, is employed, in order to effect the reduction of the chromic salt to the condition of ses-quioxide. The sulphur method yields at once a good pigment, which needs nothing but a thorough washing, first with very dilute sulphuric acid and then with water, followed by grinding, to fit it for use, but a second calcination is required in the starch process.

In order to prepare oxide of chromium by the wet way, a solution of an alkaline chromate or bichromate is to be reduced by sulphur or other reducing agents, or a sesquisalt of chromium is precipitated by ammonia or a fixed alkali. The hydrated sesquioxide is thrown down; after having been washed, this substance is gently calcined. The hydrate itself, when air-dried, constitutes one of the forms of 'transparent oxide of chromium.'

The ordinary or opaque oxide of chromium usually occurs in the form of a greyish-green powder of considerable body. It is quite permanent under all conditions of exposure and of commixture with other pigments, and is available in all the processes of painting. Its tints with flake-white, and the yellowish greens which it yields with aureolin, are peculiarly valuable to the landscape-painter.

Green oxide of chromium is rather imitated than adulterated. This pigment when genuine is not altered in colour either by ammonium sulphide or caustic potash. But under the name of chrome-green mixtures of Prussian blue and lead chromate are constantly sold. These preparations generally contain some barium sulphate, often a considerable quantity of gypsum, and, occasionally, alumina. The term 'chrome-green' cannot be justly applied to these mixtures, which are greatly inferior in stability to the pigments under discussion, and are all discoloured by lime and alkalies.