Mixtures of a yellow and a blue pigment produce a green; according as one or the other predominates colours are obtained inclining to yellow or blue.

In some cases the mixing can be accomplished in the actual production of the pigments, so that a green precipitate is directly produced. This is, however, rarely the case; generally the compound pigments are obtained by simply mixing the two colours. The mixing can be done either in the dry or the wet way; colours in the wet state are more easily mixed than when dry, so that even when dry colours are employed the mixing is done with the addition of water. Although the mixture then requires a second drying, this method is still to be recommended, in the first place, because the mixing can be more quickly done in consequence of the greater mobility of the mass, and in the second, because the formation of poisonous dust is completely avoided.

The mixing is generally accomplished by mechanical arrangements. If dry colours are to be mixed, rotating cylinders may be used; these are filled with the materials to be mixed, well closed and rotated about the axis as long as may be necessary. It should be observed that when colours which have a very different specific gravity, such as chrome yellow and Prussian blue, are to be mixed, the cylinders must be rotated for a much longer time than when colours of approximately equal specific gravity are to be united.

In working in the wet way, which is to be preferred, sufficient water is added to the colours to make a paste thin enough to be mixed by spatulas; the mixture is thoroughly stirred and the pulp ground in ordinary mills until the mixture is quite uniform. Already whilst on the mills the pulp becomes thicker in consequence of evaporation; when the grinding is finished it is spread out in thin layers so that it may dry as quickly as possible. This is important in the case of mixtures whose constituents have very different specific gravities, otherwise the heavier pigment may sink to the bottom of the paste and the mass thus lose its uniformity.

The compound green pigments come into the market under most varied names, which are often entirely without connection with their chemical composition. Such names are: mineral green, English green, oil green, green vermilion; the most common are chrome green and Brunswick green. It should be noted that these names are more strictly applied to simple colours previously described.

Chrome Green is generally made by mixing deep chrome yellow and Prussian blue; it can naturally be obtained in all possible shades. The brightness of this already handsome pigment can be considerably increased by the addition of a small quantity of indigo carmine. In this case the mixing is best done by adding a solution of the indigo carmine to the stiff paste and again sending the mixture through the mill.

By additions of white substances paler shades of chrome green are obtained; levigated terra alba or white pipe clay can be advantageously employed.

Elsner’s Chrome Green is obtained by preparing a solution of yellow prussiate of potash and potassium chromate and another solution of lead acetate and ferric chloride. The two liquids are mixed with vigorous stirring; according to the proportions of the materials a blue or a yellow shade of green is produced.

Silk Green.—Forty-one parts of lead nitrate are dissolved in 20 to 30 times the weight of water, the solution is boiled in a copper pan, and, according to the shade required, 10 to 30 parts of fine Chinese blue are added. After well stirring, a solution of 10 parts of potassium bichromate and 1 part of nitric acid is poured into the boiling liquid, the mixture is again well stirred, the precipitate allowed to settle, washed and dried. The green pigment obtained in this way has a peculiar silky lustre, hence the name ”silk green”.

Natural Green is a mixture of Guignet’s green with picric acid; it is used in making artificial flowers instead of emerald green.

Non-arsenical Green was proposed as a substitute for emerald green, which, however, it does not equal in brilliance. It is obtained by mixing copper blue (basic copper carbonate) with chrome yellow, chalk and ferric oxide, in somewhat variable proportions. It usually contains 80 to 82 per cent. of the copper compound and 13 to 15 per cent. of chrome yellow.

In making mixtures of pigments it is to be remembered that chemical actions may occur in the mixture; colours which act upon one another should not be mixed: the mixture would contain the cause of its own destruction. For example, a lead pigment should not be mixed with one containing sulphur in the form of a sulphide or sulphate. By following this important rule the number of pigments which may be mixed together is considerably diminished; the advantage is that durable colours are produced, which do not change in a short time to an unrecognisable shade.