When the solution of potassium manganate is mixed with a solution of a barium salt, a handsome violet precipitate immediately forms. This is washed with water and quickly ground with ¾ to 1 part of its weight of barium hydrate. The mixture is heated in a copper dish with constant stirring to a low red heat; the colour changes gradually to a very fine green. When the mass has reached the proper shade it is treated with cold water to remove excess of barium hydrate, until the washings show no trace of alkaline reaction. =Manganous Oxide= is occasionally used as a green pigment, especially for painting metal work; it is obtained in the following manner: Manganese sulphate solution is precipitated by soda solution, and the manganous carbonate obtained strongly ignited in a crucible. In order to avoid oxidation of the manganous oxide, which would readily occur in the process, it is necessary to prevent air from reaching the substance. With this object the crucible in which the heating is conducted is covered by another from which the bottom has been removed and which is filled with coal. The air entering the crucible on cooling must pass through the layer of glowing coals, by which it is deprived of oxygen. =Manganese Blue= is obtained, according to G. Bong, by igniting a mixture of 3 parts of quartz, 6 parts of soda ash, 5 parts of limestone and 3 parts of manganese oxide, or of 3 parts of quartz, 8 parts of barium nitrate and 3 parts of manganese oxide, air being admitted and reducing gases excluded. All materials must be free from iron. The quantity of manganese oxide regulates the depth of the colour, but not its shade; by increasing the soda a greener, by increasing the quartz a more violet, shade is obtained. CHAPTER XXXV. COMPOUNDED GREEN PIGMENTS. 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.