Chrome Green.—This name is often applied to any green in which chrome enters as an element, but more particularly to the modern Brunswick greens described on pp. 114-118; and to the green which bears the name of its first maker, Guignet, and described under the title of Guignet’s Green, see [p. 125].
Cobalt Green.—This remarkably stable, but somewhat costly, pigment is also known by the names of Rinmann green and zinc green, the former after the name of the chemist who first prepared it, and the latter because it contains a large proportion of zinc. It is in fact a combination of the oxides of cobalt and zinc, and was originally produced in the following manner:-½ lb. pure cobalt ore was dissolved in 4 lb. concentrated nitric acid, and added to a solution of 1 lb. zinc in 5 lb. nitric acid; the mixture was diluted with water, and a solution of potash carbonate was added, throwing down a pinkish precipitate, which was washed on a filter, dried, and calcined at a high temperature.
Wagner found that an indispensable condition was to have a protoxide of cobalt as free as possible from foreign metals, with which object he practised the following method:—Cobalt oxide is dissolved in three equivalents of hydrochloric acid, and the solution is evaporated to dryness; the residue is dissolved in six equivalents of water, and through the solution is passed a current of sulphuretted hydrogen gas, so long as any precipitate is formed. This precipitate consists of sulphides of the foreign metals. The clear solution is siphoned off, evaporated to dryness, and the residue is dissolved in water. As required, this solution is treated with carbonate of soda, and the precipitate, washed, and while still wet, is mixed with zinc white. The reddish mass produced in this way is dried and calcined. The best tone is attained by combining 9 to 10 parts of zinc oxide with 1 to 1½ parts of cobalt protoxide.
Louyet has shown that if the cobaltic solution be precipitated by the phosphate or the arseniate of potash, the corresponding salt of cobalt thus produced possesses the property of imparting a green colour to zinc white at a much lower temperature than is required in the case of ordinary protoxide of cobalt: moreover, the pigment gains in body, and the colour gains in purity and brightness. If a small quantity of arsenious acid is added to the ordinary mixture before calcination, the calcined mass will assume a remarkably bright green colour; and its structure being loosened by the disengagement of fumes of arsenious acid, it will be easy to grind.
According to Barruel and Leclaire’s method, 1 lb. of pure dry sulphate of cobalt, dissolved in hot water, is mixed with 5 lb. of zinc oxide. The mixture is dried, and calcined for three hours at a clear red heat in a muffle; when cooled, it is thrown into water, washed, and dried.
The composition of cobalt green has been shown by Wagner to vary considerably, as is to be expected from the methods of its preparation. The proportion of zinc oxide ranges from 71½ to 88 per cent., and the cobalt protoxide from 11½ to 19 per cent.; in addition, there will be fluctuating percentages of phosphoric acid, soda, oxide of iron, &c., according to the process followed.
With the single exception of its costliness, cobalt green possesses advantages over most other green pigments. It has a bright colour, sometimes inclining to a yellowish tint, or, when phosphates are used in its preparation, leaning to a blue shade. But it is always permanent, not only under the influence of air and light, but also in the presence of alkalies and any but concentrated acids; thus it may safely be compounded with other pigments.
Douglas Green.—This pigment, which is fairly permanent, and possessed of considerable covering power, owes its name to the chemist who proposes its use, and its colour to the oxide of chromium. The method by which it is prepared is as follows:—Solutions of barium chloride and potassium chromate are mixed together. To the barium chromate thus produced is added one-fifth of its weight of concentrated sulphuric acid, whereby partial decomposition is brought about, resulting in a mixture of barium chromate, barium sulphate, and chromic acid. This mixture is dried, and calcined in a crucible at bright red heat, the effect of which is that the chromic acid is converted into green oxide of chromium, and, being scattered throughout the mass, imbues it with a green colour.
Emerald Green.—This is quite an old-fashioned pigment, having been in use some 80 years. It is a combination of acetate and arsenite of copper, and varies in tint from a dark to a pale green, always with a bluish cast. It possesses good covering power, and can be used either as an oil-or as a water-colour, but particularly as the latter, and is much used in paper staining. In composition it varies considerably, as there are some half-dozen industrial methods of making it; but in general terms it usually contains over 50 per cent. of arsenious acid, and about 30 per cent. of oxide of copper, together with various impurities. Following are some of the processes by which it is manufactured.
(1) According to the method introduced by Liebig, 1 part of verdigris is heated in a copper kettle with sufficient distilled vinegar to effect its solution, and to this is added a solution of 1 part of arsenious acid in water. The result is a precipitate of a dirty green colour, which is dissolved in a new quantity of vinegar and boiled for some time. In this way is obtained a new precipitate, granular and crystalline, and exhibiting a splendid green colour. When this has been filtered off, washed, and drained, it is boiled with one-tenth of its weight of commercial potash, in order to deepen and brighten the colour and destroy the bluish tint. Should the waste liquor obtained after the filtration of the pigment from the second boiling in vinegar contain any remaining copper, arsenious acid is added; and if arsenious acid be present, copper acetate is added; while if acetic acid survives it may be used again for dissolving another lot of verdigris.