All varieties of Vandyke brown are stable pigments, without any disturbing influence when used in admixture with other colours, and quite proof against any change on exposure to air and light. Next to the umbers they are the most generally useful browns.
CHAPTER V.
GREENS.
Green pigments form an important and numerous class, but many of those which possess the most brilliant and durable qualities contain highly poisonous ingredients, and some of the most beautiful are not permanent. All things considered they are perhaps the least satisfactory group of colouring matters. The following list comprises all worth notice.
Baryta Green.—It is said that the manganate of baryta makes an excellent green pigment, which may with advantage replace for many purposes those greens which contain arsenic. Several methods of preparing it have been published:—(a) One consists in igniting together the nitrate of baryta and manganese oxide or dioxide. (b) Another consists in fusing a mixture of pyrolusite or black oxide of manganese, caustic bartya, and chlorate of potash. (c) According to a third method, mix 2 parts caustic soda and 1 part chlorate of potash, and gradually add 2 parts very finely powdered manganese; heat gradually up to dull redness, then allow to cool, powder, and exhaust with water; filter and cool, and add a solution of nitrate of baryta to the filtrate; a violet-coloured baryta precipitate forms; this is carefully washed, dried, and treated with ½-1 part of caustic baryta, hydrated, and gradually heated up to redness, with constant stirring. The cooled mass is powdered, and finally washed to remove any excess of baryta.
By either process a green mass is obtained, but the second method seems to yield a more beautiful and homogeneous product. In experimenting with other and more direct methods for preparing a baryta green of great purity and beauty, Fleicher has made several observations of its properties. If a green solution of manganate of potash be precipitated, while boiling, by chloride of barium, a heavy, granular, but not crystalline, precipitate of manganate of barium is obtained. This precipitate has a violet colour, approaching blue, can be washed by decantation at first, and afterwards may be collected on a filter. On drying the precipitate, its colour grows lighter with the increase of temperature; and on being heated to a dark red heat, it looks almost perfectly white, with only a shade of greyish blue. If, then, it be heated still higher with free access of air, or in an oxidising flame, it gradually turns green; by carrying the process farther the colour becomes a beautiful greenish-blue, and finally, at a very high heat, a dirty greyish-brown mass is formed from the reduction of the manganic acid to binoxide of manganese. On adding chloride of barium to a solution of the permanganate of potash, and boiling, a precipitate is slowly formed of a peach-blossom colour, while the liquid retains a deep violet colour. By decanting and bringing the mass, diluted with water, on a filter, the precipitate is not decomposed, and can be dried at 212° F. without changing colour. When the dry permanganate of barium is gradually heated, its colour also grows paler, but does not, like the manganate of baryta, acquire a green colour at a still higher temperature; for after the colour has once vanished, an increase of temperature soon converts it into the greyish-brown mixture of the binoxide of manganese and baryta or carbonate of baryta. Hence it is impossible to prepare the green manganate of baryta from the permanganate.
In regard to the colour itself, experiments have shown that the most beautiful green is that formed by igniting the manganate as described above. The green prepared by Rosenstiehl’s process—fusing together caustic baryta, chlorate of potash, and binoxide of manganese—is less beautiful than the above; while that attained from nitrate of baryta and binoxide of manganese is far inferior to either of the others. Perhaps, however, this colour could be improved by preparing it in a reverberatory furnace with a strong oxidising flame.
The blue-green baryta pigment has different shades, according to its preparation, some being almost pure blue with only a shade of green, and resembling the light blue quill feathers of many parrots. The greener the colour the more it gains in intensity, but it loses in fineness, although still surpassing the green manganate of baryta.
The production of the blue or bluish-green baryta is due entirely to the alkaline property of the mass. Whether each definite colour is due to a definite composition is doubtful, since the temperature, which must not exceed that of a bright red heat, exerts a greater influence on the colour. This much is however certain, that both manganic acid as well as the permanganate of baryta, when mixed with about 20 per cent. of hydrate of baryta and ignited at a red heat, will always produce this blue-green colour. It is evident that the blue-green colour is dependent entirely on its basic character; for on placing this powder in weak acids, it first turns green and is then gradually decomposed. The baryta pigment is quite permanent, and may be subjected to the action of strong sulphuric acid for hours, at the ordinary temperature, before the colour will be destroyed. Boiling potash solution has no perceptible effect upon it. The permanence, especially of the blue shade, is increased by adding a little baryta, which increases its alkalinity. It is also worthy of remark that the pigment prepared from the nitrate of baryta is much less permanent, because the nitrous acid present will after a time exert a reducing action.
The baryta pigments seem especially adapted to fresco painting, because they appear very bright and lively on stone, and especially on lime, where many other pigments lose their beauty or are entirely destroyed.
Bremen Green.—This old-fashioned pigment is a basic carbonate of copper, and has been produced in several ways. At first a basic chloride or oxychloride was used, its mode of preparation varying somewhat but without affecting the character of the result, the great essential being that no subchloride of copper should be present. Therefore, in some factories, it was the practice to prepare the magma of basic oxychloride even a year in advance; or, to subject it to repeated wetting and drying in order to ensure prefect oxidation. The method has now become obsolete, and is superseded by the following:—