have ever been used on a practical scale is doubtful; these will be briefly described. BRIGHTON GREEN is the name given to a pigment made by grinding together in the dry condition 7 lbs. of copper sulphate, 3 lbs. of acetate of lead, and 24 lbs. of whiting; during the grinding chemical decomposition sets in, resulting in the formation of a basic acetate of lead. It was a pigment of no great depth of colour or permanency. DOUGLAS GREEN.—Mr. Thomas Douglas has described in the Chemical News, vol. xl., p. 59, a green prepared from barium chromate. The latter compound, prepared in the usual way by mixing solutions of barium chloride and potassium chromate, is mixed with 20 per cent. of its weight of strong sulphuric acid, which partially decomposes it, forming a mixture of barium chromate, chromic acid, and barium sulphate; the mixture is dried and then calcined at a bright red heat in a crucible; the chromic acid is thereby decomposed into the green oxide of chromium, which, being disseminated throughout the mass of barium sulphate and chromate, colours it green, forming a pigment possessing considerable body and permanency. Nothing definite is known as to its having been used as a pigment. CHINESE GREEN or LOKAO.-This is a green pigment, as yet but little used, made from the juice of various Chinese species of buckthorn trees by extracting the juice from the berries by pressure, mixing this with alum, &c., and drying. It comes into commerce in the form of bluish-green slabs, which are easy to break, but somewhat difficult to powder. Chinese green contains from 27 to 47 per cent. of mineral matter, principally lime and alumina, and, probably, consists of the lake formed by the combination of those bases with the colouring principle of the juice from the buckthorn berries, named by Kayser lokaonic acid, C42 H48 O27. According to the same authority, the colouring principle consists of a glucose, which he calls lokaose, to which he assigns the formula C, H12 O, and lokanic acid, a body having the composition C3 H36 021 The colouring principle has also received the name lokain and the formula C28 H34 017 6 SAP GREEN.-This pigment is prepared from buckthorn berries. Two methods are adopted in its preparation. In one the berries are allowed to ferment slightly by placing them in a warm place for a few days; they are then pressed, the juice collected, and alum added in the proportion of from an ounce to 1 ounce per pound; the mixture is then boiled down and evaporated to dryness at the boiling heat. Another plan is to boil the berries in water for two or three hours with constant stirring; the liquors are then strained through cloths in order to separate the woody and other insoluble particles; the clear liquor is boiled down to a syrup, 5 ozs. of alum per gallon added to the syrup, and the mixture carefully evaporated to dryness. For some purposes the mass is left in the pulpy condition. Sap green is a dark yellowish-green pigment; when dry it breaks with a glossy fracture; it is very transparent, and hence is not used as a body colour, but chiefly as a glazing colour; another use for it is in colouring confectionery and beverages. It works well as a water-colour, but not as an oil-colour; and fades on exposure to light. An analysis of sap green made by the author shows it to have the following composition: Water, Mineral constituents, Organic constituents, 12.95 per cent. 100.00 Of the organic constituents a quantity equal to about 29.34 per cent. of the original colour are soluble in alcohol. Its composition and general properties somewhat resemble those of a lake. 5 MANGANESE GREEN.-This pigment was patented in 1864 by Schad, who prepared it by the following process:14 parts of oxide of manganese, 80 parts of nitrate of barium, and 6 parts of sulphate of barium, are intimately mixed together. The mixture is heated in a crucible in a suitable furnace to a bright red heat until it has assumed a green colour; it is then ground in a mill with water to a fine powder; a small quantity of gum arabic, dextrine, or similar substance, amounting to about per cent. of the original material, is added, and the mass is dried at from 190° to 212° F.; or it may be used in the form of a paste. Instead of the above mixture there may be used one of 24 parts of nitrate of manganese, 46 parts of nitrate of barium, and 30 parts of sulphate of barium. The addition of the gum or dextrine is said to be essential for its stability, a factor which cannot but have an adverse influence on its value as a pigment, for which purpose it has probably not been used. It consists principally of manganate of barium. TITANIUM GREEN.-This pigment is the ferrocyanide of titanium, prepared by mixing solutions of potassium ferrocyanide and of a titanium salt; the pigment must be dried at a low temperature, as decomposition sets in above 100° C. It has a pale green colour, and was proposed as a substitute for the arsenical greens; owing to its cost it has never come into use. ZINC GREEN.-Under this name there is frequently sold greens made in a similar way to the Brunswick greens, by mixing together zinc chrome, Prussian blue, and barytes; such greens possess the advantage that they are not affected by sulphur as much as the Brunswick greens. They are best made in the dry way (see p. 151). The green lakes and the pigments made from coal-tar colouringmatters will be found described in the chapter on Lakes. 180 CHAPTER VI. BLUE PIGMENTS. Blue, as a colour, enters very largely into the decoration of objects, both alone and in combination with other colours, to form a large and very useful series of tints and shades. Although so important as a colour, yet there are few blue pigments; but these possess the merit of being more permanent, and, therefore, more useful than any other group of pigments. The list of blue pigments includes ultramarine (a curious compound of silica, alumina, and soda, which was at one time obtained exclusively from natural sources, but is now mostly prepared artificially), and Prussian blue with its varieties (a most valuable blue, whose base is iron), which are the most predominant blue pigments used. Cobalt is the base of cobalt blue and smalts, while copper forms the basis of several unimportant pigments. ULTRAMARINE. Ultramarine is one of the most important pigments possessed by the painter; being used in painting, in printing of all kinds (letterpress, wall-papers, calico), and in bleaching; it is undoubtedly the best blue for the laundry, and in soapmaking it is used to produce the blue mottled soap. Ultramarine has been known for centuries, but its extended use has only been possible during the last half century. Prior to about 1820 the natural supplies were small, and the processes so expensive that it could only be used by artists who did not find the cost prohibitive; but about the year named, discoveries were made by several chemists, which resulted in ultramarine being made artificially at such a cheap rate that it is the cheapest blue pigment known; consequently, its consumption is now measured by tons. Natural Ultramarine.—The source of natural ultramarine is a blue mineral, lapis lazuli, found in small quantities in Persia, China, Siberia, and a few other places. This mineral is found in streaks and small patches distributed through an earthy matrix or gangue, from which it has to be separated by mechanical means. The production of natural ultramarine has declined very much during the last fifty years, it having been displaced by artificial ultramarine; but the mineral is still sought for in fair quantities, for use in the production of inlaid ornamental work, as the peculiar blue colour of the mineral cannot be obtained by other means. The process of extraction of the pigment from the mineral consists in grinding the mineral to a fine powder, after separating as much of the gangue as possible; it is then mixed with a compound of resin, wax, and linseed oil, and the mixture put into cloths and kneaded under hot water; the colour comes through the cloth into the water, several waters being used; after the working, the waters are placed on one side for the colour to settle. The blue thus obtained varies in shade in the different waters; that which settles out of the first water is the deepest in colour, and the brightest, and is sold as ultramarine; that which comes from the last waters has a blue-grey colour, and is sold as ultramarine ash. After the colour has settled, it is usual to grind it still finer, so that the beauty of the pigment shall be developed as much as possible. No better process for extracting ultramarine has been devised, although it is so tedious and gives such poor results. Very little is now so produced, as the natural variety has been almost replaced in European and other countries by the artificial variety. The chemical composition and constitution of ultramarine early became the subject of research by chemists, which researches were partly undertaken with a view to its artificial production; for it was recognised that, from the beauty of its colour and its permanent qualities, ultramarine would, if it could be produced cheap enough, have a wide field of use. Several analyses were made by different chemists, but these vary very much, owing, as is probable, to the difficulty of obtaining the pigment quite free from its matrix. Those by Clement and Desormes and by Gmelin, which are, perhaps, the most typical, are here given. |