Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/766

692  Pigment Colours are so named because the tinctorial agents employed are coloured lakes, and the insoluble mineral powders used otherwise as painters colours. Only a limited number of painters colours are so used, as, for a variety of reasons, many of them are not suitable for calico- printing. Colours containing arsenic, for example, and some others which produce brilliant effects, cannot be used on account of their highly poisonous nature. Others are excluded on account of their cost, some are too dull and muddy in colour, and some are liable to tarnish or darken on exposure to light, air, moisture, or other influences. The pigments which are most extensively useful are ultramarine blue, Guignet s or chrome green, and chrome orange, all of which are very largely used by themselves or in combination with other colours. Lamp black is also employed for the production of a solid grey, and vermilion- red with some other metallic sulphides are sometimes, though rarely, printed as pigment colours. The principal lakes used are carmine, corallin lake (a derivative of phenol or carbolic acid), black logwood lake, and several others prepared from the dye - woods with tin and alumina salts. The aniline dyes on their first introduction were also worked as pigment colours, and printed with albumen. The first medium employed for fixing mineral pigments and lakes to calico was a solution of India rubber in coal naphtha, an agent which, so far as clearness and permanency of the printed colour is concerned, was perfectly satisfactory. The steaming dissipated the highly volatile naphtha and left the thin film of caoutchouc mixed with the colour firmly adherent to the tissue. But the inflammability of the copious naphtha fumes evolved gave rise to many serious accidents, and the method had on that account to be abandoned. No other medium has been found to give so satisfactory results as the protein compounds, of which albumen obtained from the white of eggs is the type. Besides egg albumen, blood albumen, lactarin or casein from milk, and gluten from wheaten flour are used as agents for fixing pigments. In printing with albumen advantage is taken of its well-known peculiarity of coagulating and becoming quite insoluble at a temperature under the boiling point. It is mixed with the colour and deposited on the cloth in its soluble state, when, by the operation of steaming, it coagulates and remains firmly attached to the tissue, imprisoning with it the particles of colour with which it was mixed. The cloth is not in reality dyed, but has only a coloured pattern mechanically fastened or glued to it. Egg albumen gives the most delicate and clear shades, but recent improvements in the preparation of blood albumen render it increasingly available for bright colours. Lactarin arid gluten, dissolved by means of caustic alkalies, are used for printing ultramarine and other pigment colours. The length of time that pigment colours are left in the steaming apparatus varies from half-an-hour to an hour. Ordinary Steam Colours.—The essential features of this style consist in printing direct on tha cloth the dyeing material, mixed in proper proportion, with any necessary mordant, and certain acids or salts to keep the mixture in solution. On the application of moist heat after printing, the acid is evaporated or a chemical decomposition takes place in the case of the salt, and an insoluble precipitate is produced in the fibre. Steam colours possess great brilliancy, but they have not the fastness and solidity of madder-dyed goods. The dyes in the case of steam colours must be in the form of decoctions or prepared extracts of the special chemical tinctorial principles. Such prepara tions have of recent years come into very wide use, and with the progress of chemical science they are daily attaining greater prominence and perfection, so that the older applica tion of crude materials is rapidly being supplanted by the use of agents of known strength and quality. Thus, r.a already mentioned, madder extract and artificial alizarin, treated as steam or &quot;topical&quot; colours, have largely taken the place of madder root as a dye colour, and by the pre paration of artificial alizarin from anthracene, printers are now rendered independent of the vegetable kingdom as a source of their hitherto most important dye-stuff. As a preparatory to printing, the cloth is mordanted or prepared by passing it through a solution of stannate of soda, and treating with a very weak solution of sulphuric acid which decomposes the stannate, combining with the soda, and leaves the stannic acid (peroxide of tin) precipitated in the fibres. Cloth thus prepared has much purer and brighter shades than simple bleached calico. The common steam colours include black and chocolate from logwood liquor, orange from annatto, yellow from Persian berry liquor and from bark liquor, green from Persian berries and yellow prussiate of potash, purple from logwood and red prussiate of potash, dark red from sapanwood and bark liquor, reds, purples, and chocolate from madder extract and alizarin, and blues from Prussian blue. Iron, alumina, and other mordants are used with these colours according to their character and the nature of the shades desired. The solvent principally employed is acetic acid, which readily volatilizes in the steaming process, but oxalic acid is also employed to keep certain special oxides in solution during the printing. Oxidizing agents, as the chlorate or bichromate of potash, are also required for the development of some colours. Steam blue is printed, not by using the Prussian blue colours ready formed, but by effecting the chemical reaction on the cloth itself, which results in the blue colour. In some cases yellow prussiate of potash is used, which yields Prussian blue ; again, when the red prussiate is employed, Turnbull blue is the result ; but a mixture of both, to which a proportion of ferro-prus- siate of tin, called tin pulp, is added, is the source of the best steam blue. The reaction by which the colour is developed will be understood by instancing the development of Prussian blue from the yellow prussiate. It is mixed with an acid tartaric, oxalic, or sulphuric or the whole three combined, and printed on the cloth. In the steaming the added acid combines with the potassium of the prussiate and liberates ferrocyanic acid, which is further decomposed into cyanide of iron, abundant fumes of hydrocyanic acid (prussic acid) being meantime evolved. On withdrawing the goods from the steaming chest after this decomposition is complete the pieces are quite colourless, but exposure to the atmosphere in an ageing chamber, or passing them through an oxidizing solution, such as the bichromate cf potash, develops the characteristic blue of Prussian blue.

Aniline Colours.—These colours now constitute the largest and most important section of steam-fixed dyeing materials, and in their behaviour and method of printing they form a class by themselves. The range of aniline colours now embraces almost every possible shade ; and in no other department of scientific and technical research has equal activity been displayed within the few years which have passed since these colours were introduced ; and the rewards of investigation have been commensurate. The number of colours introduced, and the methods of preparing them which have been suggested are beyond computation, and the list of those which are now in current use is exceedingly extensive. In addition to the dyes procured from aniline many more of an allied nature are prepared from other derivatives of coal-tar, phenol, naphthalin, and anthracene, some of which have also come into extensive use, and the applicability of others has been demonstrated. The topical use of these colours in connection with extract of madder, Guignet s green, ultramarine, &c., has exercised a powerful influence in improving the art of design in con- 