Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/601

DYEING 579 are nearly always composed of some of the red and yellow dyes mentioned in the preceding pages, such, for instance, as cochineal and fustic, which are applied in one bath, the same mordant serving for both. A motto orange. A warm solution of arnotto in weak alkalies is used without mordant to impart to silk an agreeable orange shade. Its colour is generally considered too yellow, but may be made redder by treatment with weak acids, or by previously giving the silk a light red foundation. Picric acid orange. Another orange on silk can be dyed by superimposing on a light pink a yellow obtained from picric acid. Nitric acid orange. Silk can also be permanently stained of a yellowish orange by means of moderately strong nitric acid, which must, however, be applied with great care, since a more than momen tary contact would be very injurious to the strength of the fibre. This method of dyeing silk was formerly much used for handker chiefs ; by protecting certain parts from the acid with melted wax or similar resists, white designs were produced upon an orange ground. PURPLE COLOURS. The purple colours may be held to include all shades produced by an admixture of red and blue, such for example as lilac, violet, mauve, &c., and are of immense variety. Aniline purples. Since their discovery aniline colours have been almost exclusively employed for dyeing silk and wool purple, yielding as they do shades which for lustre and purity surpass any obtainable from the older colouring matters, and possessing also a fair amount of stability. An aqueous solution of the dye without mordant is all that is required, and the goods when dyed need very little subsequent treatment. The aniline purples, violets, and mauves do not dye upon cotton without previous mordanting, and even then are so loose and unstable that they are only fitted for use where great fixity is not demanded, as for linings of clothing, &c. The most general mordant for the aniline purple colours on cotton consists of a taimate of tin obtained by first steeping the cotton in a solution of tannic acid, or in decoction of gall-nuts, sumach, or myrobalans, all of which contain tannic acid; after a few hours contact a considerable quantity of tannic acid has become firmly attached to the cotton, and the goods, being now treated successively with stannate of soda and dilute sulphuric acid or in other ways, acquire a certain proportion of oxide of tin, and are prepared to receive the colours. Madder purple. But the purple colour par excellence upon cotton is obtained from madder or alizarin, the mordant being oxide of iron or a sub-salt of iron deposited on the fibre by treatment with the commercial pyrolignite of iron, commonly called iron liquor. This purple is remarkable for great permanency. It is very largely used in combination with black and white in the best kind of printed calicoes. Archil purple. Archil and cudbear are sources of purple colours on wool and silk. The shades produced are rich and beautiful; they are not, however, very permanent, and have been nearly superseded by the aniline colours. Of the few instances that can be cited of stuffs dyed purple by the direct union of red and blue colouring matters, the violet or purple woollen cloth used for ecclesi astical purposes is an example. The indigo colour is first fixed and cleansed, and then the cloth is dyed with cochineal and tin mordants in the way already described for dyeing scarlet. The purple thus obtained is a fast colour, but is very costly, and on that account is not much worked. The common shades of purple, violet, lilac, &c. , upon wool are obtained from logwood with a mordant of alum and tartar; the red woods are sometimes employed in conjunction with logwood for tlie.se colours, which are "topped " with archil to give them more brilliancy. The extensive range of colours, comprising all the shades of brown, bronze, chocolate, nut, wood, drab, and grey, which may be considered as compounded of the three elementary colours, some one of the three predominating, can only be briefly treated of in this article. Most of them are actually produced by the use of dye-stuffs yielding the three simple colours ; but there are colour ing matters like catechu, which themselves yield brown colours, and others, such as logwood, which may be held to contain two or more of the simple colours, the blue predominating. A few illus trations will show how these triply compounded colours are pro duced by the dyer. BKOWN COLOURS. Bronze brown on wool. The wool is mordanted with alum and tartar in the usual way, and is then dyed in a mixture of fustic and madder or other equivalent red and yellow dye-stuffs; for fast colours a blue part can be communicated to it by the indigo vat. For a lower class of colours no indigo is used, but instead, a mixture of yellow wood (fustic or quercitron) with madder for the red, and logwood for the blue part; or again, the sulphate of indigo may be employed for the blue. Tan brown. According to Mr Jannain, the wool is mordanted by boiling it for an hour with one per cent, of its weight of bichro mate of potash ; it is then washed, and transferred to the dyeing vessel, with the following percentages of its weight of materials : madder, 3 2 ; fustic, 4 8; camwood, 2; barwood, 175; sumach, 2 1 ; with these materials it is boiled for two hours. Dark drab. From the same authority we take the following as the weights required to dye lOOlb wool, previously mordanted with 1 ft of bichromate of potash: camwood, 64 ft; sumach, 2 lb; madder, 2 ft ; fustic, 4 ft ; logwood, 2J ft ; boil for one hour and a half, and afterwards, to darken the colour, pass into water containing 1 Ib of sulphate of iron. BLACK COLOURS. Black, from a dyer s point of view, is compounded of the three simple colours, red, yellow, and blue, in a state of concentration ; but in reality the blue predominates in all good black colours, and gives them their density and at the same time their lustre. What is called a dead black, crape black, or jet black, is the nearest approach to a neutral black, but even this would be brownish if the blue did not predominate. It is often extremely difficult to obtain a black dye to suit a particular market. Of ten pieces ap pearing equally black to the uninitiated, an expert would, perhaps, 1 iron ounce one to be sooty, another purple, another red, another brown, another green, and so on. We should have to go back some years in the history of dyeing, to find a time when black was actually dyed with the three elementary colours. In some processes blue from indigo was first applied, and then, upon an alum mordant, red and yellow from madder and weld respectively ; such a colour was unexceptionable for stability, but its great cost caused it to be disused. At the present day, logwood is the chief dye-stuff for blacks upon wool or cotton, and gall-nuts and other astringents for silks. Ani line black, on account of obstacles to its application, cannot be said to have yet established itself in dyeing proper, though it is much and highly valued in calico printing. Black dye upon silk. Silk easily takes a black by treatment first with decoction of gall-nuts, and subsequently with a salt of iron. For blue blacks the silk is usually first dyed with Prussian blue, and then with gall-nut black . Extract of chestnut-wood with an iron mordant gives a good black. In modern black silk dyeing, materials are heaped upon the fibre which are not necessary to its colour, but which increase its weight in an extraordinary manner, so as not onlv to compensate for the loss of 25 per cent, of natural gum in the silk, but even, in some cases, to double or treble the original weight. The silk is, of course, much injured by the ac cumulation of foreign matters upon it, the fibre becoming harsh and brittle, and soon showing the effects of wear. The chief sub stances used for weighting are lead salts, catechu, iron, and galls, with soap or fatty matter, to soften in some degree the harshness these occasion. Black upon wool. Upon woollen cloth of fine quality, the black is dyed upon a basis of indigo blue, and, from the use of woad for this colour, such blacks are in England called ".woaded blacks." The first process, therefore, in producing the best black is to dye the wool in the indigo vat of a tolerably deep shade of blue, and afterwards boil it in a mixture of logwood and sumach, treating it with sulphate of iron ; the latter process being two or three times repeated, a very perfect and durable black is obtained, provided the indigo basis is sufficiently deep, and only a minimum quantity of logwood has been employed, say about one-fourth the weight of the sumach. Common black. Common blacks upon wool haveiioindigo in their composition, but are dyed chiefly with logwood ami iron salts; the wool and logwood are heated together for some time, and then sul phate of iron is added to the dye-bath. In other blacks of somewhat better quality, the woollen is boiled for some time with solution of iron, copper, and aluminium salts, together with tartar, and when the mordanting oxides have been fixed, the colour is dyed up in logwood. The bichromate of potash mordant can also be used for the black dye, and the cloth can be "bottomed" with camwood or barwood ; it is then dyed up with logwood, to which fustic or sumach may be added. Black upon cotton. Almost the only ordinary black in cottrn dyeing is obtained from logwood with iron mordant ; sumach is sometimes used, and very rarely the black is dyed upon an indigo blue basis by means of sumach or galls and iron. As before stated, aniline black has not yet been practically applied in dyeing cotton. A common method is to first heat the goods for some hours with decoction of sumach, wash mordant in sulphate of iron, and then dye in logwood; another method consists in fixing an iron basis upon the cotton by the method given above (page 573), and dyeing in logwood, along with a portion of sumach or fustic, according to the shade required. Velvet dyeing. The most important branch of black dyeing upon cotton goods, is that employed for cotton velvets and vel veteens, in which it is desired to produce a rich lustrous effect; the

process is long, tedious, and uncertain, consisting of successive