Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/600

578 578 DYEING be used. Dissolve 9 fb ot yellow prussiate of potash in hot water, and add the solution to the required quantity of water ; then add 1 4 ft sulphuric acid, 6 ft sal-ammoniac, and about 6 oz. of crystals of protochloride of tin; the merino is placed iu the mixture, and the temperature of the dye-bath gradually raised to the boiling point in five hours. The blue gradually formed on the cloth requires brightening in a fresh bath consisting of alum, persalt of tin, and cream of tartar, heated to nearly the boiling point. Red prussiate of potash is used in nearly the same way to dye dark Prussian blues upon wool, but as it is more easily decomposed than the yellow prussiate a weaker acid-bath suffices. These blues are frequently finished off with logwood to give them a deeper tone. Prussian blues can also be obtained on such woollen goods as merinoes, by a process of padding, and the use of a colour nearly identical with the so-called French or royal blue used by calico printers. Amixture is prepared as follows. Halfapoundof wheaten starch is boiled with about half a gallon of water; in the thin paste thus made 13 oz. of powdered yellow prussiate are dissolved, and afterwards 6 oz. of tartaric acid ; when the mixture is quite cold 1 ft of prussiate of tin in paste is added, 1 oz. oxalic acid, and 3 oz. sulphuric acid; the whole is well mixed and strained. The woollens to be treated are first &quot; prepared,&quot; as it is called, by impregnating them uniformly with oxide of tin, and then the above thickened mixture is applied by means of rollers, so that it shall be evenly and smoothly spread over the whole stuff; the cloth is then dried and exposed to the action of steam, which causes the acids to react upon the prussiates, and from a nearly colourless mixture develops an intense blue, which is found to be permanently fixed in the fibre. Aniline blues. There are several artificial blue dyes made from aniline and similar bodies, which yield very brilliant colours on wool and silk. They can be easily applied, the goods simply requiring to be worked in their aqueous solution until they have acquired a sufficiently dark tinge. An artificial dye called Nichol son s blue is differently applied ; it is dissolved in an alkaline liquid, and forms then a colourless or nearly colourless solution, with which the goods to be dyed are impregnated ; they are then passed into dilute acids, which develop the blue colour. Litmus and logwood blues. The other substances which have been used for blue colours, such, for example, as litmus, are of little importance, and are now nearly unknown to the practical dyer. A blue can be obtained from logwood which has some resemblance to indigo blue upon wool, but it is of a very low character both as to stability and shade, and is hardly ever employed by respectable dyers. Yellow Colours. Yellow textiles, being less pleasing to the eye, and more readily soiled, are not nearly so much iu use as those dyed with the two simple colours blue and red. The chief yellow dyes, besides fustic, are quercitron bark or its concentrated extract flavine, Avignon or Persian berries, and the now almost disused indigenous product, weld. The general mordant for these is tin, sometimes with addition of alum. One or two illustrations will suffice to show the methods of using them. Fustic yellow. Fustic is probably the most generally employed yellow dye-stuff for wool; it gives yellows inclined to orange. For light shades it is not necessary to mordant the wool ; it is simply well cleansed, and then heated with fustic decoction and some cream of tartar. For darker shades the wool is boiled with solution of tin and tartar, washed, and then worked in the decoction of fustic. Picric acid yellow. Picric acid, one of the artificial colouring matters, gives pure though not deep yellow shades upon silk and wool without the aid of a mordant, the cleansed material being dyed by working it in a warm solution of the acid. Chromate of lead yellow. The yellow most commonly employed for cotton goods is obtained by the use of salts of lead and bichromate of potash. The method of obtaining this colour differs somewhat from any previously described. The cotton, having been properly bleached, is impregnated with a salt of lead, usually by employing a solution of the acetate or sub-acetate of lead. The goods are next passed into a milk of lime solution, to which it is pru dent to add some acetate of lead, in order to prevent the lime from dissolving the oxide of lead at first precipitated; the result of the lime treatment is that oxide of lead is evenly fixed upon the cotton, the excess of lime and lead is then well washed away, and the goods are passed into a solution of bichromate of potash, where they quickly acquire a bright and deep yellow colour, owing to the formation of the well-known pigment chrome yellow. To facilitate the combination, the bichromate of potash is mixed with as much sulphuric acid as suffices to liberate the whole of its chromium as chromic acid. The yellow-dyed goods require no further treatment than a good washing, the colour being quite fast. This yellow is, however, in very little demand, and in ninety-nine cases out of a hundred it is immediately converted into an orange, by passing it through boiling lime-water, which produces the basic chromate known as chrome orange, which has alway been in demand for many articles of wear. Compound Colours. The so-called simple colours red, blue, and yellow having now been dealt with, it remains to treat of their combinations, and this may be done briefly, the processes employed being for the most part similar to those already described. The compound shades in Chevreul s chromatic nomenclature amount to nearly 1 5,000, and it is very pro bable that fully that number are produced by the dyers of the present day. For practical treatment, however, the compound colours can be reduced to comparatively few classes. Mixing the simple colours one and one we obtain three compound colours, blue and yellow give green, blue and red give purple, yellow and red give orange ; while there may be a normal green, purple, and orange, it is evident that all the varieties of these several colours will depend upon the proportions of their constituents. If the three simple colours be mixed together, say in equal pro portions, we may get a normal brown, or even a black ; but if in unequal proportions, an immense number of shades, varying from the imagined normal brown to grey and drab, are produced. Although in many cases compound shades are produced by means of two or more simple colours, there are many natural as well as artificial dye stuns which yield them ready formed, and frequently purer than they can be otherwise obtained. Most of these will be found mentioned in the following brief notice of practical processes in use. GKEEX COLOURS. Lo-kao or Chinese green. Until about the middle of the present century there was not an instance known of any green on textiles which was not composed of the two separate colours blue and yellow. About that time some green-dyed cottons, imported into France from China, attracted the attention of chemists, who were surprised that they could not separate the green into blue and yellow constituents. Inquiries showed that the Chinese employed a green colouring matter called Lo-kao, until then unknown in Europe. It was a costly dye-stuff, selling in China for its weight of silver. Some quantity of it was imported and used in silk-dveing by the French ; it was not, however, found altogether satisfactory, and has at length been quite abandoned for the aniline greens, which are in every respect preferable. Aniline yrcen. There are two or three kinds of artificial green dyes in use, of which that known as methyl-aniline green, applied in silk dyeing, is most in request. The so-called iodine green has also been somewhat extensively employed for all kinds of fabrics. These artificial and unstable materials are the only dye-stuffs for green possessed by the dyer, who is compelled to produce the colour by means of blue and yellow elements. The arsenical yellows are afforded by Persian berries, quercitron, fustic, or the yellow chromate of lead. The processes employed consist, for the most part, in the separate application of the blue and yellow: for example, in dyeing a fast green upon wool from indigo and any of the yellow dye-stuffs, the blue is first produced as previously described, and the proper mordant for the yellow is then applied to the cloth, which is afterwards placed in the yellow colouring matter ; the two colours are so intimately mixed as to be indistinguishable even by high magnifying powers. It may be observed that the reception of the blue does not to any perceptible extent diminish the power of the cloth to combine with the yellow. Prussiate green. Prussian blue is employed as a basis in the same manner, only not being capable of resisting chemical agents so well as indigo blue, it demands more care. The greens with Prussian blue bases are more lively than those made with indigo, but are not so fast. Sulphate of indigo is even less stable than Prussian blue. It is, however, cheap and easy of application, and gives rich colours. The greens made with chromate of lead are for the most part con fined to cotton goods, and are not in much demand. OUANGE COLOURS. For cotton the chief orange-dye is the chromate of lead compound already described. For other materials the orange colours employed