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 DYE I N G a dark blue powder. After drawing off the supernatant liquid, the sediment of indigo is boiled with water to prevent further destructive fermentation, and then collected on filtering frames, the drained pasty indigo being finally pressed ; the press-cakes are cut into the form of cubes, which are finally dried in open-air sheds. In 1878 Professor Baeyer succeeded in preparing indigo by artificial means from the coal-tar product toluene. Two years later he devised another method, the initial substance being cinnamic acid. This process, known as the “ propiolic-acid ” method, was for a brief period carried out on a commercial scale, the product ‘ ‘ propiolic acid ” being employed by calico-printers and only converted by them into indigo-blue during the process of application. Since then, chemists have found it possible to prepare artificial indigo by various methods, but few of these have given any great promise of commercial success. In 1890, however, Heumann astonished the chemical world by preparing indigo from such simple materials as aniline, acetic acid, chlorine, and alkali, the immediate mothersubstance, prepared from the first three, being so-called phenylglycocoll, which requires simply to be melted with alkali and dissolved in water to yield the indigo. Difficulties as to the yield of indigo obtained soon caused this process to be abandoned in favour of one closely akin to it, in which the carboxylic acid derivative of phenyl-glycocoll is similarly melted with alkali. This substance is derived by successive stages from anthranilic acid, phthalimide, phthalic acid, and finally naphthalene. This last-mentioned substance, which has now therefore become the initial material, is obtained in large quantity by the direct distillation of coal-tar. Since 1897 the manufacture of indigo from naphthalene on a commercial scale has been carried on by one of the large German colour-manufacturing firms (Badische Anilin and Soda Fabrik) with every appearance of success, so much so that the indigo-planters of India have been compelled to adopt improved methods in order to reduce the cost of production. At the present time natural and synthetic indigo are entering upon a period of commercial competition with each other, and time alone can show which can be produced at the lowest price and be ultimately successful in the conflict. It is well to emphasize the fact that the colouring matter indigotin is identical in both the artificial and natural products, but in the latter it is accompanied by other substances, notably indigo-red (indirubin), indigo-brown, and indigo-gluten. The first is a dyestuff similar in general properties to indigo-blue, but yielding purple shades instead of blue; the rest may be regarded as impurities. An exaggerated importance has been attached by some dyers to the presence of these accompanying substances in natural indigo, as giving fulness and depth to the dyed colour, but these effects may also be obtained with artificial indigo by employing suitable means ; and in the case of indigo-red this, too, can be made artificially if required. As to the methods of application in dyeing, they are the same with both dyestuffs, and these may be now briefly described. Indigo, being insoluble in water, would be of no use in dyeing if it were not capable of being rendered soluble. This is effected in two ways, corresponding to which there are two methods of dyeing with indigo. One method consists in dissolving the indigo in very strong sulphuric acid, whereby it is converted into indigotindisulphonic acid (Indigo Extract), which is readily soluble in water. This substance belongs to the group of Acid Colours ; hence it is applied to the animal fibres, wool and silk, by boiling in a solution of the colouring matter slightly acidified with sulphuric acid. The second and most important method is based on the fact that under the influence of reducing agents (i.e., substances capable of yielding nascent hydrogen) indigo-blue is changed into indigowhite, which is soluble in alkali, the solution thus obtained being called a “vat.” If textile materials are steeped in a clear yellow solution of the reduced indigo and then exposed to air, the indigowhite absorbed by the fibre is oxidized and reconverted into indigoblue within and upon the fibre, which thus becomes dyed blue; this is the so-called “ indigo-vat ” method of dyeing. Comparing the two methods, the “indigo-extract method” is only applicable to the animal fibres, and although it gives brighter colours, they are fugitive to light and are decolorized by washing with alkaline solutions ; the “vat method” is applicable to all fibres, and gives somewhat dull blues, which are very11fast to light, washing, &c. lime and copperas vat” the 11 Cotton is dyed by means oftl the zinc powder vat,” or the hydrosulphite vat.” In the firstmentioned vat the ingredients are quicklime, ferrous sulphate, and indigo ; the lime decomposes the ferrous sulphate and precipitates ferrous hydrate; this quickly absorbs oxygen from the water present, with the liberation of hydrogen, which at once combines with the indigo to form indigo-white, this in turn dissolving in the excess of lime present. The ingredients of the zinc powder vat are zinc powder, lime, and indigo; in the presence of the lime and indigo the zinc takes up oxygen from the water, liberating the hydrogen necessary to reduce the indigo, as in the previous vat. The constituents of the hydrosulphite vat are hydrosulphite, or more correctly hyposulphite of soda, lime, and indigo. The requisite hyposulphite of soda is prepared by allowing zinc powder (13 lb)

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to act upon a cold concentrated solution of bisulphite of soda (17 gallons of sp. gr. 1'225), taking care to avoid, as much as possible, access of air and any heating of the mixture, to prevent decomposition. The solution thus obtained is thoroughly neutralized by the addition of lime ; and after settling, the clear liquor is used for the vat, along with indigo and lime. Here again the hyposulphite takes up oxygen from the water and liberates the necessary hydrogen. It is found convenient to prepare, in the first instance, a very concentrated standard of reduced indigo, and to add as much of this to the dye-vat as may be required, along with lime and a little hyposulphite of soda. The advantages of this vat are that it is easily prepared and that there is very little sediment; moreover, it can be employed in dyeing wool as well as cotton, and it is now very generally in use. The vat usually employed for dyeing wool is the so-called “woad vat,” which differs from the foregoing in that the hydrogen necessary to reduce the indigo and bring it into solution is furnished, not by the action of chemical agents, but by means of fermentation. The ingredients of the woad-vat are indigo, woad, bran, madder, and lime. The woad here employed is prepared by grinding the leaves of the woad plant (Isatis tinctoria) to a paste, which is allowed to ferment and then partially dried. It serves as the ferment to excite lactic and butyric fermentation with the aid of the bran and madder, the necessary hydrogen being thus evolved. Excessive fermentation is avoided by making timely additions of lime ; sluggish fermentation is accelerated by additions of bran and slightly raising the temperature. When the reduction and complete solution of the indigo is effected, the vat is allowed to settle, and the woollen material is immersed and moved about in the clear liquor for half an hour to two hours, according to the shade required, then squeezed and exposed to the air in order to develop the blue colour on the fibre. Aniline Black differs from other dyes in that it is not sold as a ready-made dyestuff, but is produced in situ upon the fibre by the oxidation of aniline. It is chiefly used for cotton, also for silk and cotton-silk union fabrics, but seldom or not at all for wool. Properly applied, this colour is one of the most permanent to light and other influences with which we are acquainted. One method of dyeing cotton is to work the material for about two hours in a cold solution containing aniline (10 parts), hydrochloric acid (20 parts), bichromate of potash (20 parts), sulphuric acid (20 parts), and ferrous sulphate (10 parts). The ferrous sulphate here employed is oxidized by the chromic acid to a ferric salt, which serves as a carrier of oxygen to the aniline. This method of dyeing is easily carried out, and it gives a good black ; but since much of the colouring matter is precipitated on the fibre superficially as well as in the bath itself, the colour has the defect of rubbing off. Another method is to impregnate the cotton with a solution containing aniline hydrochlorate (35 parts), neutralized with addition of a little aniline oil, sodium chlorate (10 parts), ammonium chloride (10 parts). Another mixture is 1'8 part aniline salt, 12 parts potassium ferrocyanide, water (200 parts), (3-5) potassium chlorate dissolved in water. After squeezing, the material is passed through a special oxidation chamber, the air of which is heated to about 50° O. and also supplied with moisture. This oxidizing or ageing is continuous, the material passing into the chamber at one end in a colourless condition, and after about 20 minutes passing out again with the black fully developed, a final treatment with hot chromic acid solution and soaping being necessary to complete the process. In this method, employing the first-mentioned solution, chlorate of copper is formed, and this being a very unstable compound, readily decomposes, and the aniline is oxidized by the liberated chlor-oxygen compounds. The presence in the mixture of a metallic salt is very important in aiding the development of the black, and for this purpose salts of vanadium, cerium, and copper have proved to be specially useful. The chemistry of aniline black is still incomplete, but it would appear that there are several oxidation products of aniline. The first product is so-called emeraldine, a dark green substance of the nature of a salt, which by treatment with alkali yields a dark blue base called azurine. The further oxidation of emeraldine yields nigraniline, also a dark green salt, but the free base of which has a violet black colour. The latter becomes greenish under the influence of acids, especially sulphuric acid, and explains the defect known as “greening ” which is developed in ordinary aniline blacks during exposure to air. By a supplementary oxidation with chromic acid such a black is rendered ungreenable, the nigraniline being probably changed into the more stable chromate of nigraniline. Catechu is a valuable brown dyestuff, obtained from various species of Acacia, Areca, and Uncaria growing in India. The wood, leaves, and fruit of these plants are extracted with boiling water ; the decoction is then evaporated to dryness or to a pasty consistency. Catechu is largely used by the cotton dyer for the production of brown, drab, and similar colours. It is seldom employed for wool. Cotton is usually dyed by boiling it for about one hour in a decoction of catechu (100 per cent.) containing