Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/595

573 DYEING 573 colour, becoming dyed, while the solution is rendered nearly colourless. During the process the fibrous material is kept in a constant state of movement, so that the dye solution shall have equal access to all portions, the temperature employed and time allowed being regulated according to the necessities of the case. The colour absorbed by the fibre has entered into an intimate state of combination with it, since it cannot be washed out again; a true dyeing has taken place. Besides the aniline colours, the older artificial dyes sulphindigotic acid, picric acid, and one or two others have the same property of combining directly with wool and silk. There are other cases of dyeing closely resembling the foregoing, in which the resulting dyed stuff may be considered as being a binary compound of fibre and colouring matter, but in which the methods of application are less simple. These may be taken generally as consisting in the use of materials or processes which bring a previously insoluble colouring matter into a soluble state ; thus the pink colours of safflower are obtained by the action of an alkali; and the dyes yielded by archil, arnoito, and indigo are also the result of the action of solvents. It is possible that during the process of solution important internal changes may take place in the composition of the above dyes, but if so, they are only of a temporary nature, for there is no reason to sup pose that the colouring matter attached to the fibre differs in chemical composition from that which is free. With regard to nearly all other colouring matters, the above simple processes are quite powerless to induce a permanent combination Avith the fibre. Let wool or silk be immersed at boiling temperature in decoctions of any of the best known natural dye-stuffs, such as cochineal, logwood, madder, quercitron bark, &c., and then washed in water, it will be found that the fibres are simply discoloured, or stained of no definite shade ; they have taken up but a small portion of colour from the decoction, and no real dyeing has taken place. Use of Mordants. To obtain permanent dyes from the great majority of native colouring materials the intervention of another class of bodies entirely different from either fibrous or colouring matter is found necessary ; these bodies are called mordants. The term mordant is found in Latin and Italian manuscripts of the 12th and 13th century, as the name of an adhesive composition by means of Avhich gold leaf could be attached to wood, marble, or metal ; early dyers appropriated the word to designate a substance by means of which colouring matters could be made to adhere to fibre, and it has been retained in that sense in all modern treatises upon dyeing. The chief mordants used in dyeing are salts of aluminium, iron, tin, chromium, copper, and a few other metals. When a decoction of a colouring matter, say logwood or cochineal, is heated with a small quantity of a properly chosen salt of one of these metals, it is found that the colouring principle loses its solubility, forms a combination with the metallic salt or its bases, and precipitates to the bottom of the solu tion, leavingthe supernatant liquid nearly or quite colourless. The precipitate is usually called the &quot; lake &quot; of the particular metal and colouring matter, which are probably in a state of chemical combination ; the lakes are insoluble in water, and are only split up again into their constituents by the action of somewhat powerful chemical agents. Fibre cannot usually be dyed by means of ready formed lakes, for the reason that they are insoluble in water and not easily soluble in any menstruum which can be safely applied to such material ; they are themselves of too coarse and gross a nature to penetrate the fibre, and when applied to it rest for the most part on the surface, and are therefore easily removable by washing or mechanical friction. It is kuowu, however, that for some colours in calico-printing lakes can be applied, but that is only in conjunction with, acid salts and at a high temperature, by means of which a sort of solution is obtained while in contact with the fibrt* itself. The art of the dyer consists in so arranging these three elements fibre, metallic salts, and colouring matter that he may obtain the formation of the insoluble coloured lake in the body of the fibre itself, whereby either by the lake being mechanically retained or chemically combined the fibre is permanently coloured. Application of Mordants. There are three principal ways in which the mordant and colouring matter can be put into contact with the fibre, the developments and modifications of which constitute the whole art of dyeing. 1. By the first method, which is by far the most common, the fibrous matter is separately impregnated with the mordant, which is by various means decomposed, so as to deposit its base in an insoluble state upon or within the fibre, and afterwards the colouring matter is applied. Take, for example, the case of dyeing a common black from logwood upon calico, which has no affinity for the colouring matter of the logwood. The first process is to pass the calico through a hot aqueous solution of sulphate of iron, some times mixed with acetate of iron, and to remove the excess by pass ing the cloth through rollers ; the cloth, either previously dried or not, is then passed through a mixture of lime and water which has the effect of decomposing the iron salts and liberating oxide of iron. A washing in water to remove the excess of lime or any loosely attached oxide of iron prepares the calico for coming into contact with the logwood. The calico, which has now a buff colour, owing to the attached mordant of oxide of iron, when placed in a hot decoction of logwood speedily acquires a dark hue and in about half an hour has become dyed of a dense black colour, and, when smoothed and finished, forms the common black calico of the shops. A variety of other cases might be adduced ; woollen cloth boiled for some time in bichromate of potash solution acquires a certain amount of a salt of chromium, which enables it to take a black colour from logwood, and other colours from other dye-stuffs. Woollen, boiled with salts of tin, is enabled to dye up a briilant scarlet in decoction of cochineal ; boiled with alum, it will take a great variety of colours in various dye-stuffs. The practice of calico-printing illustrates in a very forcible manner the action of mordants ; by the aid of apparatus described in the article upon that subject, portions of a piece of calico are impregnated with mordants, and these portions alone acquire colour from the dyeing solution, and thus designs or patterns are produced upon a white ground. The most usual method of impregnating the fibrous matter with mordant, consists in heat ing it with the required metallic salts, and it will be seen hereafter that easily decomposed salts are those preferably used ; or substances such as chalk, alkalies, or tartar are added to some more stable salt, such as alum, to induce the formation of comparatively unstable compounds, which, under the influence of a high temperature and contact with fibrous matter undergo decomposition, the metallic oxide or some basic insoluble compound of it becoming intimately combined with the fibre, which is then said to be mordanted. 2. A second method, less general than that above described, is to apply the colouring matter before the mordant. It is resorted to only with heavy goods which absorb a large quantity of liquid, or with light colours upon other fabrics ; dyes produced in this way are super ficial in their character, and not so permanent as those produced by the first method. In dyeing by that method it is in many cases cus tomary to add a small quantity of mordant to the dye-bath when the process is quite or nearly finished, or to pass the dyed goods, as a final operation, through a diluted mordant. 3. A third method is to apply the mordant and the colouring matter together to the fibrous substance. In common piece-dyeing in weak liquids this plan is seldom followed, on account of the ten dency to form insoluble lakes in the solution, which, depositing only on the external part of the fibres, give inferior results, alike as to sta bility of colour, depth of shade, and evenness or regularity of the dye. In calico printing or in padding, this method is of extended appli cation and the inconveniences experienced in common dyeing are not perecptible, owing to the greater concentration of the mordant ing salts and the use of thickening matter. Lakes are very probably formed to some extent during the preparation of the mixtures, but, the combination taking place in the presence of a fluid made viscous with gum or starch, the insoluble lake is in an extremely fine state of division ; in such a mixture there is always present an acid or an acid salt, such as acetic acid, oxalic acid, tartaric acid, or alum, chloride of tin, cream of tartar, or binoxalate of potash. These tend, in the first instance, to restrain the formation of a lake, and after wards, when the fibre and the mixture of mordant and colouring matter are submitted to heat, as in the process of steaming or stoving, facilitate the solution of any laka formed, which thus finds entrance into the fibrous matter, and there undergoes combinati en with it,