Dyeing

Dyeing. The art of dyeing consists of the fixing in fabrics of colours in such a manner that the colour is of a fairly permanent character. As many passages in older writings show, the art was practised by the ancients; thus the Tyrian purple is alluded to by Pliny, and was probably known 1000 B.C. The Phoenicians and Egyptians were well acquainted with various processes for dyeing materials, and manufactured some of their colouring preparations on the large scale. In Europe the knowledge appears for some time to have been restricted to the Jews. In the 15th century, however, dyeing in various shades and colours was well known in Europe, and the first text-book upon the subject appeared in 1540, while other books of this period are extant, containing recipes for the preparation of many dyes. The artificial preparation of innumerable dyes from coal tar, which followed Perkins preparation in 1856 of the first dye from that source, has had the greatest possible influence upon dyeing, and marks the present century as a most important epoch in the history of the industry.

Before dyeing a fabric it is essential that it should be perfectly white and scrupulously clean, as not only would any colour upon the material serve to materially detract from the brilliancy and alter the shade of the dye, but it is also found that a small quantity of impurity has often a great and generally deleterious effect upon the colour. The first process, therefore, in dyeing, is the bleaching of the fabric, which should be of a thorough character, followed by washing in several changes of water. [Bleaching.] From the above also it is seen that it is necessary that the water employed throughout should be of a high degree of purity, for although in certain cases the presence of particular salts may be unprejudicial, or even desirable, it is always better to add them, as required, to the pure water.

The method employed varies according to which of two classes of dyes is used. Those of the first class have the power of directly fixing themselves upon the cloth, without the assistance of any other substance. In this case the dye is dissolved or finely suspended in water, and the material placed in the solution, and kept in constant motion in order to ensure uniformity in the dyeing. When the desired shade of colour is obtained the material is taken out, well washed in a large supply of water, and dried. This mode of dyeing is of more frequent application with wool and silk than in the case of cotton, etc. In the other and larger class of dyes, however, simple immersion does not suffice, and it is necessary to fix in the cloth some substance - the mordant - which causes the adherence of the dye to the fabric. In many cases, also, the dyestuff itself is colourless or nearly so, and union with the mordant is necessary for the development of the colour, which may vary according to the particular mordant employed. In such cases the cloth is generally impregnated with the mordanting substance before the application of the dyestuff. This is generally done by boiling with, or passing through, a hot solution of the mordant, and pressing off the excess by rollers. It is then treated with the dye in the ordinary manner. In some cases, however, the fabric is treated with the dye first, and in other cases both mordant and colouring material act simultaneously upon the cloth. With regard to the theory of the nature of the process, it seems probable that in some cases there is merely a physical attraction between the colouring matter and the fibres of the fabric, while in others true chemical combination occurs. When mordants are employed their function seems to be to combine readily with both the fibre and the colouring matter, so that the colour produced by its union with the latter is fixed in the fabric. It thus appears evident that the nature of the mordant must vary with that of the dyestuff; in the case of dyes of an acid nature, basic mordants are required, and with basic dyes, acid mordants must be used. The principal mordants for general usage are the following: - (1) Metallic salts, which are used as basic mordants, e.g. alum, chromium salts, nitrate or sulphate of iron (copperas), and tin salts, more especially the chloride SnCl₂, known in the trade as tin crystals. (2) Acid mordants, employed for fixing basic dyes, e.g. tannic acid and various fatty acids generally prepared by the addition of sulphuric acid to a fat or oil.

The dyestuffs employed are exceedingly numerous. They may be divided into (1) those obtained directly from natural products, as from decoctions of barks of trees, animal products, etc.; (2) inorganic pigments, or coloured salts, such as lead chromate, which are frequently formed from their constituents during the process itself; (3) artificial organic dyes, now the most important class. The greater number of these latter are obtained from coal-tar, and known as coal-tar colours. Many of them are of the chemical nature of amines (q.v.), and require basic mordants; others are related to phenol, and require basic mordants; some are obtained from naphthalene or anthracene (q.v.); and a large number are related to a compound known as azobenzene (C₆H₅N.N.C₆H₅).

These latter have within recent years acquired considerable importance, as very many have the power, possessed by few other dyes, of dyeing cotton without a mordant. Some of the commoner dyes required for particular colours are the following: -

Red. Chief are cochineal for wools, and madder or the artificial alizarin for cotton. In the first case, the wool is mordanted by boiling with tin salts, and, after washing, brought into a boiler containing the cochineal in suspension, and there boiled until the colour of the liquid is discharged. This gives a scarlet, but other mordants give other colours; thus by the use of alum a crimson is obtained. For cotton the madder is now largely replaced by alizarin, as better tints may be obtained by the use of the artificial colour. The mordants employed are generally tin-salts or alum. The cotton is treated before the operation with an oily emulsion, and after dyeing the colour may be brightened by boiling with soap or alkalis.

Blue. Indigo is the chief dye employed. This dye is insoluble, and cannot therefore be used by itself. If reduced, however, a substance known as indigo white is obtained, and this, dissolved in an alkali, is generally employed. The cloth is agitated in this solution, and after a sufficient time withdrawn. Though not coloured when taken from the liquid, the colour makes its appearance when the fabric is exposed to the air. A sulphate of indigo is also much employed for wools and silks. Prussian blue is also employed for blues. The material is first soaked in nitrate of iron, and then washed. Some oxide of iron is thus formed in the material, and this, when the fabric is treated with yellow prussiate of potash (potassium ferrocyanide) and an acid, yields Prussian blue, which is hence formed in the fibres of the cloth. Many aniline blues, as spirit blue, methyl violet, water blue, are also largely employed with wool or silk, and give beautifully brilliant colours. They have all a complicated composition, but are mostly derivatives of triphenyl methane, C(C₆H₅)₃H.

Yellow. The chief natural yellow dyes are fustic yellow, a dyestuff obtained from the bark of a Central-American tree, and Quercitron bark. A number of artificial yellows are known. Amongst these may be mentioned - lead chromate, which is formed in the fabric by treating with (1) lead acetate, (2) potassium chromate; picric acid; naphthol yellow, derived from naphthalene; rosolic acid and aurine, related to the triphenyl methane mentioned above; and a number of dyes derived from azobenzene.

Greens, purples, etc., can be often obtained by a judicious mixture of two dyes. Fine greens and purples are also given by different coal-tar colours, while madder used with an iron mordant also gives a purple.

For black dyes, logwood is generally used for wool and cotton, gall-nuts for silk, while aniline black may be also employed. Wool is heated in decoction of logwood, and then with sulphate of iron, and may or may not be first dyed with indigo. In silk-dyeing a decoction of gall-nuts takes the place of that of logwood. [Calico-printing.]