The art of tinging or imbuing various substances with different colours at pleasure. The principal application of thi3 art is to fabrics of wool, silk, and cotton; but it is also applied to the colouring of leather, marble, ivory, and various other substances. We shall limit our description to the process of dying the first-mentioned substances. The simple colours employed in dying are mostly either of animal or vegetable origin; but although the number of possible dyes which might be obtained from these sources is almost infinite, the number employed in the regular manufactories of Europe is small, the infinite diversity of tint which is obtained being the result of the combination of two or more simple colouring substances with one another, or with certain chemical reagents. Of the great variety of known dyes, some (though comparatively few) may be applied to animal or vegetable stuffs without any other preparation than that of cleansing the stuff, and then immersing them in a decoction or infusion of the dye, which then becomes so permanently combined with the fibre as to resist the effect of washing and the bleaching power of the sun and air.

But the greater number of dyes have such feeble affinity for fibre that no permanent combination can be effected between them in their simple state; to effect the combination, recourse is had to various substances having a strong affinity both for the fibre and the colouring matter; and the cloth being previously steeped in a solution of some of these substances, and afterwards immersed in the dye, an intimate union of the cloth and the colouring matter is effected. The substances which serve to fix the colouring matter in the cloth are termed mordants; and in the management and selection of them is the chief art of the dyer, as their effects are not limited to the fixing of the colour, but they, in most cases, produce some alteration in the natural hue, - a circumstance which the dyer avails himself of in numerous cases, to vary and improve the colour obtained from simple dye stuffs: thus aluminous mordant changes the dull red of madder to a bright crimson; and the solutions of tin not only fix cochineal in wool, but change it from crimson to bright scarlet; but if the oxide of iron be substituted for the tin as a mordant, the colour becomes changed to a black.

In dying, then, it is necessary not only to procure a mordant for the colouring matter and the cloth, and a colouring matter which passes the desired colour in perfection, but to procure a mordant and colouring matter, which, when combined, shall produce the colour sought. It is also evident that a great variety of colours may be produced from a single dye stuff, provided the mordants can be sufficiently varied. Mordants are generally composed of earths or metallic oxides, tannin, and oil. Of earthy mordants, the most important and generally used is alumine, either in the state of common alum, in which it is combined with sulphuric acid, or in that of acetate of alumina, which answers much better than alum, as the cloth is more easily saturated with alumine, and takes, in consequence, a richer and more permanent colour. Almost all the metallic oxides have an affinity for cloth, but only two of them are extensively used as mordants, viz. the oxides of tin and of iron. The oxide of tin was first introduced in dying by Kuster, a German chemist, who brought the secret to London in 1543. It is generally employed in the state of nitro-muriate, muriate, or acetate of tin.

Iron is generally employed in the state of the sulphate or the acetate; the first being chiefly used for wool, and the latter for cotton. Tannin has a very strong affinity for cloth, and for several colouring matters. It is principally obtained from nut galls, or sumach, which contain it in great quantities. Tannin is also often employed along with other mordants to produce a compound mordant. Oil is also used for the same purpose in dying cotton and linen. We now proceed to a consideration of the dyes, and commence by observing that innumerable as are the different colours and shades of colours communicated, they all originate from four or five primary dyes, modified according to the colour intended to be produced. These primary or simple dyes are as follow: - blue, yellow, red, black, and fawn, or brown colour.