The only two substances used for dying blue are woad and indigo-Indigo has a very strong affinity for wool, silk, cotton, and linen, all of which may therefore be dyed with it without the assistance of any mordant. But indigo is soluble only in sulphuric acid; it is therefore necessary either to employ the sulphate of indigo, or to render it soluble in water, by depriving it of its oxygen. The first process is frequently employed for dying wool or silk; but for linen or cotton, the latter is generally resorted to. When the sulphate is employed, one part of indigo is to be dissolved in four parts of concentrated sulphuric acid, and one part of dry carbonate of potash added to the solution, which is then to be diluted with eight times its weight of water. The cloth must be boiled for an hour in a solution of five parts of alum and three of tartar, after which it must be removed to a bath containing a greater or smaller proportion of the sulphate of indigo, according to the shade which the cloth is to receive; and in this bath it must be boiled until it acquire the desired colour. The alum and tartar are not intended to act as mordants, but to facilitate the decomposition of the sulphate of indigo.
The alkali added to the sulphate answers the same purpose.
But the most common method of employing indigo is to deprive it of the oxygen to which it owes its blue colour, and thus reduce it to the state of green pollen, and then to dissolve it in water by means of alkalies or alkaline earths, which act very readily upon it in that state. It is deprived of oxygen either by admixture with other substances possessing a greater affinity for oxygen, as the green oxide of iron, or various metallic sulphurets; or it may be mixed in water with certain vegetable substances which readily undergo fermentation: the ferments most commonly employed are woad and bran. During this fermentation, the indigo is deprived of its oxygen, and is then dissolved by means of quicklime or alkali added to the solution. The first of these methods is usually followed in dying cotton or linen; the second, in dying wool and silk.
This colour is most commonly obtained from weld, fustic, or quercitron bark. The cloth requires to be prepared before dying, by combining it with some mordant; that most commonly employed for this purpose is alumina.
The materials employed for this colour are lac or kermes, cochineal, archil, madder, carthamus, and Brazil wood; and the ordinary mordants are alumina, and the oxides of tin; various shades are produced by intermixture of the dying materials above named, or by first dying the stuff with one or more of them, and subsequently passing it through a yellow bath.
The substances employed to give a black colour are red oxide of iron and tannin. These two substances have a strong affinity to each other, and, when combined, assume a deep black colour, not liable to be decomposed by light. Logwood is usually employed as an auxiliary, because it communicates lustre, and adds considerably to the fulness of the black. Cloth, before it receives a black colour, is usually dyed blue, which renders the colour much fuller than it would otherwise be; for inferior cloth, a brown colour is sometimes given by means of walnut peel.
Of Brown, or Fawn Colour. - Various plentiful and cheap substances are employed to give a brown or fawn coloured ground, as birch, sumach, alder bark, but more especially decoction of walnut peels, or walnut bark or root. The shades produced by the bark or rind of the walnut tree are particularly fine, the colours solid, and it renders the wool, when dyed in it, flexible and soft. From the above colours variously combined are derived the endless gradations of tint imparted to the various fabrics of silk, wool, cotton, and linen. Of these compound colours we shall notice a few of the principal.