The above-mentioned metallic mordants do not merely mediate the absorption of the dye, but modify the shade produced, rendering it purer and brighter, as well as faster. Hence their use is often needful, even in the case of substantive colours.
Two mordants of a different class have recently been discovered, and promise to become of great practical value. The first of these is gelatinous silica, which will open out quite a new era, especially in the fixing of aniline colours upon cotton. All that is needed is to pass the yarns through a solution of silicate of soda, the so-called soluble glass, the more neutral the better, provided it retains its solubility. The yarns are steeped in a dilute acid, so as to precipitate the silica upon the fibre. After careful washing, they are plunged into the dye-beck, where they rapidly take bright, full colours, which are decidedly faster than those obtained with the various mordants previously known. The other new mordant,—also, it is believed, primarily due to Prof. Reiman, of Berlin,—is finely divided sulphur. The yarns, either woollen or cotton, are boiled in bypo-sulphite of soda, along with, or followed by, very weak sulphuric acid. Sulphur in a state of very fine division is thus deposited upon the fibre, and acts as a mordant, not merely for the aniline greens, but also for magenta, the violets, phosphine, &c.
There still remain the organic mordants. If vegetable fibres be coated with certain animal products, such as albumen (whether obtained from eggs or from blood), dissolved caseine (lactarine), gelatine, &c., they assume superficially the characters of wool or silk, and absorb a great variety of colours in the same manner as animal fibres do. These animal mordants play a more important part in calico-printing, than in dyeing. The so-called " oil-mordants " or "emulsive oils," originally introduced in Turkey-red dyeing, are found very widely applicable in fixing refractory colours upon vegetable fibre, and giving them a more permanent character.
Further details concerning mordants, and instructions for their manufacture, will be found in a separate article—Mordants.
The tendency of the art of dyeing is all towards simplification. Pure colouriog-matters, such as alizarine, are employed, instead of crude vegetable or animal products, and thus a great saving of time and of labour is effected.
like all vegetable fibres, is more easily injured by acids than is wool. Consequently, neither mordants nor colours of a strongly acid character can be employed ; otherwise the goods will be corroded, and the colours will fail to be duly absorbed. The solutions employed must be very feebly acid, neutral, or even alkaline. Another important feature is the temperature at which cotton is dyed. Wool almost invariably requires a boiling-heat ; but cotton, in the majority of cases, is worked in the cold, or at a " hand-heat," i. e. at about (90°-100° F.). Cotton is most extensively dyed in the state of yarn, but a large quantity is also dyed after being woven (see Cotton Manufactures). This especially relates to the mixed fabrics, known as Brad ford goods, the warps of which are cotton, and the weft worsted. The perfection of cotton dyeing is to produce on these warps the same tone and depth of colour as are found on the worsted, so that the entire piece may appear level, and free from any cheeky character.
It will now be convenient to give a series of approved recipes, for producing the principal colours upon cotton, selecting such processes as may best illustrate the resources of the modern dyer, and having especial regard to the recently discovered tinctorial substances ;—