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Red oxide of iron

This is also used as a mordant in dyeing; it has a very strong affinity for all kinds of cloth, of which the permanency of red iron-spots, or iron-moulds on linen and cotton is a sufficient proof. As a mordant it is used in two states: in that of sulphate of iron, or copperas, and that of acetite of iron. The first, or coperas, is commonly used for wool. The copperas is dissolved in water, and the cloth dipped into it. It may be used also for cotton, but in most cases acetite of iron is preferred, which is prepared by dissolving iron, or its oxide, in vinegar, sour beer, or pyroligneous acid, and the longer it is kept the better.

Tan, &c.

Tan is very frequently employed as a mordant. An infusion of nut-galls, or of sumach, or of any other substance containing tan, is made in water, and the cloth is dipped in this infusion, and allowed to remain till it has absorbed a sufficient quantity-Tan is often employed also; along with other mordants, to produce a compound mardant. Oil is also used for the same purpose, in dying cotton and linen. The mordants with which tan is most frequently combined, are alumine, and oxide of iron.

Besides these mordants, there are several other substances frequently used as auxiliaries, either to facilitate the combination of the mordant with the cloth or to alter the shade of colour; the chief of these are, tartar, acetate of lead, common salt, sal ammoniac, sulphate of copper, &c. Mordants not only render the dye permanent, but have also considerable influence on the colour produced. The same colouring matter produces very different dyes, according as the mordant is changed. Suppose, for instance, that the colouring matter is cochineal; if we use the alumincus mordant, the cloth will acquire a crimson colour; but the oxide of iron produces with it, a black.

In dyeing, then, it is not only necessary to procure a mordant which has a sufficiently strong affinity for the colouring matter and cloth, and a colouring matter which possesses the wished-for colour in perfection; but we must procure a mordant and a colouring matter of such a nature, that when combined together, they shall possess the wished-for colour in perfection; and even a great variety of colours may be produced with a single dye-stuff, provided we change the mordant sufficiently.

To determine the effects of various salts or mordants on colours.