This section is from the "A Complete Dictionary of Dry Goods" book, by George S. Cole. Also available from Amazon: A complete dictionary of dry goods and history of silk, cotton, linen, wool and other fibrous substances,: Including a full explanation of the modern processes ... together with various useful tables.
Mordant. A substance used to fix colors; a substance which has an affinity for, or which can at least penetrate the fibre of the material to be colored, and which possesses also the property of combining with the dye-matter, and of forming with it an insoluble compound within or about the fibres. Albumen, gluten, gelatin, tannin, certain oils, certain acids, alumina, soda and lead salts are used as mordants. To obtain permanent dyes from the great majority of coloring materials, the use of mordants is found absolutely necessary. Fiber cannot usually be dyed by means of ready formed lakes, or native dyeing extracts, for the reason that they are of too coarse and gross a nature to penetrate the fiber and remain there. When they are applied to any sort of fibres without the use of a mordant, they rest for the most part on the surface, and are, therefore, fugitive and easily removable by washing or friction. The art of the dyer, therefore, consists in so combining these three elements - cloth, mordant and coloring matter - that he may obtain the formation of a color in the body of each individual fiber whereby it will be chemically combined with the fibre and permanently retained. There are three principal ways in which the mordant and coloring matter can be put into contact with the yarn or cloth, the modifications of which constitute the whole art of dyeing:
1. By the first method, which is by far the most common, the cloth is separately impregnated with the mordant, which is by various means decomposed, so as to deposit its base in an insoluble state within the fiber, and afterwards the coloring matter is applied. Take, for example, the case of dyeing a common black from logwood upon calico, the cotton composing which has no affinity for the coloring matter of logwood. The first process is to pass the calico through a hot solution of sulphate of iron, and to remove the excess by passing the cloth through rollers; the cloth, either previously dried or not, is then passed through a mixture of lime and water which has the effect of decomposing the iron salts. A washing in water to remove the excess of lime prepares the calico for coming into contact with the logwood. The calico, which now has a buff color, when placed into a hot decoction of logwood speedily acquires a dark hue in about half an hour, and has become dyed of a dense black color, and, when smoothed and finished, forms the common black calico of commerce. A variety of other cases might be cited. Woolen cloth boiled for some time in bichromate of potash acquires a certain amount of salt of chromium, which enables it to take a black color from logwood, or other colors from other dyestuffs. Woolen cloth, boiled with salts of tin, is enabled to dye up a brilliant scarlet in a decoction of cochineal; boiled with alum, it will take a great variety of colors in various dyestuffs. The practice of calico printing illustrates in a very forcible manner the action of mordants. By the aid of proper machinery described in an article upon that subject, portions of a piece of calico are impregnated with mordants, and these portions alone acquire color from the dyeing solutions, and thus designs or patterns are produced upon a white ground. The most usual method of impregnating the cloth, composed of any sort of fiber, with mordant, consists in immersing it in a hot bath containing the required salts, which, under high temperature, decomposes and is absorbed by the fiber. It is then said to be mordanted and the cloth is ready to receive the dyestuff.
2. A second method, less general than that above described, is to apply the coloring matter before the mordant. It is only resorted to with heavy goods which absorb a large quantity of the dye-color, or with light tints upon other fabrics; dyes produced in this way are not so permanent as those produced by the first method.
3. A third method is to apply the mordant and coloring matter together, to the cloth or yarn. In common piece dyeing this plan is seldom followed, on account of unsatisfactory results. It consumes less time, however, and calico is frequently colored by this process. [See Cochineal, Calico, Logwood, Indigo, Madder, Turkey-red]