MORDANT, in dying and calico-print ing, denotes a body which, having a two fold attraction for organic fibres and coloring particles, serves as a bond of anion between them, and thus gives fixity to dyes ; or it signifies a substance which, by combining with coloring par ticles in the pores of textile filaments, renders them insoluble in hot soapy and weak alkaline solutions. In order pro perly to appreciate the utility and the true functions of mordants, we must beat in mind that coloring matters are peculiar compounds possessed of certain affinities, their distinctive characters being not to be either acid or alkaline, and yet to be capable of combining with many bodies, and especially with saliflable bases, and of receiving from each of them modifica tions in their color, solubility, and alter ability. Organic coloring substances, when pure, have a very energetic attrac tion for certain bodies, feeble for others, and none at all for some. Among these immediate products of animal or vegeta ble life, some are soluble in pure water, and others become so only through pecu liar agents. We may thus readily con ceive, that whenever a dye-stuff possesses a certain affinity for the organic fibre, it will be able to become fixed on it, or to dye it without the intervention of mor dants, if it be insoluble by itself in water, which, in fact, is the case with the color ing matters of safflower, annotto, and in digo. The first two are soluble in alkalis •, hence, in order to use them, they need only be dissolved in a weak alkaline ley, be thus applied to the stuffs, and then have their tinctorial substance precipi tated within their pores, by abstracting their solvent alkali with an acid. The coloring matter, at the instant of ceasing to be liquid, is in an extremely divided state, and is in contact with the organic fibres for which it has a certain affinity. It therefore unites with them, and, being naturally insoluble in water, that is, hav ing no affinity for this vehicle, the subse quent washings have no effect upon the dye. The same thing may be said of in digo, although its solubility in the dye bath does not depend upon a similar cause, but is due to a modification of its constituent elements, in consequence of which it becomes soluble in alkalis. Stuffs plunged into this indigo bath get impregnated with the solution, so that when again exposed to the air, the dye ing substance resumes at once its primi tive color and insolubility, and washing can carry off only the portions in excess above the intimate combination, or which are merely deposited upon the surface of the stuff.
Such is the result with insoluble color ing matters • but for those which are so luble it should be quite the reverse, since they do not possess an affinity for the or ganic fibres, which can counterbalance their affinity for water. In such circum stances, the dyer must have recourse to intermediate bodies, which add their affinity for the coloring matter to that possessed by the particles of the stuff, and increase by this two-fold action the intimacy and the stability of the combi nation. These intermediate bodies are
the true mordants.
Mordants are in general found among the metallic bases or oxydes ; whence they might be supposed to be very nu merous, like the metals ; but as they must unite the two-fold condition of pos sessing a strong affinity for both the coloring matter and the organic fibre, and as the insoluble bases are almost the only ones fit to form insoluble combina tions, we may thus perceive that their number may he very limited. It is well known, that attliough lime and magnesia, for example, have a considerable affinity for coloring particles, and form insoluble compounds with them, yet they cannot be employed as mordants, because they possess no affinity for the textile fibres.
Experience has proved, that of all the bases, those which succeed best as mor dants are alumina, tin, and oxyde of iron; the first two of which, being naturally white, are the only ones which can be employed for preserving to the color its original tint, at least without much va riation. But, whenever the mordant is itself colored, it will cause the dye to take a compound color quite different from its own. If, as is usually said, the mordant enters into a real chemical union with the stuff to be dyed, the application of the mordant should obviously be made in such circumstances as are known to be most favorable to the combination taking place ; and this is the principle of every day's practice in the dye-house.
Mornmsr is also the name sometimes given to the adhesive matter by which gold-leaf is made to adhere to surfaces of wood and metal in gilding. Paper, vellum, taffety, &c., are easily gilt by the aid of different mordants, such as the following: 1, beer in which some honey and gum arabic have been dissolved 2, gum arable, sugar, and water ; a, the viscid juice of onion or hyacinth, strength ened with a little gum arabic. When too much gum is employed, the silver or gold is apt to crack in the drying of the mordant. A little carmine should be mixed. with the above colorless liquids, to mark the places where they are ap plied. The foil is applied by means of a dossil of cotton wool, and when the mor dant has become hard, the foil is polished with the same.
The best medium for sticking gold and silver leaf to wood is the following, called mixtion by the French artists :-1 pound of amber is to be fused, with 4 ounces of mastic in tears, and 1 ounce of Jewish pitch, and the whole dissolved in 1 pound of linseed oil rendered drying by litharge.
The above is used by distemper paint ers and paper-hanging manufacturers for attaching gold and silver leaf to walls or paper.