The essential circumstances in dyeing are, to ascertain the affinities of the co louring substance ; first, to the solvents ; secondly, to those substances which mo dify its colour, increase its brilliancy, and strengthen its union with the stuff; third ly, to the different agents, which may change the colour, and principally to air and light.
The colouring matters possess chemi ral properties, that distinguish them from all others : they have attractions peculiar to themselves, by means of which they unite with acids, alkalies, metallic oxides, and some earths, particularly alumen. They frequently precipitate oxides and alumine from the acids which held them in solution ; at other times they unite with salts, and form supra-compounds, which combine with the wool, silk, cot ton, or linen : and with these their union is rendered much more close by means of alumine, or a metallic oxide, than it would be without their intervention.
The qualities of the uncombined co louring particles are modified when they unite with any substance ; and if this com pound unites with a stuff, it undergoes new modifications. Thus the properties of the colouring particles of cochineal are modified by being combined with the oxide of tin; and those of the substance thence resulting are again modified by their union with the wool or silk : and all these modifications are analogous to what is observed in other chemical com binations.
Of .11ordantr.
The title of mordant is applied to those substances which serve as intermedes be tween the colouring particles and the stuff to be dyed, either for the purpose of facilitating, or of modifying their com bination ; and by their means, colours are varied, brightened, made to strike, and rendered more durable.
Was it possible to procure a sufficient number of colouring matters, having a strong affinity to cloth, to answer all the purposes of dyeing, that art would be exceedingly simple and easy. But, ex cept indigo, there is scarcely a dye-stuff which yields of itself a good colour, suffi ciently permanent to deserve the name of a dye. This difficulty is obviated by employing an intermediate substance, which has a strong affinity both for the stuff and the colouring matter, and this is the principal purpose for which the mordant is used.
A mordant is not always asimple agent; new combinations are sometimes formed by the ingredients which compose it: so that the compounds resulting from the mixture, and not the simple substances that compose it, are the immediate agents which produce the effect.
Sometimes the mordant is mixed with the colouring particles, sometimes the stuff is impregnated with it, and on other occasions both those modes are united ; and, finally; stuffs are dyed successively with liquors containing different substan ces, the last of ivhich only can act on that with which the stuff is impregnated.
The principal substances employed as mordants are, aluminons salts, lime, me tallic oxides, some astringent substances, and animal matters.
Formerly sulphate of alumen was the only species used as a mordant in dye ing; but of late years acetite of alumen has been introduced with excellent ef fect, particularly for cottons or linens, whose attraction to the a'unien being weak, they require it to be applied, com bined with a substance to which it has not so strong an union as it has to sul phuric acid ; and its attraction to acetous acid is found to be sufficiently inferior to that for the cotton or linen, that it readily quits the acetous acid to combine with them.
Acetite of alumen isprepared by pour, ing acetite of lead into a solution of alum, in the proportion of one part of the ace tite of lead to three of the alum in weight, *sixteenth of potash, and as much of powdered chalk, are also added. In this mixture, the sulphuric acid combines with the lead, and is precipitated ; and the alumen, or base of the alum, combines with the acetous acid, as it parts from the lead, and forms acetite of alumen ; the chalk and potash serve to saturate the ex cess of acid.
The final effect of alumining, in what ever way performed, and whatever che mical changes may have taken place in it, consists in the combination of alumen with the this union is probably im perfect, and the acids but partially sepa rated at first, but becomes complete when the stuff is afterwards impregnated with the colouring substance.