DYEING. Dyeing is the art of staining or coloring yarn or cloth in such a manner that the color is to a measurable degree permanent. The art was practised among the Eastern nations from time immemorial, and in the Scriptures we read of the vestments of the high priest being dyed purple and of linen cloths being dyed blue, purple and scarlet. The famous Tyrian purple is believed to have been discovered by the in habitants of Tyre 1500 a.c., and immediately afterward Tyrian purple became the badge of royalty and cloth dyed with it commanded a very high price. The Egyptians, Grecians and Romans practised the art of dyeing and it gradually became more widespread as civiliza tion advanced. In earlier times dyeing was much more extensively followed as a domestic art than at present. The process of dyeing is accomplished in two stages: (1) absorption; and (2) fixation. The first is a simple diffusion of the dye throughout the fibre; the second is the result of a chemical action in which the dye becomes insoluble in the material of the dye bath. Where mordants are used it is the mordant which is applied to the fibre in the absorption stage. The textile fibres upon which the dyer exercises his art are of two groups: (1) animal fibres, as wool and silk; and (2) vegetable fibres, as cotton, linen, jute, ramie and the artificial °silks'> Wool fibre is the most complex, being composed of three forms of cells. Silk has no cellular formation, being practically a long double cylinder. In cotton each fibre is a single hollow cylindrical cell, flattened and twisted. In linen and jute similar hollow cells are grouped in clusters to form the individual fibres. In chemical composition and reaction there are still wider and more signifi cant differences.
If the various coloring matters used in dye ing had an affinity for the fibre in its natural state the process would be very simple. It would be necessary only to make a solution of the dye drug and immerse the goods to ensure their being dyed. But so far from this being the case, if we except indigo and safflower, there is scarcely a dyestuff that imparts its color directly to goods. The greater part of the dye drugs have so weak an affinity, for cotton goods especially, that they impart no color suf ficiently permanent to deserve the name of a dye. The reason for this is simple. For exam ple, in a decoction of logwood the color matter is held in solution by the water. Upon putting a quantity of cotton into this solution the fibre becomes filled with the colored solution, and if the cotton has no power to render that coloring matter insoluble within its fibres it is plain that by taking out the cotton and putting it into water the coloring matter within it will be diffused in the water. In other words, the dye having no attraction for the fibre is washed out. This lack of primary affinity makes dyeing in tricate and renders it more dependent upon sa ence. Indeed, it is only by the careful arrange ment of processes in accordance with certain chemical laws that the dyer is enabled to advantage the various coloring matters of which he is in possession. When the dyer finds there
is no affinity between the goods and a coloring substance he wishes to use he endeavors to find a third substance which has a mutual attrac tion for the fibre and coloring matter, so that by combining this substance with the fibre and then passing the cloth through the dyeing solu tion the coloring matter combines with the sub stance which is upon the goods and creates a color. This third substance used, which acts as a mediator, combining two inimical bodies, is termed a mordant.
All the mordants with one or two exceptions are found among the metallic oxides. In order that a substance may act as a mordant it must have an attraction for the coloring matter so as to form with it an insoluble colored compound, and it must be held easily in solution. It may also have an affinity for the fibre and a tendency to unite with it, but this property is not essen tial. The first two properties limit the mordants almost entirely to what are termed the insoluble bases, that is, substances which are not by them selves soluble in water. The bases or oxides which are in general use as mordants and which appear to succeed best are alumina and the oxides of tin, chromium and iron. An important part of all dyeing operations is the proper choice and ap plication of mordants; there being a chemical union between them and the coloring matter a new substance is formed, not only differing in properties but differing in color from any of the originals. Consequently, a very little alteration in the strength or quality of a mordant muses a decided variation in the shade of color. As, for example, logwood alone gives no color to cotton worthy the name of a dye, yet by ju dicious application of a few different kinds of mordants all the shades, from a French white to a violet, from a lavender to a purple, from a blue to a lilac and from a slate to a black, are obtained from this substance. Before any chemical union takes place between bodies they must not only be in contact, but they must be reduced to their ultimate molecules. Mordants that are insoluble of themselves must be dis solved in some appropriate menstrua before their particles can enter the fibres of the goods or combine with the coloring matter. In doing this the dyer must respect the degree of affinity between the solvent and the mordant to deter mine what force it will exert against the com bining of the mordant with the fibres of the cloth. Otherwise, a powerful mordant may be weakened by the attraction of the solvent. For example, common alum even though much con centrated is but a weak mordant for cotton goods, owing to the great attraction between the alumina and the sulphuric acid of the dye bath. But if acetic acid, which has comparatively a weak affinity for the alumina, be substituted for sulphutic acid it becomes a very powerful mordant.