Dyeing

the process and art of imparting permanent color to yarn and cloth. Any animal, vegetable, or mineral substance, sufficiently porous to absorb coloring matter in solution, may be dyed; but, as commonly used the word has reference to fabrics, or to the animal and vegetable fibers of which fabrics are composed. The art of dyeing is as old as civilization. Tyrian purple is supposed to have been used in Tyre 1,500 years before the Christian era. The ancient Egyptians, Greeks, and Romans employed dyes of various colors. During the Dark Ages the knowledge of this art seems to have died out in Europe. It is probable that the Jews preserved the secrets of the art. We learn from one account that, about the year 1160, of the 200 Jews then resident in Jerusalem, all were employed in wool dyeing, the trade being wholly in their hands. By the close of the thirteenth century the practice of the art of dyeing had spread again throughout Europe. Dyers' gilds existed in many cities, and knowledge advanced rapidly, until most of the natural dyes of the old world were well understood. The discovery of America resulted in the addition of two important dyestuffs--cochineal and logwood. Little improvement in methods was made until, with the development of chemistry in the nineteenth century, dyeing became an applied science.

If the various solutions used in dyeing possessed an affinity for the raw fiber, the process of coloring would be simple. Nothing but immersion in the dye bath would be required. As a matter of fact, this affinity is lacking, especially in cotton fiber. If a cotton fabric be put in the dye, the coloring matter is absorbed by the fiber to some extent; but if the cloth thus treated be washed, the dyestuff in most cases is dissolved again, and the fiber left in its natural color. This fact makes dyeing a difficult matter. The dyer must find some third substance which has a mutual attraction for the cloth or fiber and for the coloring matter; so that, acted upon by this substance, the fibers of the cloth will unite with the coloring matter and be permanently dyed. This third substance is called a mordant, from a French word meaning to bite; the old idea being that the mordant bit into the fiber, thus opening it up to the action of the dye. Every natural dye has its own particular mordant, most of the mordants being found among the metallic oxides. Before chemical research threw light upon dyeing processes the dyer worked blindly. If successful in a particular instance the recipe was preserved, and followed carefully for similar results. Often failure resulted, the cause of which it was impossible to explain. At present chemical laboratories are erected in connection with the larger dyeing establishments, and methods are regulated with scientific precision.

Certain questions connected with the principles of dyeing have occasioned much discussion. Some scientists regard the union between fiber and dye as a chemical action. Since, however, any dye may be removed from cloth or yarn and dissolved by certain agents, while the fiber remains utterly uninjured, it would seem that the action is mechanical, rather than chemical. A view held by some chemists is that the dyeing of wool is a chemical combination, but that the dyeing of cotton is merely a fixation of the color in the pores of the fiber.

It is evident that the principal part of every dyeing operation is the finding of a proper mordant and its successful application. Sometimes the fabric or yarn is treated with the mordant and then immersed in the dye bath. Sometimes the cloth is dyed first and then treated with the mordant. Sometimes mordant and dye are combined and applied to the fabric simultaneously. A little change in the strength of a mordant may effect a decided change in tint or shade. Thus a wide field for variety of shades is offered the dyer. For example, logwood alone will not dye cotton. With the proper use of mordants, logwood may be made to dye cotton in all shades of lavender, violet, purple, lilac, and slate.

In the manufacture of textile fabrics the dyeing may be done at various stages. Sometimes the raw fiber receives the dye. By this method, called stock dyeing, the color is fixed more firmly and is less likely to fade or lose its brilliancy. The spun yarn may be dyed, but it is difficult to be sure the dye enters the inner fibers of a tightly twisted yarn. The cheapest and most common method is called piece dyeing, which means that the woven fabric is dyed. The labor, in this case, is accomplished by machinery, and there is less waste of expensive dyestuffs than occurs in stock and yarn dyeing. As has been said, cotton has less affinity than wool for natural dye solutions. Until artificial dyes came into use, therefore, the dyeing of cotton was a difficult process. The azo dyestuffs, a class of coal-tar colors, however, can be applied to cotton without a mordant, and produce brilliant and lasting colors. This fact causes azo dyes to be regarded as a most important class of artificial dyes.