The object of the dyer is to produce on a given material any desired color, with a certain standard of fastness. The demand may be for fastness to light; it may be for fastness to washing or, better, for fastness to both light and washing. The manufacturer must also consider the cost of the dyestuffs and of the process necessary to set the dye on the fiber. Here is one great cause of our many fugitive colors, for too often the finished material must be cheap.
There are two distinct types of dyestuffs: the natural dyes and the artificia - the aniline, or coal-tar dyes; the first class as old as the hills, and the second comparatively new. The Tyrian purple so highly prized in ancient days and used as a sign of royalty was obtained a drop at a time from a shellfish found on the shores of Tyre. The Egyptians early used gorgeous colors, obtained from plant and mineral sources, dyeing being a well-developed art even then.
The Oriental peoples, most famous for their beautiful rugs, guard sacredly the secrets of their dyes, which have been handed down and improved by generation after generation, until they are surpassed in beauty by no others. The famous tapestries of the Middle Ages and of more modern times have been colored with these dyes taken directly from nature and brewed by the skilled dyester, while the simple materials woven by our grandmothers were dyed with bark of walnut, leaf of sumac, or root of indigo.
All this has been changed as a resuit of the marvelous discovery made by the chemist, Perkin, in the year 1856. While experimenting in his laboratory with aniline from coal tar he found the dye substance mauveine. This discovery, followed by many others, in time completely revolutionized the dyeing industry. There are at present hundreds of dyestufls made from coal-tar products, which produce an enormous variety of shades, hues, and pure colors. The manufacturers of these dyes employ constantly an army of chemists to discover new dyes and to perfect the method of using those already known.
Considerable has been written in criticism of aniline dyes, and much of this criticism is just. The chief difficulty is that these dyes are so much purer than the natural dyes that the resulting colors are crude and hard. Also, many of the colors are fugitive and fade to ugly tones. When properly used, however, beautiful and fast colors may be obtained with greater certainty and much more cheaply than with the vegetable dyes.
Vegetable Dyes are still used to a certain extent by craft workers and by those who dye on a small scale. For commercial purposes, however, with the exception of logwood and indigo, natural dyes have almost entirely disappeared, and these two are rapidly being replaced by chemically prepared dyes. Cochineal, the coloring matter extracted from the bodies of the female of a small bug which grows on cactus plants, is still used to color the red coat of the British soldier.
Primitive peoples still use the natural dyes. When they use yarns dyed with aniline dyes, as some of our Western Indians have done of late years, their products are anything but artistic, since they do not know how to handle these colors.
It is a capital offence to carry aniline dyes into Persia; in this way the law has protected the beauty of the Persian rug.
Aniline dyes especially prepared in small packages for vegetable or for animal fibers are known to the housekeeper. Those dyes are chosen which will produce the best results in unskilled hands, and are combined with the chemicals necessary to produce this result. The dyer needs to add only common salt or vinegar as an assistant.
Certain dyestuffs produce a fast color on a fiber, while others require the action of some chemical to bind the dyestuff to the fiber. The chemical which is used to combine with and fix a dye upon a fiber is called a mordant. A dye which requires a mordant for one fiber may not require it for another. In general, vegetable fibers require mordants more often than do animal fibers.
The theories of dyeing are not entirely established yet, some authorities holding that there is a chemical reaction between the fiber and the dyestuff by which a compound called a lake is produced. Others hold that the process is merely a mechanical one, and that the color particles are deposited on the fiber. Both theories have evidence in their favor. A third theory is that the fiber is capable of dissolving the dyestuffs and producing a solid solution. Whichever theory is the true one, conditions of heat, amount of liquid, and elements in the dyebath greatly affect the result produced.
It has been said that a mordant is a chemical substance which combines with the dye to produce a fixed color on a fiber. Certain metallic salts have this power. They are absorbed by the fiber, and then mordant and dye combine to form an insoluble color lake. Vegetable fibers, however, do not have the power of absorbing these metallic salts to any great extent, but do combine with tannic acid. The tannic acid may then combine with a mordant, and finally the dye-stuff is brought into combination. In some cases the mordant is applied to the fiber first, then the dye added, or the mordant and dye may both be put into the same bath. Common mordants are salts of aluminum, chromium, iron, and tin. In the old methods of dyeing, alum and chrome were common mordants. Tannic acid was obtained from gallnuts or sumac.
Glauber salt, common salt, acids, or alkalies are used with different dyestuffs to hasten or delay the process or to regulate the distribution of the dyestuff, according to the properties of the dye or of the fiber.
Certain colors are produced on fibers by after-treatment with various agents. The color may be merely a deeper tone of that originally produced by the dye, or it may be a different color. The object in producing colors in this manner is to gain fastness.,