Within the last sixty years the processes of bleaching and dyeing have changed from rule of thumb methods, handed down from generation to generation, and carried on most successfully by those who inherited the skill from their forefathers, to a most scientific method, requiring for best results the skill of the textile chemist. The workman who mixes the dyes and watches the machine is not necessarily as intelligent as the Oriental dyer in his native mountains, but the dyestuffs are not gathered from nature by the man who mixes them, and he is not even intrusted with the matching of shades.
The processes of bleaching and dyeing differ for the different textile fibers. The vegetable fibers have similar chemical reactions, and the animal fibers are somewhat alike in their chemical characteristics, yet each fiber must be treated in the way best suited to its individual needs. So many and such complicated reactions are involved in these processes that only a general statement of methods and principles will be given here. Since the chemical and physical structure and characteristics determine the treatment of the fibers in bleaching and dyeing, these may be briefly reviewed.
Cotton consists of cellulose 87 per cent, water 8 per cent, and the remaining 5 per cent vegetable wax, oil, albuminous matter, pectic acid, and coloring matter. The object of bleaching cotton is to remove this 5 per cent impurity, so that the fiber may be white or that it may absorb dyes readily; the oil and wax prevent this ready absorption.
Cotton is a single cell, hollow in the center, a collapsed tube.
Linen is not so largely pure cellulose, having about 15 to 30 per cent of foreign substance, besides 6 to 8 per cent water. The foreign matter is mostly pectic in nature and is difficult to remove. The linen fiber also differs in being composed of many cells held together by a gummy substance which may be dissolved by chemicals, making it more easily disintegrated than cotton.
In general, the treatment of cotton and linen in dyeing and bleaching is the same. Acids weaken or destroy the fibers, while alkalies, except in special cases, have little effect on them. Mercerized cotton has a greater affinity for dyestuff than ordinary cotton, and linen is more difficult to dye than cotton. Cellulose has very weak acid and basic properties, so weak that it does not combine directly with most dyes. Cotton has a very slight affinity for metallic salts.
Wool, being an animal fiber, is a nitrogenous compound. Much foreign matter is present in raw wool; in some cases less than 30 per cent of the weight of wool is fiber, 14 per cent is water, and the rest impurity. Most of this foreign matter is, however, removed before the fiber reaches the bleaching process.
Raw silk is composed of two substances. The fiber, fibroin, is a nitrogenous compound; sericine, the gummy substance surrounding the fiber, is soluble in warm, soapy water and is removed in the cleaning process. About 73 per cent of silk is fibroin, 25 per cent sericine, and 12 per cent water.
The chemical actions of wool and silk are practically opposite to those of cotton and linen. Wool disintegrates readily in warm, rather dilute alkali; silk is not so readily affected. Acids, even when fairly concentrated, have very little effect on wool, but injure silk more readily. Both wool and silk have great affinity for metallic salts. The action of wool towards bleaching powder in cold and dilute solutions is one of chemical combination; the product, "chlored" or "chlorinated," wool, has a high luster, loses its felting property, and has an increased affinity for dyes. The absorption of metallic salts by silk gives the process of weighting already mentioned.
Wool sometimes acts as an acid, sometimes as a base, therefore combines readily with different classes of dyestuffs.
Fibers are made weak, or tendered, by certain chemicals. In the case of vegetable fibers, dilute acids and strong alkalies in the presence of air have this effect. Animal fibers are weakened by caustic alkalies, alkaline carbonates in boiling solutions, and concentrated mineral acids.
Owing to its physical structure, the single cell of cotton, with its central canal, doubtless absorbs dyes by capillary action. This, however, is probably largely due to pores which run from the surface inward. Mercerized cotton, in which the central canal has almost entirely disappeared, has a greater affinity for dyes than ordinary cotton. Linen in the unbleached state has the cells of the fibers surrounded by a pectic substance that retains its natural coloring matter and resists dyes, and this is true to a certain extent of bleached linen.
The structureless silk fiber readily absorbs dyestuffs. So great is the affinity of this fiber for some classes of dyes that it is necessary to add the dyestuffs in several small amounts to prevent uneven absorption. The central or cortical layer of cells of the wool fiber takes up the dyestuff, while the outer, scaly layer does not dye so readily. This explains the depth of color obtained when looking into a pile material, where the cross section of the thread appears darker than the surface.