When the fibers come into the hands of the dyer, either in the form of raw fiber, yarn, or cloth, they are in a more or less impure condition; besides the natural impurities there are present oil and dirt from the machinery and sizing in the cloth.
Three general operations must be gone through in this process:
1. The material must be thoroughly wet, so that every portion will be uniformly penetrated by mordant or dye, and not be protected from contact with it by bubbles of air, etc.
2. All impurities, such as vegetable or animal oils, which would prevent the fiber from taking up the mordant or dye, must be thoroughly removed.
3. The natural coloring matters, when they would interfere with the brilliancy of color to be produced, must be removed.
The process for cotton, bleached more commonly in the yarn or cloth state, consists of the following operations:
2. Treatment with bleaching powder solution to destroy natural coloring matter, and also to break down various non-cellulosic matters associated with the cellulose of the cotton.
3. Treatment with a dilute solution of acid called "souring," to dissolve lime compounds left in fiber from bleaching powder and to decompose any chlorine compounds which may have been formed.
4. Washing. For the purpose of removing all soluble matters resulting from the action of the bleaching powder and acid; also for the removal of the acid from the fiber.
5. Soaping and tinting, for cloth to be left white, neutralizes the last trace of acid, softens the cotton, and gives a slightly bluish tone to the white.
Various methods are used for carrying out these processes, and also different chemicals.
Lime preceded by resin soap was formerly used for boiling out. In the madder bleach seven operations, viz.: I, wetting; 2, boiling with lime water; 3, rinsing; 4, treating with acid; 5, rinsing; 6, boiling with soap and alkali; 7, rinsing, were carried out before treatment with bleaching powder; then followed the treatment with acid and the final rinsing.
1 Matthews. Laboratory Manual of Dyeing and Textile Chemistry, p. 45.
Caustic soda is now commonly used for boiling out.
Bleaching powder, or chloride of lime, has the chemical formula CaOCl2. It is a yellowish-white powder, partly soluble, used in a cold bath. Care must be taken that no undissolved particles are allowed in the bath, and that the cloth is entirely immersed, as the combined action of bleaching powder and air weakens the material.
Souring is usually done in a cold bath of very weak sulphuric acid, so that the cotton may not be made tender. Hydrochloric acid seems for several reasons to be better suited than sulphuric acid.
Soaping and tinting are done with dilute lukewarm solution of soap and a small quantity of blue dyestuff, as Prussian blue.
Recently hydrogen peroxide has come into use as a cotton bleach. Although expensive, it does not weaken the fiber and is used for some high grade cottons.
Linen Bleaching may be done in the same general manner as cotton, but the fiber is much weakened by the process. The pectic acid, or gum which glues the cells of the fiber together, is partially dissolved.
The old method of bleaching linen included bleaching both in the yarn and in the cloth. Mrs. Alice Morse Earle, in "Home Life in Colonial Days," says some forty processes of wetting, washing, bleaching with acids rinsing, soaking, bleaching on the grass, etc., were gone through with before the linen was pure white. These processes required many weeks.
The modern linen bleach is usually as follows for half bleach: first, the linen is boiled in sodium carbonate or in soda ash; then treated with bleaching powder, then with dilute sulphuric acid. After each of these operations it is thoroughly washed and the processes repeated; then if three-fourths bleach is required the cloth is spread on the grass and the sun and air do their work. For a full bleach, or pure white, the processes are repeated two or three times.
Linen loses from 25 to 30 per cent in bleaching, and becomes weaker as it becomes whiter. This weakening is more marked with chemical bleaching than with the old grass bleach methods, and this accounts in part for the great difference in wearing quality between the old homespuns and the modern machine-made linens.
The operations preparatory to bleaching wool really begin when it first comes to the mill with the wool scouring, for here it is that the large per cent of dirt, perspiration, and oil is removed from the fiber. Later, when the yarn or the cloth is to be bleached, it is again washed in soap solution, and stretched to prevent tangling. Final bleaching is accomplished by hanging it, thoroughly moist, in brick chambers, into which the fumes of burning sulphur or sulphurous acid are brought. This operation requires from ten to twenty hours. The sulphuric acid formed must be thoroughly washed out of the wool when the bleaching process is completed.
Peroxides are becoming widely used as bleaching agents for wool. Hydrogen peroxide has been used, but the expense is almost prohibitive. Sodium peroxide is much less expensive, but must be handled with care, as when moist it is quite explosive.
Silk bleaching differs somewhat from that of wool in that a large part of the coloring matter is present in the soluble gum which constitutes about 30 percent of the fiber.
This gum may be entirely removed, as in boiled-off silk, where the material is boiled for an hour in a 25 to 35 per cent soap solution, rinsed, then boiled again from one-half hour to three hours in a 10 to 15 per cent soda solution. Souple silk has about one-sixth of the gum removed, and in this case the first soap solution has a strength of only about 3 or 4 per cent and the second bath consists of a weak solution of soda crystals. In each of these classes of silks the boiling is followed by a bleaching process, in which either the sulphur bleach or the hydrogen peroxide bleach is used.
Another class of silk, called "ecru," has but one-twelfth of the gum removed by scouring, and has no bleaching process.
Smoothing by the use of a 4 per cent cream of tartar solution softens the silk after these processes.
Since the cost of silk is greater than the cost of wool, it is more consistent to use the peroxide bleaches for it than for wool.
Silk that has a rustle has been treated with a weak acid, usually acetic or tartaric, which gives it this property known as scroop. The acid is not washed off, but is allowed to dry on the silk.