As an animal fiber, silk has some characteristics resembling wool, but in its physical structure it is very different. While wool consists of layers of cells of different kinds, silk has no cell structure. The length of the silk fiber makes it unlike any other. A carefully reeled fiber from a perfect cocoon may be from three to four thousand feet long. In evenness, strength, and fineness the fiber will differ slightly in different parts of the cocoon, the best part being in the center. The first silk reeled from the cocoon, together with the innermost layers, is less strong and even, and somewhat finer.
The pure silk fiber is transparent and lustrous, but as it comes from the cocoon it is covered with a coating of gum, which conceals the luster shown by the fiber when the gum has been removed. Pure silk is very strong, being sometimes compared to an iron wire of the same diameter. It is also very elastic.
Scroop is the term applied to the rustle peculiar to silk, but is not a quality inherent in the fiber. Scroop is produced by treatment with dilute acid, acetic or tartaric, which is allowed to dry on the surface of the silk.
The hygroscopic power of silk is high; 30 per cent of water may be taken up and a great amount of dyestuff and metallic salts may be absorbed without in any degree changing the appearance of the silk. This property is made use of in adulteration by the process of weighting, when a large amount of foreign matter, mineral salts and dyes, is added to the silk. In the case of heavy fringes as much as 200 to 300 per cent of the weight of the fiber is sometimes added.
Fig. 24. Raw Silk Fiber, Gum Adhering.
Fig. 25. Silk Fiber, Gum Boiled off.
The microscopic appearance of cultivated or mulberry silk shows two fibers, each smooth, round, and structureless, united by the gum secreted at the same time that the fiber is formed. There are no markings on silk from which the gum has been removed, but threads from woven cloth usually show some grains of dyestuffs or weighting material.
Wild or tussur silks are very different from cultivated silks in microscopic appearance. Instead of being round, regular fibers, they are flat and somewhat irregular. While true silk has rarely any longitudinal stri-ations, the wild silks are quite definitely marked, the fibers sometimes having a twisted appearance, while the ends often appear frayed, as if the fiber were composed of small fibrils which had begun to separate.
Fig. 26. Wild Silk Fiber.
Chemically, silk is composed of fibroin, a protein substance differing from wool in containing no sulphur. Its reactions are similar to those of wool. Alkalies do not act as readily on silk as on wool, but when concentrated dissolve the fiber, while dilute alkalies weaken it. Concentrated mineral acids rapidly dissolve the fiber, but dilute acids have little injurious effect. Silk readily absorbs these acids from dilute solutions. Acetic and other dilute organic acids produce the scroop, or rustle, already spoken of.
Solutions of common salt have a deleterious effect on silk, greatly weakening the fiber. In some climates, on islands in the sea, for example, silk is so weakened by the salt in the atmosphere that it drops to pieces. This action is increased when the silks have been heavily weighted. Other salts produce this same result to a certain extent, as is shown by the rotting of heavily weighted silk when laid away for some time and the weakening of silk by perspiration.
Silk has, perhaps, a greater affinity for coloring matters in general than any other fiber, and may be dyed a great number of tones and hues.
The silk industry may be divided into two distinct parts, the culture of the silkworm and the manufacture of the fiber into thread and cloth.
Only one of the two types of silk mentioned, the Bombyx Mori, or mulberry silk, has been extensively cultivated. The wild silk cocoons are gathered from trees where the worms have spun them, and little attention is paid to the worm during its growth. The cultivation of the mulberry silkworm is, however, a delicate operation, and the value of the silk depends largely on the care and food of the worms.
The silk culture is usually carried on in rooms or sheds devoted to its use, and requires much hand labor. In July the moth lays several hundred eggs about the size of mustard seeds, which, by virtue of a gummy substance secreted with them, stick to a cloth that has been provided for this purpose. This cloth is hung up to dry, then put away in the dark and cold until February or March, when it is brought out and hung in a warm place. In about thirty days the tiny worm, one-eighth of an inch in length, hatches; it immediately begins to eat the mulberry leaves which have been provided for it. These worms are kept on trays in a room of the right temperature, and are supplied with fresh leaves two or three times a day. The silkworm is a voracious eater and grows very rapidly. After about five or six days it molts; this process is repeated four times during the thirty to thirty-five days from the hatching of the worm to the spinning of the cocoon. At the end of this time the worm stops eating. Its length is now about three inches, but it begins to shrink and becomes almost transparent before spinning its cocoon.
Fig. 27. Mulberry Leaf and Silkworm Eggs. The dark eggs are unhatched.
From two glands, the openings of which are on either side of the mouth, a viscous substance is secreted, which hardens upon contact with the air. At the same time two other glands secrete a gummy substance which causes the two fibers to adhere. The single fiber is called brin; the double fiber, bave. As the worm spins, its head is thrown in such a way that the thread is spun in the form of a series of figure eights, this regular arrangement aiding the unwinding later on. Before spinning the cocoon proper, however, the worm fastens a few threads about itself to form a connection with the nearest twig or leaf; then the compact cocoon is spun about the worm, completely concealing it. The spinning, howrever, continues until the worm is well protected by its little case, when it goes into the dormant or pupa stage. If undisturbed, after a period of rest the moth emerges from the cocoon fully developed, lives long enough to mate and lay eggs, then dies. Thus the cycle of the life of the silkworm is completed - egg, worm, pupa, and moth.
The exit of the moth from the cocoon injures the value of the silk for the market, since the moth moistens the threads at one end of the cocoon and pushes its way out, entangling them in such a way as to interfere with the successful reeling of the silk. For this reason the cocoons are heated in a warm oven or steamed to kill the pupa within, enough cocoons being saved to provide moths for the next season's eggs.
During the cultivation the silkgrower must exercise great care that the worms are kept clean, that they are not crowded, that sufficient food is provided, and the rooms kept at the proper temperature. Diseases due to irregular feeding, to germs and fungi, may reduce the production of silk enormously.
The very rapid growth of sericulture in Southern Europe during the first half of the nineteenth century led to carelessness and overcrowding, which resulted in a serious epidemic among the silkworms. In 1865 Louis Pasteur, the famous French bacteriologist, began the study of this epidemic and discovered that the disease was caused by a germ, and also that by microscopic examination and rejection of the eggs which carried the infection the disease might be avoided. This discovery was the salvation of the silk industry, not only in France and Southern Europe, but in other silk-growing countries as well. By careful selection and cultivation of the mulberry plants, breeding, microscopic inspection of the eggs, and improved methods of handling the worm the industry has increased rapidly in Southern Europe in the last half century.