3. Modern Sericulture
Countries Producing Raw Silk
Silk raising, or sericulture, has been a leading industry in Japan and China for a great many years, while the Near East and such countries as France and Italy have also played a part in the industry—but to a lesser degree. A great deal of this work, particularly in Japan and China, has been carried on as a home industry, but with the growth of modern business methods more and more of the silk raising has come into the hands of companies operating on a highly scientific basis.
Cocoonery
Japanese Supervision
After many years of experience and experimentation the breeding and care of the silk worm has been put on a very technical and closely regulated schedule that minimizes the chances of loss by waste or the spread of disease. In Japan the industry has been encouraged and fostered by the Government; a special division of the administration is devoted to its attention, and numerous organizations and associations conduct experimental stations for research and study. In fact since about the middle of the 19th century everything possible has been done to foster this highly profitable branch of the country’s industry.
Modern Methods of Cultivation
Hatching the Eggs
The merest outline of the modern methods of cultivation will show how highly they have been developed. To begin with, the eggs are placed on sheets of paper or muslin directly after they are laid. These sheets are hung for a few days in a damp atmosphere, and then placed in cold storage for about six months, the period of cold being advantageous for later hatching, which is done by heat.
Growth of the Worms
After hatching, the worm sheds its skin four times. The periods between the “molts,” or ages, vary with different silk worms, but the total process takes about a month. Worms of different ages are always kept separate, being held on large cloth trays which are carried in tiers along the walls of the rearing rooms. The cocooneries where best results are obtained are quiet, spacious, well-ventilated rooms where an even temperature can be maintained. Each worm is kept absolutely clean and has plenty of room, as overcrowding brings disease. As a precaution mild fumigation is resorted to from time to time. Heavy odors or smoke of any sort are not allowed, as these are disturbing to the worms.
Feeding the Worms
The Mulberry Tree
Nourishment is, of course, a very important item for the growing worms, and the best form of food for the Bombyx mori is the leaf of the white mulberry, which must be young, fresh and dry, but never withered. For this purpose mulberry tree raising has become an important by-industry in itself. Three varieties are found, classified according to the time of budding—early, medium and late. The leaves, therefore, can be found in the correct condition for the various stages in the growth of the worm. The late budding trees are cultivated in more abundance, as the worms are larger at that time and consume more leaves. The soil in which the trees grow is important, as it has been found that one which is rich in certain minerals provides leaves that keep the worms in better physical condition. A cold winter followed by a warm spring develops the leaves well, and the condition of the leaves is one of the most important factors in the whole process.
Early Development
Careful selection of the eggs is another matter of prime importance. When the leaves are almost ready, the eggs are brought out of cold storage and subjected to heat for about a month before they hatch out. When the worm hatches it is about the diameter of a hair and less than three-fourths of an inch long. It gnaws a hole through the end of the egg from which it issues. Nourishment at first is taken by sucking the sap of the leaves, which are at this stage chopped into fine pieces; later the leaves are consumed without the necessity of their being chopped up.
Spinning the Cocoon
The development is very rapid, the worms consuming their own weight daily. During this period, as has been stated, they shed their skins four times, and after the fourth molt—about one month after hatching—they have assumed their full size. From then on hunger lessens, restlessness grows, and the lifting of the fore part of the body indicates the desire to climb and spin cocoons. For this purpose brush and twigs are provided in the trays, to which the worms attach themselves and begin the spinning process. The silk thread is expelled in a semi-liquid form from two openings underneath the mouth, hardening as it comes in contact with the air. The first threads issued are coarse and rough, having the necessary strength to serve as supports and guys for the cocoon. Gradually the worm is enclosed in the cocoon after three days of continuous spinning, during one of which it is visible and then slowly disappears, though it can be heard working inside.
Worms Feeding
The worm wastes away as its silk glands are exhausted, and becomes a chrysalis, from which the moth escapes after fifteen to twenty days.
A Nest of Cocoons
Cocoons for Breeding
Cocoons to be used for silk manufacture must be submitted to heat soon after they are completed, in order to kill the chrysalis and prevent it from forcing its way out, thus breaking the fibres. A certain proportion of the cocoons of each crop, however, is allowed to go through the natural process, for reproductive purposes. There is an active market in these breeding cocoons, particularly in Japan and China, and a large quantity are exported annually to Europe and the Near East.
Construction of the Silk Thread
The female cocoon is oval and the male cocoon is peanut shaped. The silk itself consists of two parts: fibroin (the silk fibre) and sericin (the gum). The thread is made up of two strands of fibre held together by the gum, and the length of thread in a single cocoon varies from three hundred to fourteen hundred yards. The cocoons are white, yellowish, or greenish, but after the boiling process the color of the Bombyx mori silk is pure white.
Japanese Cocoon Market
Tussah and Wild Silk
The preceding description is concerned more particularly with the latter type, from which the largest proportion of the world’s silk supply is derived, but in a general way it applies to the other types also. The most important of these are the Tussah silk of India and China and the Japanese wild silk. The products are coarser and harsher than those of the Bombyx mori, and the natural color is brown.
A Japanese Filature