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Silk

Silk, a fine, lustrous fiber produced by silkworms and other insect larvae, generally to form their cocoons. The breeding of silkworms and the making of silk began in prehistoric times. China is generally credited with the first silk culture, although some claims have been made that it originated in India.

Development of the Silk Industry
Traditional Chinese accounts ascribe the cultivation of silkworms and the weaving of silk to the wife of the legendary emperor Huang-ti, who is supposed to have lived in the 3d millennium B.C. In any event, silk culture flourished by the time of the Shang dynasty (c. 1523:1027 B.C.).

Aristotle and Pliny described the silkworm, but for centuries after silk fabrics were known in the West, the prevalent opinion was that silk was either a fleece that grew on a tree or the fiber from the inner bark of some tree or shrub. Some persons, deceived by the glossy, silky fibers of the seed vessels of Asclepias and the silk-cotton tree, believed that it was the product of one of these. A few came near the truth with the conjecture that silk was spun by a spider or beetle. So carefully did the East keep its secret that it was not exposed until 552 A.D., when two Nestorian monks, who had lived in China, concealed a small quantity of silkworm eggs and brought them to Constantinople.

From Constantinople, sericulture spread through the Balkan Peninsula, and Byzantine silks soon became prized in Europe for ecclesiastical vestments. Although the looms of Constantinople declined in the 8th and 9th centuries, those of Thebes and other Greek and Syrian cities increased their production and improved their methods of manufacture. The Greeks maintained their supremacy until 1204, when the Latin Empire of Constantinople was established. Meanwhile, the Arabs had introduced the industry into northern Africa, Spain, Portugal, and Sicily. Spanish and Sicilian production was substantial by the 11th century. When the Normans conquered Sicily, beginning in about 1060, they encouraged the local silk industry. The second Norman ruler of Sicily, Roger II (reigned 1105:1154), invaded Greece in 1147 and took captive a number of silk weavers. He established them in Palermo and Calabria, where they were induced to teach the Greek methods of silk culture.

In the reign of Louis XI (1461:1483), silk was manufactured in Tours and later in Lyon, Montpellier, and Paris. Cocoons were first raised successfully in France in the reign of Francis I (1515:1547). The French silk industry, encouraged by successive rulers, expanded until 1685, when the revocation of the Edict of Nantes drove into exile about 400,000 Huguenots, many of whom were silk workers. Not for many years did the industry regain the ground it had lost, and then it was almost destroyed again during the French Revolution. Those Huguenots who settled in England tried to manufacture silk there, but without much success until 1718, when a new method of throwing was introduced. Thereafter, the English industry expanded rapidly, and English silks replaced French in the European market. The French industry revived with the aid of protective tariffs in the Napoleonic and Restoration periods. Conversely, the industry in England was almost ruined after 1860, when that country negotiated a commercial treaty that admitted French silks duty free. Silk manufacture also flourished in Switzerland and Germany in the 19th century. The Belgian and Dutch industries, dating from the late Middle Ages, remained productive during the 19th century. The Russian industry received a great impetus in the 1890s with the enactment of a protective tariff.

With the opening of the treaty ports in China in the mid-19th century, large quantities of raw silk began to reach world markets from Shanghai and Canton. China exported only its surplus, the great bulk of its raw silk being woven and sold in the home market. The same has been true of the minor producing areas in Asia. There has been some export of Asian manufactured silks, particularly those of China, India, and Japan. In 1857 Japan entered the raw silk export field and rapidly assumed leadership.

The first silk mill in the United States was established in 1810 in Mansfield, Conn. However, the American industry did not prosper until protective tariffs on manufactured silk were introduced in the latter half of the 19th century.

The development of human-made fibers in the 20th century greatly reduced the demand for silk fabrics and hosiery. Japan is by far the principal consumer of raw silk, with the United States, Italy, India, France, South Korea, Switzerland, Germany, and Britain consuming relatively small amounts by comparison. The leading silk producers are Japan, China, India, South Korea, and states of the former Soviet Union.

Types of Silkworms
Any caterpillar that spins a fibrous cocoon that can be used to make cloth is known as a silkworm. By far the most important and most widely disseminated species of silkworm is Bombyx mori, which belongs to the family Bombycidae, a native of northern China or Bengal.

Among the less important species of silkworms, the following are probably best known. The Japanese oak silk moth (Antheraea yamamai,) produces green-tinted silk, which is used in Japan and China for embroidery. A close relative, Antheraea peryn, i, is a native of northern China, where its large grayish brown cocoons are used for threads and various fabrics. Both wild and cultivated cocoons of Antheraea assamensis, and Antheraea paphia, are also used extensively in Asia, as is the silk of the ailanthus silkworm (Philosamia cynthia,). Since the thread of the ailanthus silkworm cannot be reeled, the silk is obtained by carding. The moth has been introduced into Europe, Africa, and the eastern United States. The Syrian silkworm (Pachypasa otus,) was the silkworm of Europe until long after the introduction of the Chinese silkworm and was cultivated extensively until 1875. The amount of silk produced by each cocoon is relatively small. Silk has been made from several common American silkworms, but quality and cost compare unfavorably with silk from the Chinese species. The cecropia moth (Samia cecropia), the largest North American moth, occurs over the whole of the United States and southern Canada. It produces a large cocoon of very strong silk. Glover's silkworm (Samia gloveri,) is similar but does not occur in the East. The promethea moth (Callosamia promethea,), common in the eastern half of the United States, has a cocoon much like that of Philosamia cynthia,, and the quality of the silk is similar. The cocoons of the polyphemus moth (Telea polyphemus,) and the io moth (Automeris io,) are somewhat like those of the Chinese silkworm, but the silken strands are copiously glued together.

Silkworm Cultivation
In the cultivation of the silkworm the first requisite is an ample supply of foliage. This is usually obtained by growing mulberry trees far enough apart to give each tree ample space to develop its branches in all directions. The distance between trees varies from 15 to 40 feet (4.6:12 meters), depending on the variety and method of training.

During the winter, the silkworm eggs are kept at a temperature of less than 50 F (10 C) in a dry, circulating atmosphere. In the spring, they are placed in a room or incubator in which the temperature is raised gradually to about 73 F (23 C). In about ten days the larvae emerge. They are then covered with sheets of perforated paper sprinkled with chopped mulberry leaves, which should be renewed about 9 times during the first 24 hours. Paper with larger perforation is necessary as the worms grow. When ready to spin, the worms should be supplied with brush, straw, or other material on which they can form their cocoons. At this time the temperature should be maintained at about 75 F (24 C) and the humidity close to 65%. Scrupulous cleanliness and abundant fresh air are essential at all times. The cocoons are sorted by quality, size, and color. After grading they are heated to kill the pupas, which, if left alive, would break the thread many times as they emerged as adult silkworm moths.

Manufacture of Silk
The filament that the silkworm spins into a cocoon is so fine that several filaments must be reeled together to produce a thread thick enough to handle. This process is undertaken in large factories called filatures. There the cocoons are placed in basins of water near the boiling temperature. Filaments from as many cocoons as are needed to make the desired size of raw silk thread are then combined, twisted, and reeled into skeins. When the filament from one cocoon has become exhausted, it is replaced by another cocoon. The reeler adjusts the size of the thread by controlling the number of fresh and partially exhausted cocoons that are forming the thread. A single cocoon of good quality may furnish from 400 to 800 yards (365:730 meters) of reelable filament. Asian silks reeled by different filatures and the different qualities of silk from the same filature are designated by colorful labels called chops.

Silk that has been reeled is called raw silk. The damaged cocoons and the parts of each cocoon that cannot be reeled are called waste silk. Waste silk is combed and spun into spun silk or schappe yarns. Japanese raw silk is prepared for shipment in books containing from 25 to 30 tightly rolled skeins, which are then packed in bales weighing 125 to 140 pounds (55:65 kg). Chinese silks are packed and shipped in a similar manner. Italian silks are packed in bales weighing 200 pounds (90 kg) or more.

The silk used in fabric making may be divided into three general classifications: (1) silk threads as they come from the filature, where several cocoon threads have been combined; (2) silk known as organzine, in which the original threads have been twisted and several of these twisted threads have been combined; and (3) silk known as tram, in which threads coming from the reeling machine have been very lightly twisted together.

When silk threads are woven as they come from the filature, without having been dyed, their natural gum gives them enough body to make them suitable for weaving. When they are dyed before weaving, the gum is removed in the dyeing process. To give these threads strength and cohesion, they must be twisted before dyeing. This process is called throwing.
   
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