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The finest thread.

The smoothness, beauty, and elegance of silk have fascinated people since the remote past. In the West, it was not known exactly what silk, a fabric that traders brought from the markets of Asia, was made from. Pliny the Elder, in his Naturalis Historiae (77 A.D.) said that silk was the fine hairs taken from the leaves of some exotic tree, and this is one of the least fanciful explanations suggested. The Egyptians, to weave the silk as they liked it, began by unpicking the fabric that arrived from the East. Processed woven silk reached the Mediterranean along the Silk Road, while the West remained in ignorance of how it was made, as the silk-growers wanted.

Silk has been produced in China for at least 4,500 years. The Empress Leizu (2500 B.C.) is said to have observed a caterpillar spinning its silken cocoon in the palace gardens. The thread could not be recovered, but one day, Leizu plunged the cocoon in hot water and showed that the fiber could then be easily unwound. She was encouraged to find that the fiber was thin, very strong, and long. And, so the legend goes, this is how silk production started. The Chinese kept the secret until the late sixth century A.D., when two Nestorian monks managed to smuggle, at the risk of their lives, a few eggs of the wonderful silkworm to the West in hollow canes.

Silk thread, produced by the silk glands of the caterpillar of the silkworm (Bombyx mori), is a protein fiber consisting of two filaments of fibroin stuck together by a gummy substance, sericin. The fiber is only a few thousandths of a millimeter thick, but is 1,969-3,937 ft (600-1,200 m) long. It is so light that a kilometer of silk thread only weighs 0.5 g. Its resistance to traction is as great as that of steel, about 45 kg/[mm.sup.2]!

When the fibers are unwound from the cocoons ("reeling"), between three and eight strands are joined together, giving the basic raw silk. Several fibers can be twisted together to give a thicker and more resistant thread. The sericin is removed by boiling the raw silk with soap, leaving it white, soft, and lustrous. Silk thread can be spun and dyed, but is also used without any further treatment, for example as dental floss, for surgical stitching, and as fishing line.

But first the silkworms have to be cultivated. The process begins with the collection of the best and healthiest eggs, which are stored over the winter in suitable conditions of temperature and humidity. When good weather arrives, the eggs are incubated in batches, so they hatch in synchronized batches from spring to autumn. For a month, the caterpillars stuff themselves with the leaves of the white mulberry (Morus alba), and then they spin their cocoons and become chrysalises. The cocoons are then plunged into hot water to kill the chrysalis and the silk is reeled and made into thread. A few chrysalises are left to hatch to provide eggs for the next season.

All these techniques were initially developed in China, but later flourished in Japan, in the Islamic countries, and in the general Mediter-ranean area. In the Middle Ages, Lyon and Valencia were leading silk producers. The world's leading producer is China, followed by Japan (which dominates silk processing and the silk trade) and Korea. World production of silk is about 77,000 short tons (70,000 t) a year, only about 0.2% of the world's total production of natural fibers. Even in the age of synthetic fibers, silk is still a luxury available to only a few.

In addition to true silk, and the synthetic fibers imitating it (artificial silk), there are other types of silk (wild silks), which have a coarser texture. Other non-cultivated moths produce wild silk. Their cocoons or gregarious nests are simply collected from their wild food plants. Examples of wild silk include the silk from the Greek island of Kos, which in antiquity was obtained from the lasiocampid moth Pachypasa otus, and several African forms of silk obtained from species of Anaphe (western and central Africa) and Borocera (Madagascar).

India produces the most different types of wild silk. In India, as in China, coarse fabrics known as fagara and eri have been manufactured for more than a century. Fagara is made with the silk of the saturniid moth Attacus atlas, one of the world's largest moths, while eri is made from the silk of the bombycid moths Samia ricini and S. cynthia. Eri silk has been produced irregularly in Korea, Japan, Egypt, Cuba, Uruguay, France, and recently in Java, while attempts are being made to produce fagara silk on the island of Java. Muga, one of the most highly valued wild silks in India, is a beautiful golden color. It is produced in the Assam region, by the saturniid moth Antheraea assamensis.

The most highly valued wild silks ("tussah") are obtained from saturniid moths ("giant silkworm moths"), whose caterpillars feed in temperate rainforests on members of the beech family (Fagaceae), such as the oak Quercus semecarpifolia. One famous wild silk, known as shantung, is produced by Antheraea pernyi and has been used by the peoples of northeast China for more than 1,000 years. It is now available in shops in America and Europe, in coats, scarfs, ties, etc. Tus-sah silk production in India from A. paphia, is also increasingly available, but this has led to protests because abusive collection of the cocoons may cause the species' extinction. Japanese wild silk, tensan, is made from A. yamamai, and is produced on a small scale in Taiwan, Korea, southeastern China, and as well as Japan, where it fetches a high price.
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Publication:Encyclopedia of the Biosphere
Geographic Code:1USA
Date:Jun 1, 2000
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