Silk

Silk is a natural protein fibre, some forms of which can be woven into textiles. The best-known type of silk is obtained from the cocoons of the larvae of the mulberry silkworm Bombyx mori reared in captivity sericulture. The shimmering appearance of silk is due to the triangular prism-like structure of the silk fibre, which allows silk cloth to refract incoming light at different angles, thus producing different colours.

Silks are produced by several other insects, but generally only the silk of moth caterpillars has been used for textile manufacturing. There has been some research into other silks, which differ at the molecular level. Silks are mainly produced by the larvae of insects undergoing complete metamorphosis, but also by some adult insects such as web spinners. Silk production is especially common in the Hymenoptera bees, wasps, and ants, and is sometimes used in nest construction. Other types of arthropod produce silk, most notably various arachnids such as spiders.

Wild silk

A variety of wild silks, produced by caterpillars other than the mulberry silkworm, have been known and used in China, South Asia, and Europe since ancient times. However, the scale of production was always far smaller than that of cultivated silks. There are several reasons for this: firstly, they differ from the domesticated varieties in colour and texture and are therefore less uniform, secondly, cocoons gathered in the wild have usually had the pupa emerge from them before being discovered so the silk thread that makes up the cocoon has been torn into shorter lengths, and thirdly, many wild cocoons are covered in a mineral layer that stymies attempts to reel from them long strands of silk. Thus previously the only way to obtain silk suitable for spinning into textiles in areas where commercial silks are not cultivated is by tedious and labour intensive carding.

Commercial silks originate from reared silkworm pupae which are bred to produce a white coloured silk thread with no mineral on the surface. The pupae are killed by either dipping them in boiling water before the adult moths emerge or by piercing them with a needle. These factors all contribute to the ability of the whole cocoon to be unravelled as one continuous thread, permitting a much stronger cloth to be woven from the silk. Wild silks also tend to be more difficult to dye than silk from the cultivated silkworm. A technique known as demineralizing allows the mineral layer around the cocoon to be removed, leaving only variability in colour as a barrier from creating a commercial silk industry based on wild silks in parts of the world where wild silk moths thrive, such as Africa and South America.

Uses

Silk's absorbency makes it comfortable to wear in warm weather and while active. Its low conductivity keeps warm air close to the skin during cold weather. It is often used for clothing such as shirts, ties, blouses, formal dresses, high fashion clothes, lingerie, pyjamas, robes, dress suits, sun dresses and Eastern folk costumes. Silk's attractive lustre and drape makes it suitable for many furnishing applications. It is used for upholstery, wall coverings, window treatments if blended with another fibre, rugs, bedding and wall hangings. While on the decline now, due to artificial fibres, silk has had many industrial and commercial uses, such as in parachutes, bicycle tires, comforter filling and artillery gunpowder bags.

A special manufacturing process removes the outer irritant sericin coating of the silk, which makes it suitable as non-absorb-able surgical sutures. This process has also recently led to the introduction of specialist silk underclothing for children and adults with eczema where it can significantly reduce itch. New uses and manufacturing techniques have been found for silk for making everything from disposable cups to drug delivery systems and holograms. To produce 1 kg of silk, 104 kg of mulberry leaves must be eaten by 3000 silkworms. It takes about 5000 silkworms to make a pure silk kimono. The construction of silk is called sericulture. The major silk producers are China 54% and India 14%.

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