Silk is a natural protein fiber, some forms of which can be woven into textiles. The protein fiber of silk is composed mainly of fibroin and produced by certain insect larvae to form cocoons. 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 colors.
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 labor-intensive carding.
Commercial silks originate from reared silkworm pupae which are bred to produce a white colored 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.
Silk has a smooth, soft texture that is not slippery, unlike many synthetic fibers.
Silk is one of the strongest natural fibers but loses up to 20% of its strength when wet. It has a good moisture regain of 11%.
Its elasticity is moderate to poor: if elongated even a small amount, it remains stretched. It can be weakened if exposed to too much sunlight. It may also be attacked by insects, especially if left dirty.
One example of the durable nature of silk over other fabrics is demonstrated by the recovery in 1840 of silk garments from a wreck of 1782: 'The most durable article found has been silk; for besides pieces of cloaks and lace, a pair of black satin breeches, and a large satin waistcoat with flaps, were got up, of which the silk was perfect, but the lining entirely gone ... from the thread giving way ... No articles of dress of woollen cloth have yet been found.
Silk is a poor conductor of electricity and thus susceptible to static cling.
Unwashed silk chiffon may shrink up to 8% due to a relaxation of the fiber macrostructure, so silk should either be washed prior to garment construction, or dry cleaned. Dry cleaning may still shrink the chiffon up to 4%. Occasionally, this shrinkage can be reversed by a gentle steaming with a press cloth. There is almost no gradual shrinkage nor shrinkage due to molecular-level deformation.
Natural and synthetic silk is known to manifest piezoelectric properties in proteins, probably due to its molecular structure.