Silk is a natural protein fiber produced by certain caterpillars in order to encase themselves in the form of cocoons. The making of silk is different from that of other natural fibers. There are many steps involved in silk manufacturing
- Cultivation of Cocoons ( Sericulture)
- Filature Operations
- Manufacturing of Silk Yarns
- Finishing of Silk Fabrics
Sericulture is the cultivation of cocoons for their filaments. The best raw silk is obtained from the species of moth called Bombyx mori. Breeding of silkworm occurs once in a year but under scientific conditions, they may be hatched three times a year. The female moth lays around 350 to 400 eggs and the moths die soon after. As they are subject to hereditary infection, the eggs from infected moths are destroyed which results into production of fine silk. Larvae of about 3mm are hatched from the eggs. For about 20 to 30 days, they are carefully nurtured and are fed five times a day on chopped mulberry leaves. In the meantime, the larvae change their skin for four times and are formed into caterpillar of about 9 cm long. Now they are ready to spin cocoon for which racks, clusters of twigs or straw are provided.
The caterpillar have small openings under their jaws called spinnerets through which they secret a protein like substance. This substance solidifies when it comes in contact with air and the filament thus formed is spun around the silkworm in the figure resembling the digit 8. In three days the cocoon gets completed which is about a peanut shell's size. The filament is held together by sericin or silk gum. The life of the worm is ended by the process of 'stoving' or 'stifling' in which the cocoons are heated. Some of the cocoons are preserved so that the pupa or chrysalis inside them develop into moths for further breeding.
The raw silk is unwound from cocoons and are collected into skeins in the factories known as Filature. Here the cocoons are sorted based on their color, size, shape and texture. Then they are immersed in hot and cold water to soften the sericin so that filament could be unwound into one continuous thread. The whole of the seracin is not removed at this stage as it protects the delicate filament in further operations. 'Reeling' is the process of unwinding the filament from the cocoon. Three to ten filaments are together reeled for producing the desired diameter of raw silk thread.
Manufacturing of Silk Yarns
The reeled silk is formed into silk yarn
or silk thread through the process called 'Throwing'. It corresponds the spinning process of other natural fibers. The raw silk skeins are sorted according to their color, size, length or quantity and washed in warm water with soap or oil for softening the seracin. After drying the skeins, they are placed on reels from where the silk is wound on bobbins. During winding, the silk strands are given desired amount of twist. The strands may be doubled and then given twist in similar or opposite directions. To get equal diameter throughout the length, the yarn is run through rollers. Many kinds of silk yarns are manufactured by giving different amount of twists. Still remaining sericin is removed from silk yarn by the process of 'Degumming' in which the yarn is washed with soap and water for bringing out its natural shine and the soft feel.
Kinds of Silk Yarns
Three to eight silk filaments are twisted together in only one direction to make 'Singles'.
A slight twist is given to two to four untwisted singles. Trams are used exclusively as filling yarns.
Individual raw silk filaments are twisted together, one in S direction and the other in opposite Z direction. They are then twisted around each other in S direction.
Two or more singles having twist in Z direction are combined by twisting them around each other in the opposite S direction. Organzine is mainly used for warp yarns.
Finishing of Silk Fabrics
Many finishing processes are applied to different silk fabrics
in order to improve their appearance, durability and feel. Calendering and Cireing is done to enhance luster, singeing is done to make them smooth, and steaming is done for raising pile weaves. Pressing and lustering removes wrinkles from the finished fabric. It is done with heated rollers and then soaking in dilute acid to bring luster.
One finish that is unique to silk fabric is 'Weighting'. The weight of silk is lost during the process of demugging. The manufacturer purchases silk by weight and to make up his loss, he does weighting of silk fabric with metallic substances such as stannic chloride, sodium phosphate, iron salt, logwood etc. Weighting is done during the dyeing process. Weighted silk is less compactly woven when compared to the unweighted silk and lesser silk is used in the fabric construction. Apart from lowering the cost of silk, weighting gives it crispness, luster and a firm feel.