Silk is an exquisitely lustrous natural fiber of exceptional strength. The origins of silk date back to Ancient China. Though the silk-making process was kept a secret by the Chinese, the secret finally leaked into the neighboring Japan in 300 A.D. and India around 400 A.D. China is the main producer of silk in the world, followed by India, Japan, Brazil and Korea. The first country to apply scientific techniques to raising silkworms was Japan, which produces some of the world’s finest silk fabrics.
Chinese ladies making silk; Source
Weaver at work at Kancheepuram, South Imdia; Source
India produces five kinds of silk – Mulberry, Eri, Muga, tropical Tassar and temperate Tassar. Mulberry silk is produced mainly in the states of Karnataka, Andhra Pradesh, Tamil Nadu, Jammu & Kashmir and West Bengal, while the non-mulberry silks are produced in Jharkhand, Chattisgarh, Orissa and north-eastern states.
Eri silkworms are multivoltine (producing several broods in one year), and reared on castor oil plant leaves to produce a brick-red Eri silk.
Tussah or Tussar silk is produced by a wild, undomesticated species of silkworm, the Antheraea mylitta, and its silk filament is heavier and coarser than that of the cultivated silkworm. These multivoltine silkworms feed on oak, Terminalia and several other host plants. The fibers are less lustrous than the mulberry silk and is usually dark orange/copper colour.
Muga silkworms are found only in the state of Assam and feed on locally available ‘som’ and ‘soalu’ leaves producing an unusual lustrous golden-yellow, attractive and strong silk.
Mulberry Silk is produced by silk worms fed exclusively on mulberry leaves. Raw mulberry silk can be either white or golden yellow in colour.
Producing Mulberry Silks
Silkworm rearing is considered to be an agro-based cottage industry. Silkworms are reared for the production of “cocoons” which is the raw material for producing silk.
Life cycle of silk worms; Source
From caterpillar to cocoons
The mulberry silk moths, Bombyx mori, start their life as tiny eggs. The fertilized eggs hatch into larvae that appear like little ants.
The larvae feed on the leaves of Mulberry tree, Morus alba, for about 20-23 days.
Silkworms molt, i.e., they shed their skin five times while growing up, and the intervening period between two sloughs is called an instar. When the larva is fully mature, it stops feeding, looks for suitable location to form a cocoon.
When they are ready to spin their cocoons, the worms are transferred to a fresh set of bamboo trays.
Silkworms produce a clear, viscous, proteinaceous fluid that is forced through openings called spinnerets on the mouth part of the larva. As the fluid comes into contact with the air, it hardens into thread which the worm uses to spin the cocoon a hammock of silk in which the cocoon will be suspended.
The larva or caterpillar moves its head and ejects a continuous thread 800–1,200 yards long in a eight figure form for about three days. The following video shows this process.
After several days in the cocoon, it is harvest time. To unravel the thread as one single strand, the cocoon must be harvested before the pupa matures and emerges as a moth. If the moth naturally emerges from the cocoon, it cuts the long undivided strands. To prevent this, the pupae are killed by boiling, steaming or baking. If water or steam is used, the cocoon must be unraveled immediately to prevent its putrefaction. If baked and dried, the cocoons can be stored for later use. To unravel, the end of the silk thread is located and the entire cocoon unwound, either mechanically or by hand. Each cocoon produces 600 to 900 meters of thread. Five to eight threads are reeled together to make a single thread for textile production.
A few of the cocoons are selected for reproduction of eggs, the rest are placed into pots of boiling water where they collapse and start to unravel. Workers remove the boiled cocoons and find the end of each strand of silk and thread it. A single thread of silk can measure up to 4,000 feet in length. Depending on the desired thickness of the silk fibers, 3–100 strands are picked from the boiled cocoons, twisted and wound together on large wooden wheels. The silk threads are boiled in huge copper vats during the dyeing process for varying lengths of time, depending on the color desired.
Take a look at the Vietnamese silk making process pictured in the following excellent video:
From cocoons to a new moth
From ten to twelve days after the completion of the cocoon, the silkworm sheds its skin one last time and becomes a elliptically shaped pupa. The pupa shell splits open and the silkworm moth emerges from it.
It secretes a special saliva which makes a hole in the cocoon, so the moth comes into the outside world. The moths cannot fly since their bodies are large and their wings are small. Within minutes of emerging from the cocoon, the males and the females mate. During mating, which lasts one day, the moths consume a lot of body energy. The female moths lay 200-300 pale-yellow eggs over a couple of days and usually die within 2 weeks. The eggs can be preserved and used later. When warmed under the right conditions, they become fertile. Fertile eggs then turn to brown or purple in a week or so.
The silk obtained out of the reeling process is referred to as raw silk. The raw silk is to be twisted before they are fed into looms.
Humane methods of silk reeling
Unlike the conventional method where the pupae are killed before reeling yarn from the cocoons, the adult moths are allowed to emerge alive from the cocoons and then the silk yarn is spun from the open ended or pierced cocoons found in the wild or from those used in breeding cycles.