- 1 What is Silk Fabric: Properties, How its Made and Where?
Environmental biology is one of the critical Biology Topics that involves understanding how humans impact the environment and how to address environmental issues.
What is Silk Fabric: Properties, How its Made and Where?
Silk is a fine, strong, soft and shining fibre produced by silkworms in making their cocoons. Silk is called ‘resham’ in Hindi. Silk is a natural fibre which is obtained from an insect (called silk moth). So, silk is an animal fibre. Silk fibre is made of a proteiii. Silk is the strongest natural fibre. The soft looking silk yarn is as strong as a comparable thread of steel ! Silk fibres are converted into silk yarn which is used for making silk cloth. This silk cloth is then used for making saris and other dresses.
The rearing of silkworms for obtaining silk is called sericulture. ‘Sericulture means ‘silk farming’. Sericulture is a very old occupation in India. India produces a lot of silk on commercial scale. Before we go further and describe the process of obtaining silk, it is necessary to know the life history of silk moth. This is described below.
Life History of Silk Moth
Silk moth is a kind of insect (which resembles a butterfly). Life history of silk moth is interesting because silk moth is not formed as such from the eggs directly. The silk moth passes through a worm-like stage called ‘larva or ‘caterpillar’ (which is also called silkworm) and an encased form called ‘pupa during its. development between the hatching of egg and formation of adult silk moth. This can be written as :
Egg → Larva (or Caterpillar) (Silkworm) → Pupa → Silk moth
The larva (or caterpillar) and pupa stages in the life history of a silk moth are totally different in appearance from the adult silk moth. It is the larva (or caterpillar) of a silk moth which produces silk and not the adult silk moth. The larva (or caterpillar) of a silk moth which produces silk is called silkworm. We can describe the life history of silk moth as follows.
(i) The female silk moth lays eggs on the leaves of a tree (such as mulberry tree) [see Figure (a)].
(ii) The eggs hatch to form worm-like larvae [see Figure (b)]. The larvae of silk moth are called caterpillars’ or ‘silkworms’. The silkworms feed on the leaves of mulberry tree and grow bigger in size. Silk is formed in liquid form in the two glands in the silkworms’s head.
(iii) When the silkworm (or caterpillar) is ready to enter the next stage of its development called pupa, it first weaves a net to hold itself. Then it swings its head from side to side in the form of figure of ‘eight’ (8). During these movements of head, the silkworm secretes silk in liquid form through the tiny opening in its head which solidifies on exposure to air and becomes a silk fibre (or silk thread). Soon the silkworm (or caterpillar) covers itself completely by silk fibres.
The silky covering spun by the silkworm (or caterpillar) of silk moth is called cocoon. The cocoon is made by silkworm to protect its development as ‘pupa. Pupa is a stage in the life history of silk moth when the caterpillar (or silkworm) becomes encased’ in a hard shell of silk fibres called cocoon [see Figure (c)]. The silkworm continues to develop in the form of pupa inside the cocoon to form the silk moth.
(iv) When the pupa (encased in cocoon) develops fully to form an adult silk moth, then the cocoon splits up and a beautiful silk moth comes out [see Figure (d)]. The adult female silk moth then lays more eggs. In this way, the life history of silk moth is completed.
Please note that in order to produce silk, the silkworm developing inside the cocoon (as pupa) is not allowed to mature into an adult silk moth. So, as soon as the cocoon is formed it is used to obtain silk fibres and the developing silkworm (as pupa) gets killed. This is because if the silkworm (as pupa) is allowed to mature into a silk moth, then the fully formed silk moth secretes a liquid to dissolve a part of silk of the cocoon to break it so as to come out of it and fly away.
This breaking of cocoon causes damage to its silk threads and hence lowers the quality of silk. This is why the cocoons having developing silkworms inside them are used to obtain silk. Some of the silkworms (as pupae) are, however, allowed to live and mature into silk moths so that they can lay eggs to produce more silkworms. Silk production involves cultivation of mulberry trees and rearing of silkworms. Mulberry is a small tree whose leaves are used for feeding silkworms. Mulberry is called ‘shehtoot’ in Hindi. We will now describe how silk is actually produced.
Production of Silk
In order to obtain silk, mulberry trees are cultivated (grown), silkworms are reared, and their cocoons collected to get silk fibres. We will now describe the rearing of silkworms, processing of cocoons to obtain silk fibres and making of silk fabrics (silk cloth) from silk fibres.
(i) Rearing of Silkworms to Obtain Cocoons
A female silk moth lays hundreds of eggs at a time [see Figure (a)]. The eggs of silk moths are stored carefully on paper strips (or cloth strips) and sold to silkworm farmers. The farmers keep these eggs at suitable temperature and humidity under hygienic conditions. The eggs are then warmed to a suitable temperature for hatching. When the eggs hatch, silkworms (larvae or caterpillars) come out of eggs.
The silkworms are fed cut-up mulberry leaves [see Figure (b)], The silkworms eat day and night and grow big in size. After about 25 to 30 days, the silkworms stop eating and get ready to spin cocoons. The silkworms climb the twigs placed near them and spin cocoons of silk fibres. The silkworms enclose themselves completely inside the silken cocoons in two or three days [see Figure (c)].
(ii) Processing of Cocoons to Obtain Silk Fibres
All the cocoons are collected at one place. The pile of cocoons is used for obtaining silk fibres. This is done as follows : The pile of cocoons is placed in hot water. Hot water makes the silk fibres of cocoons to separate out. The long silk fibres are obtained by unwinding the threads from cocoons. The process of taking out silk fibres from the cocoons for use as silk is called reeling. Reeling is done in special machines which unwind the fibres of silk from cocoons.
(iii) Converting Silk Fibres into Silk Cloth
Silk fibres obtained from cocoons are spun (twisted) to form silk threads called ‘silk yarn. The silk yarn is then woven on looms into silk cloth by the weavers.
Different Varieties of Silk
There is a variety of silk moths which look very different from one another. The silk produced by the silkworms of different varieties of silk moths is different in texture (coarse, smooth, shiny, etc.). Some of the varieties of silk are : Mulberry silk; Tassar silk ; Mooga silk ; Kosa silk ; and Eri silk. These silks are obtained from cocoons spun by the silkworms of different types of silk moths. The most common silk moth is the mulberry silk moth. The silk obtained from the cocoons of mulberry silk moth is called mulberry silk. Mulberry silk is soft, lustrous (shiny) and elastic, and can be dyed in beautiful colours. Thus, the most common variety of silk is mulberry silk.
Natural Silk and Artificial Silk
Natural silk is obtained from the cocoons of silkworms and it is made of a ‘protein. Natural silk is an animal fibre. Artificial silk (called rayon) is obtained from wood pulp and it is made of modified plant material ‘cellulose’ (Paper is also made of cellulose obtained from wood pulp). We can distinguish between natural silk (or pure silk) and artificial silk by performing the ‘burning test’.
To Distinguish Between Natural Silk and Artificial Silk
Take a piece of natural silk fabric and another piece of artificial silk fabric, burn them separately and observe the smell produced :
- The fabric which burns giving a smell of burning hair will be natural silk (or pure silk).
- The fabric which burns giving a smell of burning paper will be artificial silk (or rayon).
Just like silk, wool is also made of proteins. So, a piece of wool (or woollen fabric) also burns giving the smell of burning hair.
Discovery of Silk
The discovery of silk was made in China a long time back. It is said that the Chinese empress Si-lung- Chi was asked by the emperor Huang-ti to find the cause of the damaged leaves of mulberry trees in their garden. The empress found some white worms eating up mulberry leaves. She also noticed that these worms were spinning shiny cocoons around them. A cocoon dropped into her cup of tea accidently and delicate silk threads separated from the cocoon. This is how silk was discovered by chance. The discovery of silk led to the beginning of silk industry in China.
The silk production in China was kept a secret for hundreds of years. Later on traders and travellers coming from China introduced silk to other countries. The route on which they travelled is still called ‘silk route’. Even today, China leads the world in silk production. India is also among the leading silk producing countries of the world. In India, a large number of women are engaged in various activities related to silk production such as rearing of silkworms, reeling of silk from cocoons and processing of raw silk into fabrics.