Animal Fibres-Silk (Discovery, Life History, Processing, Properties)

Animal Fibres- Silk: Silk is a strong, light, and silky fabric with a shimmering appearance. Silk fibres are also animal fibres derived from silk moth cocoons (silkworm). Sericin and fibroin are two proteins that makeup silk fibre. These proteins give the fibre a beautiful sheen. Sericulture is the practice of breeding silk moths to extract the silk.

In this post, we will study everything about silk, an animal fibre.

Define Animal Fibres-Silk

Silk is a very fine natural fibre. The silk fibre is produced as a filament by the caterpillar of a silk moth. 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.

Discovery of Silk

Silk was first developed in China. According to a legend, in about \(3,000\,{\rm{BC}}\), the bride of Chinese Emperor Hunang Di, named Xi-Chung-Shih developed the first silk reel. It was kept a closely guarded secret for hundreds of years.

Knowledge of making silk reached India only in \({\rm{AD}}\,{\rm{300}}\) through traders and travellers. Silk moths were also traded to European countries, but they could not survive due to cold weather. However, Italy became a silk centre for western countries of Europe. China is a leading silk-producing country in the world. Other major silk-producing countries are Japan, India and Italy.

Sources of Silk

1. The best quality of silk is obtained from the cocoons of the silk moth, Bombyx mori. Its caterpillar feeds on the leaves of the mulberry tree. This variety of silk moths is cultivated in China and the mountainous regions of Northern India.
2. Wild silk called tassar is obtained from silk moths that feed on oak leaves. Oak trees grow wildly in India and China.
3. Munga silk is obtained from another wild variety of silk moths found in Brahmaputra valley.

Life Cycle of a Moth

While learning about the silk and the silk-moth, we will come across certain terms which represent the different stages in the life cycle of a moth. These stages are the same in both the moth and the butterfly. To have a better idea about these terms (names), let’s go through the life cycle of a moth.

A female moth lays thousands of eggs on the green leaves of the mulberry tree. The moth begins its life as a hatched fertilised egg, and a larva or a caterpillar worm comes out of the egg. The worm grows quickly while feeding voraciously on the mulberry leaves.

A caterpillar is a silkworm.It bears two silk glands along the sides of the head of its body. The caterpillar or the silkworm grows without food for some time and then starts exuding (pushing out) liquid silk from the glands into the air. The liquid silk dries immediately, forming a long, thin, and hard filament of silk. This way, a silkworm forms nearly a thousand meters of silk filament.

While exuding silk, the worm moves its head from side to side and silk thread is woven around the body of the worm (caterpillar). This covering is called a cocoon in which the animal sleeps. A cocoon with the worm inside is also called a chrysalis, the pupa stage. The worm or pupa inside the cocoon then changes into a moth with antennae and wings. The moth inside forces its way to come out of the cocoon as an adult moth. There are four stages in the life cycle of a moth: (1) eggs, (2) larvae or caterpillars, (3) pupa or chrysalis, and (4) adult moth (winged).

Silk secreted by a mulberry silkworm is a little yellowish fluid producing a prismatic (triangular in cross-section) filament. This filament refracts light and gives a lustrous shimmering appearance to the silk cloth. This makes silk special and highly prized.

Processing of Silk

Silk is processed in the following steps:

1. The factory where silk fibre is unwound from the cocoons into bundles of yarn or thread is called filature.
2. The process of unwinding the filament from cocoons is called reeling.
3. As the filament from a single cocoon is too fine for use, \(3\) to \(10\) filaments are reeled at a time to produce the desired raw silk thread. The reeled silk is made into bundles and is passed on for spinning.
   The process of spinning silk is similar to that of cotton yarn spinning. To manufacture silk thread, the silk yarn is twisted to achieve the appropriate strength and quality. Silk yarn does not need to be combed or dragged out like wool because its filaments are long and clean. Silk mills are where the spinning takes place.
4. Weaving of silk: Silk yarn is weaved into the fabric on looms (weaving machines). While weaving cloth, silk yarn is used both as warp and weft.

Properties of Silk

The properties of silk are discussed below:

Appearance

It is soft, smooth, and lustrous.

Strength

Silk fibre has good tensile strength, which allows it to withstand great pulling pressure. Silk is the strongest natural fibre.

Absorption of Moisture

The silk fabric’s absorptive capacity makes it pleasant to wear even in hot weather.

Insulating Nature

In the summer, it’s cool to wear, and in the winter, it’s warm.

Cleanliness and Washability

Because of its smooth surface, silk cloth does not attract dirt.

Elasticity

Silk is an elastic fibre that can be stretched from \(1/7\) to \(1/5\) times its original length before breaking. It returns to its original size but loses little of its elasticity.

Heat Conductivity

Silk is a protein fibre. Like wool, it is a non-conductor of heat. Thus, it is suitable for winter apparel.

Quality of Silk

The quality of the silk fabric depends on many factors such as the type of silkworm, quality of the mulberry leaves fed to the silkworms, selection of cocoons, weaving and finishing of the silk thread.

Occupational Hazards of Silk Industry

1. Various chemicals used for degreasing, disinfecting, bleaching and dyeing silk cause irritation of the eyes and skin. Moreover, workers handling raw silk may get allergic skin reactions.
2. The scales of the moth and silk fibre proteins can initiate asthma.
3. Continuous contact with hot water used in reeling silk may soften the skin of the hands and cause burns and scalds.
4. Many of the dyes used to colour silk are easily absorbed by our digestive system if ingested and can cause health problems.
5. Workers develop infectious skin diseases due to the dipping of hands in boiling water while killing the pupae.
6. Severe headaches, fever and pain in the neck and lower back have also been observed in the workers of the silk industry.
7. Leg deformity and bow-leggednessare also found in some workers.

Such conditions can be prevented by using less dangerous chemicals, having proper exhaust and ventilation, personal protective equipment, good washing facilities (including shower baths where practicable) and strict personal hygiene.

Summary

Silk, like wool, hemp, linen, and cotton, is a natural fibre that was one of the first to be discovered by man. Silk is a fibroin made up of proteins that are produced as a single thread in a fluid form by the silkworm caterpillar. To live, these silkworms feed the selected plants and spin cocoons as a ‘protective shell.’ This article taught us about silk, the silk moth’s life cycle, its processing, and its qualities.

FAQs on Animal Fibres- Silk

Q.1. Why are silk and wool are considered animal fibre?
Ans: Silk comes from silkworms, and wool is obtained from sheep, goats and yak. Hence silk and wool are animal fibres.

Q.2. What are the uses of silk?
Ans: The uses of silk are:
a. Silk is commonly seen in dresses, tees, shirts, pants, pyjamas, underwear, and folk costumes. Because of its capacity to keep body heat contained while remaining lightweight, it is frequently utilised in ski clothes.
b. Silk is also used to manufacture lightweight and hypoallergenic silk comforters and duvets. Upholstery, curtains, rugs, and bedding, such as sheets, are all made from it.

Q.3. What are the sources of silk?
Ans: a. The best quality of silk is obtained from the cocoons of the silk moth, Bombyx mori. Its caterpillar feeds on the leaves of the mulberry tree. This variety of silk moths is cultivated in China and the mountainous regions of Northern India.
b. Wild silk called tassar is obtained from silk moths that feed on oak leaves. Oak trees grow wildly in India and China.
c. Munga silkis obtained from another wild variety of silk moths found in Brahmaputra valley.

Q.4. What are the properties of silk?
Ans: Appearance: It is soft, smooth and lustrous.
Strength: Silk fibre has good tensile strength, which allows it to withstand great pulling pressure. Silk is the strongest natural fibre.
Absorption of moisture: The absorptive capacity of the silk fabric makes comfortable apparel even for a warmer atmosphere.
Insulating nature: It is cool to wear in the summer and warm to use in the winter.
Cleanliness and Washability: Silk fabric does not attract dirt because of its smooth surface.
Elasticity: Silk is an elastic fibre.
Heat conductivity: Silk is a protein fibre. Like wool, it is a non-conductor of heat. Thus, it is suitable for winter apparel.

Q.5. What are different kinds of silk?
Ans: Different kinds of silk are produced by different kinds of moths. The most common types are eri, mulberry, tassar and mooga.

Q.6. What is sericulture?
Ans: Rearing of silkworms for the production of silk is called sericulture.

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