Silk is one of the most favorite fabric materials in the world. It has been so for thousands of years. The soft and shiny material is almost a prized item in many places. Silk has been known for several millenniums. The earliest known specimen of silk fabric was discovered from China, which dates back to 3630 BC. China is also the largest producer of the material. The historical Silk Route says how much importance silk had in bygone eras.
Silk is essentially a natural protein fiber. It is made by several insect larvae to form cocoons. However, the industrial silk is produced from the worms of moth species Bombyx mori. An unfortunate side effect of this is that the pupae have to be killed to obtain the material. It takes the killing of about 3000 caterpillars to get a pound of silk.
|Silk cocoon from silk worm (probably Bombyx mori)|
The cultivation of silkworms is called sericulture. The eggs of moths are incubated for 10 days. They hatch to produce caterpillars which are fed on mulberry leaves for about five weeks. The caterpillars then begin to spin the cocoon. This time, they would weigh about 10,000 times heavier than they did at the time of hatching. While making cocoons, the caterpillars move in a certain pattern. They secrete a liquid protein named fibroin from two glands and force it out through openings in head called spinnerets. They are coated in a protective agent called sericin, which hardens with air contact. The caterpillar would produce about 1 mile of filament in three days.
The cocoons are then soaked in boiling water to kill the pupae and soften the sericin. The fibers are then unwound to form a continuous thread. This process is called reeling. Since a single strand is too thin for commercial use, a few (usually between 3 and 10) are twisted together to form a single silk thread. This is called reeled silk. There are different twisting methods, and each produces a distinct type of silk. The shining appearance of silk is from the triangular prism-like structure of the fiber. Owing to the structure, the incoming light refract at different angles, thereby producing different colors.
Since numerous pupae have to be killed to produce silk, many animal right movements have raised their voice against the industry. The scientific world is trying to come up with methods that don’t require killing of the worms, which may come to fruition soon.
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