The origins of silk date back to 3000 BC China. The Chinese used silk fabrics for arts and decorations as well as for clothing. Silk became an integral part of the Chinese economy and an important means of exchange for trading with neighboring countries. Caravans traded the prized silk fabrics along the famed Silk Road into the Near East. By the fourth century BC, Alexander the Great is said to have introduced silk to Europe.
Initially, the Chinese were highly protective of their secret to making silk. However, the mystery of the silk-making process was eventually smuggled into neighboring regions, reaching Japan about 300 AD and India around 400 AD. By the eighth century, Spain began producing silk, and 400 years later Italy became quite successful at making silk, with several towns giving their names to particular types of silk.
The first country to apply scientific techniques to raising silkworms was Japan. Other countries that also produce quality silks are China, Italy, India, Spain, and France.
Silk is highly valued because it possesses many excellent properties. Not only does it look lustrous and feel luxurious, but it is also lightweight, resilient, and extremely strong—one filament of silk is stronger then a comparable filament of steel.
The commercial process of silk making is highly complex and labor intensive.
- Sericulture– Cultivation of the silkworm cocoon for their filaments is known as sericulture. Only the filament produced by Bombyx mori, the mulberry silk moth and a few others in the same genus, is used by the commercial silk industry. Breeding of silkworm occurs once in a year but under scientific conditions, they may be hatched three times a year.
- Hatching the eggs- The laying of silkworm eggs occurs in a controlled environment such as an aluminum box, where they are then examined to ensure they are free from disease. The female deposits 300 to 400 eggs at a time. The tiny eggs of the silkworm moth are incubated (about 10 days) until they hatch into larvae (caterpillars). At this point, the larva is about a quarter of an inch long.
- The feeding period- Once hatched, the larvae are placed under a fine layer of gauze and fed huge amounts of chopped mulberry leaves during which time they shed their skin four times. Larvae fed on mulberry leaves produce the very finest silk. The larva will eat 50,000 times its initial weight in plant material. For about 6 weeks the silkworm eats almost continually. After growing to its maximum size of about 3 inches at around 6 weeks, it stops eating, changes color, and is about 10,000 times heavier than when it hatched. The silkworm is now ready to spin a silk cocoon.
- Spinning the cocoon (Pupating)- The silkworm attaches itself to a compartmented frame, twig, tree or shrub in a rearing house to spin a silk cocoon over a 3 to 8 day period. Liquid secretions from the two large glands in the insect emerge from the spinneret, the mouthpart of the larva. The secretions harden on exposure to the air and form twin filaments composed of fibroin, a protein material, which is produced as a long, continuous filament. A second pair of glands secretes a gummy binding fluid called sericin which bonds the two filaments together. Steadily over the next four days, the silkworm rotates its body in a figure-8 movement some 300,000 times, constructing a cocoon and producing about a kilometer of silk filament.
- Reeling the filament- At this stage, the cocoon is treated with hot air, steam, or boiling water. The silk is then unbound from the cocoon by softening the sericin and then delicately and carefully unwinding, or ‘reeling’ the filaments from 4 – 8 cocoons at once. As the sericin protects the silk fiber during processing, this is often left in until the yarn or even woven fabric stage. Raw silk is silk that still contains sericin. Once this is washed out (in soap and boiling water), the fabric is left soft, lustrous, and up to 30% lighter. The amount of usable silk in each cocoon is small, and about 2500 silkworms are required to produce a pound of raw silk.
- Creating different types of silk- This process of creating the silk yarn is called “throwing,” and prevents the thread from splitting into its constituent fibers. Four different types of silk thread may be produced from this procedure: crepe, tram, thrown singles, and organzine.
- Crepe is made by twisting individual threads of raw silk, doubling two or more of these together, and then twisting them again.(Used for weaving crinkly fabrics)
- Tram is made by twisting two or more threads in only one direction. (Used for the weft or filling)
- Thrown singles are individual threads that are twisted in only one direction. (Used for sheer fabrics)
- Organzine is a thread made by giving the raw silk a preliminary twist in one direction and then twisting two of these threads together in the opposite direction. (Used for the warp threads of materials)
Broken or waste filaments and damaged cocoons are retained, treated to remove the sericin, and combed. This is then processed into yarn, marketed as spun silk, which is inferior in character to the reeled product and much cheaper.