Silk is produced by several insects, but generally only the silk of moth (Bombyx Mori) caterpillars has been used for textile manufacturing. The silk worm isn’t really a worm, but the caterpillar (larva) of a moth, Bombyx mori. The caterpillar spins a cocoon before turning into a moth and it is this cocoon that yields silk.
Four types of silk threads are produced from the raw fibers, such as crepe, tram, thrown singles, and organzine. Crepe silk is produced by twisting two or more threads of silk together in multiple directions. Tram is made by twisting threads of silk in just one way. Thrown singles include just one silk thread twisted in a single direction, and organzine is made of two or three threads twisted in one direction before twisting in the opposite direction. Crinkly fabrics are made with crepe, while sheer cloth is made with single thread. Leftover silk is spun into yarn and sold as spun silk. These fibers have lower quality than reeled silk, and spun silk yarn is much cheaper.
LIFECYCLE OF A SILK WORM
1 – 3 weeks: From a tiny egg a small caterpillar hatches.
3 – 4 weeks: The larva grows in four stages, or instars, as it eats mulberry leaves or artificial food and sheds its skin.
2 – 4 days: The caterpillar then spins a cocoon of silk around itself. In silk production, the cocoon is then heated to kill the caterpillar inside and use the cocoon for its silk.
2 – 3 weeks: It turns into a pupa which emerges from the cocoon.
1 – 3 days: The pupa then becomes a non-flying moth, mates and, if female, lays hundreds of eggs. After this the moth will die.
The silk farmers then heat the cocoons to kill them, leaving some to metamorphose into moths to breed the next generation of caterpillars. Harvested cocoons are then soaked in boiling water to soften the sericin holding the silk fibers together in a cocoon shape. The fibers are then unwound to produce a continuous thread. Since a single thread is too fine and fragile for commercial use, anywhere from three to ten strands are spun together to form a single thread of silk.
And despite advances in production methods and new possibilities for cultivation, still today the only reasonable way to glean the thread in mass quantities is by killing the thing that made it.
Sericulture is the cultivation of cocoons for their filaments. The best raw silk is obtained from the species of moth called Bombyx mori. Breeding of silkworm occurs once in a year but under scientific conditions, they may be hatched three times a year. The female moth lays around 350 to 400 eggs and the moths die soon after. As they are subject to hereditary infection, the eggs from infected moths are destroyed which results into production of fine silk. Larvae of about 3mm are hatched from the eggs. For about 20 to 30 days, they are carefully nurtured and are fed five times a day on chopped mulberry leaves. In the meantime, the larvae change their skin for four times and are formed into caterpillar of about 9 cm long. Now they are ready to spin cocoon. The caterpillar have small openings under their jaws called spinnerets through which they secret a protein like substance. This substance solidifies when it comes in contact with air and the filament thus formed is spun around the silkworm in the figure resembling the digit 8. In three days the cocoon gets completed which is about a peanut shell’s size. The filament is held together by sericin or silk gum. The life of the worm is ended by the process of ‘stoving’ in which the cocoons are heated. Some of the cocoons are preserved so that the pupa or chrysalis inside them develops into moths for further breeding.
The first stage of silk production is the laying of silkworm eggs, in a controlled environment such as an aluminum box, which are then examined to ensure they are free from disease. Silk moths lay eggs on specially prepared paper. The eggs hatch and the caterpillars (silkworms) are fed fresh mulberry leaves. The female deposits 300 to 400 eggs at a time. Only the healthiest moths are used for breeding. Their eggs are categorized, graded, and meticulously tested for infection. Unhealthy eggs are burned. The healthiest eggs may be placed in cold storage until they are ready to be hatched. Once the eggs are incubated, they usually hatch within seven days. They emerge at a mere one-eighth of an inch (3.2 mm) long and must be maintained in a carefully controlled environment.
Under normal conditions, the eggs would hatch once a year in the spring when mulberry trees begin to leaf. But with the intervention of sericulturists, breeding can occur as many as three times per year.
FEEDING THE LARVAE
The silkworms feed only on the leaves of the mulberry tree. The mulberry leaves are finely chopped and fed to the voracious silkworms every few hours for 20 to 35 days. During this period the worms increase in size to about 3.5 inches (8.9 cm). They also shed their skin, or molt, four times and change color from gray to a translucent pinkish color.
SPINNING THE COCCOON
A straw frame is placed over the tray of caterpillars, and each caterpillar begins spinning a cocoon by moving its head in a pattern. During its 3 to 8 day pupating period, the silkworm secretes fibroin, a sticky liquid protein, which is the actual silk. Pushed through a spinneret (opening on the mouth), the twin pair of continuous filaments gets harden when they come into contact with the air. Next, the silkworm also secretes sericin, a bonding gum, to hold the two filaments together. While constructing its cocoon, the silkworm twist in a figure-8 motion about 300,000 times and within 2-3 days it produces around 1 kilometer of filament and is completely encased in a cocoon. The caterpillar spins a cocoon encasing itself completely. It can then safely transform into the chrysalis, which is the pupa stage.
It takes about 2,500 silkworms to make 1 pound of raw silk.
STOVING THE CHRYSALIS
Since hatching from the cocoon destroys the thread. To harvest the silk, while protecting the silk filament from being destroyed as the silkworm will try to break the cocoon to get free out of it. Hence to avoid the breaking of silk filaments, the cocoons are placed in either boiling water, or blasted with steam or hot air, all processes that kill the pupae. Less lethal methods were tried in the past, such as pulling the silk as the worms spun it, but the worms resisted and bit off the filaments. Besides killing the pupae, the heat softens the binding agent (sericin), so that the filaments may be unwound. Sometimes, the softened sericin is left on the fibers, and this product is called raw silk.
SORTING AND SOFTENING OF COCCOONS
In this Step, the cocoons are sorted by various characteristics, including color and size, so that the finished product can be of uniform quality. The cocoons must then be soaked in hot water to loosen the sericin. Although the silk is about 20% sericin, only 1% is removed at this stage. Since the presence of this gum facilitates the further mechanical processing of converting filaments into yarns.
REELING THE FILAMENT
Reeling can be done manually or through reeling machines. The cocoon is brushed to locate the end of the fiber. It is threaded through a porcelain eyelet, and the fiber is reeled onto a wheel. Meanwhile, diligent operators check for flaws in the filaments as they are being reeled. During Reeling, As each filament is nearly finished being reeled, a new fiber is twisted onto it, thereby forming one long, continuous filament.
The raw silk reeled filaments are then formed into skeins. These skeins are packaged into bundles weighing 5-10 pounds (2-4 kg), called books. The books are further packaged into bales of 133 pounds (60 kg) and transported to manufacturing centers.
Silk thread, also called yarn, is then formed by throwing, or twisting, the reeled silk. First the skeins of raw silk are categorized by color, size, and quantity. Next they are soaked in warm water mixed with oil or soap to soften the sericin. The silk is then dried. As the silk filaments are reeled onto bobbins, they are twisted in a particular manner to achieve a certain texture of yarn. For instance, “singles” consist of several filaments which are twisted together in one direction. They are turned tightly for sheer fabrics and loosely for thicker fabrics. Combinations of singles and untwisted fibers may be twisted together in certain patterns to achieve desired textures of fabrics such as crepe de chine, voile, or tram. Three to ten filaments are together reeled for producing the desired diameter of raw silk thread. The silk yarn is put through rollers to make the width more uniform. The yarn is inspected, weighed, and packaged. Finally, the yarn is shipped to fabric manufacturers.
To achieve the distinctive softness and shine of silk, and impart sufficient dyeability to the silk, the remaining sericin must be removed from the yarn by soaking it in warm soapy water. Degumming decreases the weight of the yarn by as much as 25%.
FINISHING OF SILK FABRICS
Many finishing processes are applied to different silk fabrics in order to improve their appearance, durability and feel. Calendering is done to enhance luster, singeing is done to make them smooth, and steaming is done for raising pile weaves. Pressing and lustering removes wrinkles from the finished fabric. It is done with heated rollers and then soaking in dilute acid to bring luster.
One finish that is unique to silk fabric is ‘Weighting’. The weight of silk is lost during the process of degumming. The manufacturer purchases silk by weight and to make up his loss, he does weighting of silk fabric with metallic substances such as stannic chloride, sodium phosphate, iron salt, logwood etc. Weighting is done during the dyeing process. Weighted silk is less compactly woven when compared to the unweighted silk and lesser silk is used in the fabric construction. Apart from lowering the cost of silk, weighting gives it crispness, luster and a firm feel.
TYPES OF SILK
There are five major types of commercially exploited silkworms producing 5 types of silk:
Mulberry: 75% of the silk is produced by the Bombyx mori which are also called mulberry silkworms since they solely feed on the leaves of mulberry plant. These silkworms are completely domesticated and reared indoors.
Tasar: Tasar (Tussah) is copperish color, coarse silk mainly used for furnishings and interiors. Tasar silk is generated by the silkworm Antheraea mylitta which mainly thrive on the food plants Asan and Arjun. The rearing is conducted on the trees in the open.
Oak Tasar: A finer variety of tasar generated by the silkworm Antheraea proyeli J. that feeds on natural food plants of oak. This silkworm is originally from India but china is the major producer of this type of silk.
Eri: Also known as Endi or Errandi, is a multivoltine silk spun from open-ended cocoons, unlike other varieties of silk. Eri silk is the product of the domesticated silkworm Philosamia ricini that feeds mainly on castor leaves. To produce this silk the pupas are not necessarily killed, the eri cocoons are spun only when they are open-mouthed. But even in this case many times the pupas are reared to be eaten or sometimes are used as fertilizers.
Muga: This golden yellow silk is reared in the Assam state of India. The silk is obtained from the semi-domesticated multivoltine silkworm Antheraea assamensis. These silkworms feed on the aromatic leaves of Som and Soalu plants and are reared on trees similar to that of tasar. The muga silk is considered as a high value silk and is used in products like sarees, mekhalas, chaddars, etc.
Still, no proper alternative has been found to save those silkworms. Some kindly researchers have recently discovered a method to harvest long filaments without killing the worms. They have noticed that when they get injured, the caterpillar engage in self-paralysis in order to give itself time to heal, the scientists found a way to isolate the biochemical used by the insect to reach that state. By extracting it and injecting it into healthy worms, the researchers were able to induce partial paralysis, after which, one end of the worm’s silk was attached to a slowly winding reel, which successfully gathered the silk. In its paralyzed state, the worm was unable to bite off the thread (as it otherwise would do). The record for gathering silk this way is 500 meters, or about half of that acquired through the traditional method.