What is silk?

Silk is a fine, soft thread produced by the larvae of the silk moth. Silk is an animal fiber but is not a staple fiber. It is obtained in a thread-like form, and is a filament fiber. Silk production is sericulture.The most common cultivated species, Bombay Mori, is the one usually used for sericulture.

Sources

China, India, Japan and the former USSR.

Mulberry leaf

Major Countries

Japan was the first country to undertake commercial production of silk. It is ranked number one, in terms of production of fine quality silk. Other countries include:

• China • India • Italy • Spain • France • Austria • Turkey • Greece • Brazil

The most common cultivated species, Bombay Mori, is the one usually used for sericulture. The worm or larva grows to maturity within a month of hatching from the egg. During this time it feeds on mulberry leaves and spins a protective cocoon of fine, silk thread. Glands in the silk-worm produce two different liquids which make the cocoon. One is fibrous (a protein called fibroin) and the other is gum (sericin). A mixture of the two liquids is extruded from two horns in the silk-worm’s head and this is what produces the filaments. The silk-worm attaches these sticky filaments to a twig or frame. Then, by moving its head in a figure of eight motion, the silk-worm wraps itself in the cocoon, spinning from the outside.

The commercial process of silk making is highly complex and labor intensive. The following will provide basic information on how silk is made.

• Sericulture• Hatching the Eggs• The Feeding Period• Spinning the Cocoon• Reeling the Filament• Types of Silk

Sericulture

Cultivation of the silkworm is known as sericulture. Although many insects produce silk, 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.

Hatching the Eggs

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. The female deposits 300 to 400 eggs at a time.In an area the size of your monitor screen, 100 moths would deposit some 40,000 eggs, each about the size of a pinhead. The female dies almost immediately after depositing the eggs and the male lives only a short time after. The adult possesses rudimentary mouthparts and does not eat during the short period of its mature existence.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. The larvae may also feed on Osage orange or lettuce. 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 six 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

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. This period is termed pupating.

Silkworms possess a pair of specially modified salivary glands called sericteries, which are used for the production of fibroin – a clear, viscous, proteinaceous fluid that is forced through openings called spinnerets on the mouthpart of the larva.

Liquid secretions from the two large glands in the insect emerge from the spinneret, a single exit tube in the head. The diameter of the spinneret determines the thickness of the silk thread, which is produced as a long, continuous filament. The secretions harden on exposure to the air and form twin filaments composed of fibroin, a protein material. 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, sometimes with a slight twist, to create a single strand.

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.

Types of Silk yarn

Raw silk is twisted into a strand sufficiently strong for weaving or knitting. 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.Tram is made by twisting two or more threads in only one direction. Thrown singles are individual threads that are twisted in only one direction. 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.In general, organzine thread is used for the warp threads of materials, tram threads for the weft or filling, crepe thread for weaving crinkly fabrics and a single thread for sheer fabrics.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.

Obtaining the fibers

In commercial production the silk-worm eggs are hatched on trays which are spread with mulberry leaves. Before the pupa (chrysalis) is able to emerge as a moth, the cocoons are subjected to either dry or wet heat which kills the insect, the cocoons are put in a bath to remove the gum, then the ends of the filament are found and are unwound with the help of revolving brush. A single filament is usually too fine to make a yarn so a number of cocoons (usually 3-8) are unwound and put on reels together to make single yarn.

If the pupa is left to develop into a moth, it dissolves part of the cocoon and escapes in order to mate. The female moth lays the eggs and then dies and the whole process starts again as more silk-worms emerge from the new eggs.

Silk Manufacturing Process

Silk is a natural protein fiber produced by certain caterpillars in order to encase themselves in the form of cocoons. The making of silk is different from that of other natural fibers.

There are many steps involved in silk manufacturing • Cultivation of Cocoons ( Sericulture) • Filature Operations • Manufacturing of Silk Yarns • Finishing of Silk Fabrics

Sericulture

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 for which racks, clusters of twigs or straw are provided.

• 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' or 'stifling' in which the cocoons are heated. Some of the cocoons are preserved so that the pupa or chrysalis inside them develop into moths for further breeding.

• Filature Operations

The raw silk is unwound from cocoons and are collected into skeins in the factories known as Filature. Here the cocoons are sorted based on their color, size, shape and texture. Then they are immersed in hot and cold water to soften the sericin so that filament could be unwound into one continuous thread. The whole of the seracin is not removed at this stage as it protects the delicate filament in further operations. 'Reeling' is the process of unwinding the filament from the cocoon. Three to ten filaments are together reeled for producing the desired diameter of raw silk thread.

•Filature Operations

• Manufacturing of Silk Yarns

The reeled silk is formed into silk yarn or silk thread through the process called 'Throwing'. It corresponds the spinning process of other natural fibers. The raw silk skeins are sorted according to their color, size, length or quantity and washed in warm water with soap or oil for softening the seracin. After drying the skeins, they are placed on reels from where the silk is wound on bobbins.

Manufacturing of Silk Yarns

• During winding, the silk strands are given desired amount of twist. The strands may be doubled and then given twist in similar or opposite directions. To get equal diameter throughout the length, the yarn is run through rollers. Many kinds of silk yarns are manufactured by giving different amount of twists. Still remaining sericin is removed from silk yarn by the process of 'Degumming' in which the yarn is washed with soap and water for bringing out its natural shine and the soft feel.

Types of silk

1. Raw silk: This is made from up to ten undamaged cocoons wound together. This is sometimes called greige silk.

2. Spun silk: This is spun from unwindable parts of the cocoons, together with other waste silk. Long raw fibers are cut to maximum lengths of 30 cm and spun.

3. Dupion silk: This is made from double cocoons where silk-worms have spun their cocoons together; the thread produced is uneven and thick.

4. Tussah silk: This is produced from uncultivated silk-worms. It is not easy to de-gum and the fineness of filaments varies.

Kinds of Silk Yarns

1. Thrown Singles: Three to eight silk filaments are twisted together in only one direction to make 'Singles'.

2. Tram: A slight twist is given to two to four untwisted singles. Trams are used exclusively as filling yarns.

3. Crepe: Individual raw silk filaments are twisted together, one in S direction and the other in opposite Z direction. They are then twisted around each other in S direction.

4. Organzine: Two or more singles having twist in Z direction are combined by twisting them around each other in the opposite S direction. Organzine is mainly used for warp yarns.

The properties of the silk

1. The fine fibers produce compact fabrics which can trap air around the body. This air is warmed by the body heat and, because it cannot escape easily, keeps the body warm. It is this action that gives silk the reputation of having high thermal insulation properties.

2. Having high moisture absorption, silk can absorb and hold up to one-third of its weight in water vapour without feeling wet.

3. The fine, soft feel makes silk comfortable to wear.

4. It is very strong because the fiber has a high degree of tenacity.

5. It has high degree of elasticity which makes it resilient fiber that resumes its shape quickly after being stretched (although this varies to some extent in relation to the type and weave of the silk).

6. It has a high resistance to static electricity; because it always contains moisture there is usually little electrostatic charge.

7. It is very easily spoiled by chemicals used in some products. For example, some deodorant sprays and perfumes can bring about color change, as can perspiration.

8. It can have its properties improved by finishing process; for example the removal of the sericin gum by gentle boiling in mild soap solution (called de- gumming) makes knitted and woven raw silk softer. The de-gummed silk is then weighted by the addition of chemicals such as metallic salts.

Moths

The moth eggs

breeding and for spinning into silk

The baskets

1.5 inches long worms