Spinning the cocoon
When the silkworm larva is fully grown and ready to pupate, it stops feeding and searches for a suitable location in which to spin its cocoon. The location may be a specially prepared frame, a bunch of twigs, a corner of a box or the cardboard core of kitchen roll.
The silkworm first starts to spin a hammock of silk to support the cocoon. The hammock is a loose, irregular protective network of silk in which the cocoon will be suspended.
Once the hammock is complete, the larva settles to produce an even, regular, lozenge-shaped and closely-knit cocoon of silk. The cocoon is much denser than the hammock and is formed from a single length of fine, strong, lustrous silk thread which is the source of commercial silk.
The silkworm produces silk by contracting its body and extruding liquid silk from two glands on the lower lip of the mouthparts. These two filaments are made from a protein called fibroin coated with the gum-like protein sericin. The two filaments solidify when they come into contact with the air and become glued together with the sericin to form a single filament.
The silk filament is continuous, very long (up to 1,300 metres in length), and extremely fine. The silkworm takes about three days to make a cocoon around itself using the silk filaments.
When the cocoon is complete, the larva starts to shrink in length, develops a hard skin and turns into a pupa, inside which the adult moth develops. This metamorphosis (change from larva to adult) takes about two to three weeks and the silkworm moth then emerges from its cocoon.
If you hold a bought cocoon and shake it you should hear the pupa rattling inside. You will also notice that the cocoon is stiff and does not look much like silk. Some people think that the silk is inside the cocoon and are surprised to find that the cocoon itself is made of silk. If you dissolve the glue that holds the cocoon together, using a process called de-gumming, the cocoon loses its shape, becomes soft, and looks and feels like silk.
Silk cocoon facts
The FAO Sericulture Manual records silk yields which vary with silkworm breed, season, rearing condition and freshness of the cocoon. Usually the weight of one cocoon ranges from 1.5 to 2.2 grams, so you would need between 450 and 700 cocoons to make one kilo. The silkworms have eaten between 18 and 20 kilos of mulberry leaves to make this quantity of cocoons. 77% to 80% of the weight of the cocoon is taken up by the pupa. The pupa is not necessarily a waste product as it is full of protein and can be used for food. Without the pupae, one kilo of whole cocoons becomes 200 to 230 grams of empty cocoons.
The process of reeling silk from whole cocoons generates waste and the one kilo of cocoons makes only 160 to 200 grams of raw silk (silk which still has sericin). The sericin is about one third of the weight of the raw silk, and after you degum the silk to remove the sericin, you are left with between 100 and 130 grams of silk. As you can see, one kilo of cocoons does not make much silk fabric.
To give you an example, you would need about 60 silkworms and two kilos of mulberry leaves to make a pongee silk scarf 150 cm by 40 cm which weighs 12 grams.
From cocoon to silk thread
There are four ways to produce thread from a silk cocoon.
1. Reeled silk
Most commercial silk is reeled from top grade cocoons. A cocoon is like a miniature, hollow ball of string, where the string is lightly glued together. To reel silk, you need to loosen the glue by placing the cocoons in warm water for a while and then unwind the thread of a several cocoons. Finally you need to twist several of these very fine threads together into a slightly thicker thread, in a process called throwing.
2. Degummed cocoons
Lower grade cocoons can be degummed. The sericin is removed by simmering the cocoons in water with soap and soda ash; the cocoons then become soft like cotton wool. You can buy degummed cocoons and spin directly from them.
3. Silk hankies
Alternatively, degummed cocoons are stretched over a frame to make silk hankies. Silk hankies have longer fibres than silk tops and they can be used for spinning or for knitting.
4. Silk tops
The residual silk from the reeling operation is degummed and carded into tops. Mulberry silk tops are made of short fibres which can be spun just like wool.
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