Silk is produced in the wild by a variety of insects such as the Cecropia moth from North America, the Tussah, Muga and Eri moths from India, the Anaphe moth from Africa and indeed by some spiders! However, commercial production is dominated by the Mulberry Silk Moth, Bombyx Mori, literally the cocoon fed on mulberry.
The natural cycle of silkworm production follows the production of leaves by the mulberry, so the tiny pinhead sized eggs, left to hibernate by the previous year’s moths, are warmed up gradually when the mulberry starts to bud in spring. They should then be ready to hatch into baby caterpillars 10 – 14 days later. Their lives are dominated by food; they eat ravenously, shedding their skins four times within a period of four weeks, moulting as they outgrow each skin. After the last moult the silkworm is 7 to 10cm long, fat, hairless and deeply unattractive!
At this point their appetites are prodigious with some 25g of eggs (around 36,000 worms) requiring as much as a ton of foliage.
After all this gluttony the silkworm stops eating and starts the production of its cocoon, suspended on the twigs or straw provided. Two modified salivary glands, or sericteries, on the caterpillar’s head produce a clear, sticky liquid which is then forced out through spinnerets and hardens on contact with the air to form a continuous filament. Working in a figure of eight the caterpillar constructs the cocoon which is held together with Sericin, a gummy substance, which literally sticks the threads together and hardens to form a distinctive shape.
During this period excessive heat or moisture have to be avoided or the hardening and therefore the quality of the silk being produced, can be adversely affected.
It takes around 10 days for the moth to metamorphose from the caterpillar. It then produces a liquid which dissolves the gum holding the threads of the cocoon together to allow the ash coloured moth to emerge, usually at dawn. The male and female moths now mate and the female lays her eggs. These then need a period of cold for hibernation and the cycle starts again.
Unfortunately the very process of the moth pushing its way out of the cocoon destroys the continuity of the thread and makes the cocoon commercially useless except for the production of spun silk. So, most moths are killed before they emerge and only a small proportion are allowed to exit normally and mate to produce the eggs for the next round of production.
The producers now have completed cocoons which are ready for reeling. Each cocoon is normally made up of one extremely fine filament. The cocoons are placed in nearly boiling water to soften the gum and release the end of the filament which is then combined with the filaments from a number of other cocoons, depending on the thickness required for the final thread. For instance, a thread for weaving would typically be made from 8-10 filaments. This process of combining several continuous filaments to form a thread is called throwing from the Anglo Saxon ‘thraw’ meaning to whirl or spin. This distinguishes it from spinning which is the combining of short lengths of silk fibres by combing and then twisting them together in much the same way as wool.
Any remaining processes depend on the use to which the fibre is to be put but can include boiling off to remove the sericin, dying, finishing, weaving or knitting.
There have been many changes in the commercial production of silk over the years, to increase the number of cycles that can be achieved in a given period. The yield of leaves from a mulberry tree or bush has been greatly increased, the surplus being dried and converted into a sort of artificial silkworm food which allows silkworms to thrive when the mulberry tree is not in leaf. Methods of fooling the silkworms into thinking that they have had a period of hibernation have also been developed and production can now continue all year round.
The European Commission for the Promotion of Silk gives some interesting statistics in their video, ‘Silk, woven into history’. Apparently, one acre of Mulberry yields around 4.5 tons of leaves which will produce around 200 kg of cocoons, giving 40 kg of raw silk. It takes around 110 cocoons to make a tie and around 630 to make a blouse. This, perhaps, goes some way to explaining why silk has always been a luxury product.