In this blog, we look at the concluding stages of the production of silk fibre.
What exactly is this fibre made of?
Silkworms have salivary glands called sericteries, which are used for the production of fibroin – a clear, viscous, protein fluid that is forced through an opening in their heads called the spinneret. The diameter of the spinneret determines the thickness of the silk thread, which is produced as a long, continuous filament. A second pair of glands secretes a gummy binding fluid called sericin which binds filaments together: more about that later.
The cocoons are then put into a shallow, woven basket made of bamboo, like the one below.
How does the cocoon turn into a fibre?
The cocoons are submerged into boiling water. The silk is then unreeled from the cocoon by softening the sericin and then delicately and carefully unwinding the filaments. In this photo, the silk worker is ‘reeling’ many fibres – the norm is between four to eight cocoons at once. A single thread filament is too thin to use on its own, so many fibres are combined to produce a thicker, usable fibre. This is done by hand-reeling the threads onto a wooden spindle to produce a uniform strand of raw silk. As the fibres are being drawn through the small hole of the wooden panel above the pot, the pot is periodically stirred which twists the fibres together. The twisted fibres, now a thread, then feed over the round barrel above the holed piece of wood, and then onto the spindle that is constantly being turned by hand. It takes nearly 40 hours to produce a half kilogram of silk.
The sericin protects the silk fibre during processing and is often left on the fibres 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.
What happens to the moth inside the cocoon?
If the moth is allowed to survive once it’s spun its cocoon, it will eventually emerge as an adult moth. To emerge, it releases proteolytic enzymes to make a hole in the cocoon. These enzymes are destructive to the silk and can cause the silk fibres to break, reducing a fibre over a mile in length to segments of random length. This seriously reduces the value of the silk threads and to prevent it, silkworms are usually not allowed to hatch from their cocoons, by being boiled or sun dried. The heat kills the silkworms and the water makes the cocoons easier to unravel.
In the next blog in this mini-series, we’ll look at silk in the UK, in Shepton Mallet and the trials the industry went through 1824-32.