A small inconspicuous caterpillar, which pupates on its way to becoming a butterfly, has been supplying one of the finest and most valuable natural fibres for millennia - silk. It is noteworthy that the protein from the cocoon of the silkworm is the only naturally occurring continuous fibre.
Silk production is thought to have originated in China and dates back several thousand years. Archaeological finds date the human use of silk as early as the year 2800 BC. There are many legends surrounding the discovery of silk as a raw material. For example, an emperor is said to have dropped a silkworm into hot tea, which led to the accidental discovery of the silk thread. Likewise, for centuries, Chinese people faced the death penalty for taking silkworms or their eggs out of the country. Two Persian monks are said to have been the first to smuggled eggs to Constantinople. Since the monks had also seen the method of silk extraction in China, this is the point at which the triumph of the fine material began in Europe.
Emergence and extraction of silk
The majority of silkworms used for silk production comes from a butterfly called the mulberry spinner (lat. Bombyx mori) which feeds on the leaves of the mulberry tree. There are still some other species which are suitable for silk production, including the Chinese oak moth (Antheraea pernyi) or the tree-spinner (Samia cynthia). In addition to caterpillars, there are also various shells which produce silk. These spin threads to attach themselves to their underground. These so-called byssus threads were mainly used in the Mediterranean for silk extraction in ancient times.
However, the main suppliers of valuable silk are silkworms. Most commonly, the larva of the mulberry spinner is used for silk extraction. This caterpillar sheds its skin a total of four times and about 35 days after hatching from the egg, it is ready for spinning. For this purpose, the caterpillar has silk glands which consist of a tube which is wound many times; from its rear, the silk substance is secreted. This consists of special proteins. The resulting silk material is passed through thin passages to a spinneret on the head of the caterpillar. The secretion emerging from this gland immediately hardens on contact with air, to form a thread. The caterpillar makes targeted head movements while the material exits, winding the thread around itself and thus building a stable cocoon from this silky web. The caterpillar places up to 300,000 turns of thread around it. The cocoon can consist of up to 900 metres of thread.
To make silk, the caterpillars are killed before hatching with hot water or steam. This is to prevent tearing of the cocoon. Afterwards, three to eight cocoons are handled together. As the individual filaments stick together due to the silk glue, a silk thread is created, also called grège. This thread can now be further processed into smooth textiles. To get 250g of a silk thread, about 3000 cocoons are needed, which is about 1 kg.
This elaborate manufacturing process is what makes silk so precious. What used to be done in painstakingly manual work during silk production is at least partially done by machine today.
To free the silk from its yellowish glue, it is cooked in soapy water. This is called de-peeling or de-gumming. After this process the silk appears whitish and is thinner, glossier and more supple. Finally, further chemical treatment processes to finish silk are possible, for example by sulphur dioxide, which bleaches the silk.
