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silk farming

The production of silk is a process that starts with a silk moth (fig.) laying its eggs. When these hatch, the tiny caterpillars (fig.) are fed on cut-up White Mulberry (fig.) leaves. In about three weeks after hatching they start to spin their silky cases, called rang mai dip in Thai, in one continuous thread, by producing silk in two salivary glands in their head, a process that lasts around 3 days to a week, depending on the silkworm's strain and climate conditions. In sericulture, the pupae are usually placed in a krajo, a traditional breeding basket arranged in concentric circles (fig.), or in a brush-like arrangement of sticks on a string, which in China is known as cao long (草龙), literally a ‘straw dragon’ (fig.). The silkworm produces the silk as a liquid secretion known as fibroin which is cemented together with a viscous gum-like substance called sericin. These substances harden when coming in contact with the air. There are two kinds of cocoons, depending on the silkworm's gender, i.e. white (to light green) and yellow cocoons (fig.). White cocoons are made by the male silkworm, have a long and smooth thread and are especially used to make crisp, smooth woven silk fabrics, like taffeta, whereas yellow cocoons are produced by the females and have a shorter thread which is rather rough. As soon as the cocoons are completed the chrysalises are killed by heat, in Thailand usually by exposing them to the sun for a couple of days, but elsewhere, in places with less sun and lower temperatures, they are either killed in an oven or by boiling them whilst simultaneously unwinding the cocoons. Anyhow, it should be done before the pupa transforms into a moth, as the moth will secrete a fluid that dissolves the silk, enabling it to emerge but thus damaging the cocoon. Some pupae are allowed to undergo the metamorphosis into moths which are used for further breeding. Each cocoon contains about 900 meters of raw silk thread. But, 48 single filaments of raw silk need to be combined to form yarn strong enough for weaving and not all raw silk is of suitable quality. That is to say, each cocoon has  three layers which vary in quality and character: the first layer is known as the outer floss and makes up about 10% of the cocoon's weight, and although it is formed of a continuous filament, it is more textured and not always suitable to be reeled; the next part or middle compact layers of the cocoon, also known as the shell, has a continuous filament which is smoother than that of the outer part and is easily reeled; the next part is the innermost layer of the cocoon, next to the chrysalis and known as the pelade or inner pelade, a thin membrane-like layer which cannot be reeled. As a result, up to 5,500 cocoons may be required to produce just one kilogram of same quality silk. Therefore, a selection takes place, eliminating damaged or imperfect cocoons (fig.), such as those with holes, spoiled ones, etc. Raw silk is obtained by boiling the cocoons in hot water (fig.). This removes the viscous gum-like sericin, thus releasing the silk filaments. After brushing  the cocoons, to find the outside ends of the filament, the threads are pulled over a puang sao (fig.), onto a nai (fig.), ready for dyeing and looming (fig.). Once the cocoons are completely unwound, inner pelades and carcasses of dead silk pupae (fig.), called dakdae or (tua) mai in Thai, are the leftovers. These pupae are edible and considered a delicacy by some. In China, they are even sold impaled on skewers at food markets, a street snack known as zha can yong (fig.). At present, China and Japan are the two main producers of silk, together annually manufacturing more than half of the world's production.