This secreted protein hardens upon exposure to the air. The resulting filament is bonded by second secretion, sericin, which forms a solid sheath or cocoon. Under natural conditions, a moth eventually breaks through the cocoon. In sericulture, the larva is killed in the cocoon by steam or hot air in the chrysalis stage before its metamorphosis. Sustained heat processing softens the hardened sericin so that the filament can be unwound.
Single filaments are too thin for utilization.
For production purposes, several filaments are combined with a slight twist into one strand. The international demand for high quality silk has multiplied. Appropriate cocoon-drying techniques and reeling operations are vital to supply good quality silk. Although new technologies are applied among major silk producing countries in Asia, many newcomers to the industry including non-traditional producers have not been transitioned to the methods that ensure high quality silk for export. World production of raw silk reached nearly one hundred thousand tonnes in 1993.