16.2.Production and Properties of Natural Fibers 335 ter about 6 weeks and four moltings,it stops eating,shrinks somewhat,and its head makes restless rearing movements,in- dicating a readiness to spin the cocoon.The silkworm is then transferred into a compartmentalized tray or is given twigs.There it spins at first a net in whose center the cocoon is spun around the silkworm.After 3 days,during which time the filament is wound in a figure-eight pattern,the completed cocoon has the shape and size of a peanut shell. The silk substance is produced by two glands and is discharged through a spinneret,a small opening below the jaws.The spin- neret is made up of several chitin plates which press and form the filament.The filament (called bave)actually consists of two strands(called brins)that are glued together and coated by silk gum(sericin),which is excreted by two other glands in the head of the silkworm.The liquid substance hardens immediately due to the combined action of air exposure,the stretch and pressure applied by the spinneret,and to acid that is secreted from still another gland.Under normal circumstances,the chrysalis inside the cocoon would develop into a moth within 2 weeks and would break through the top by excreting an alkaline liquid that dis- solves the filament.Male and female moths would then mate within 3 days and the female would lay 400-500 eggs,after which time the moths would die.The life cycle is,however,generally interrupted after the cocoon is spun by applying hot air or boil- ing water(called stoving or stifling)except in limited cases when egg production is desired.The filaments of 2-7 cocoons are then unwound (called reeling)in staggered sequence to obtain a ho- mogeneous thread strength;see Plate 16.1.The usable length of the continuous filament is between 600 and 900 meters.Shorter pieces are utilized for spun silk.It takes 35,000 cocoons to yield 1 kg of silk.[Note in this context the silk fibers depicted in Fig- ure16.2(b)]. The raw silk is usually degummed to improve luster and soft- ness by boiling it in soap and water,which reduces its weight by as much as 30%.(Sericin is soluble in water whereas fibroin is not.)The silk is subsequently treated with metallic salt solutions (e.g.,stannic chloride),called weighting,which increases the mass (and profit)by about 11%and adds density.Excessive weighting beyond 11%causes the silk to discolor and decom- pose.Likewise,dying adds about 10%weight.Silk fabric treated with polyurethane possesses excellent wet wrinkle recovery and dimensional stability during washing.Silk is more heat-resistant than wool (it decomposes at about 170C);it is rarely attacked by mildew but degrades while exposed extensively to sunlight. Silk can adsorb large quantities of salts,for example during per-ter about 6 weeks and four moltings, it stops eating, shrinks somewhat, and its head makes restless rearing movements, in￾dicating a readiness to spin the cocoon. The silkworm is then transferred into a compartmentalized tray or is given twigs. There it spins at first a net in whose center the cocoon is spun around the silkworm. After 3 days, during which time the filament is wound in a figure-eight pattern, the completed cocoon has the shape and size of a peanut shell. The silk substance is produced by two glands and is discharged through a spinneret, a small opening below the jaws. The spin￾neret is made up of several chitin plates which press and form the filament. The filament (called bave) actually consists of two strands (called brins) that are glued together and coated by silk gum (sericin), which is excreted by two other glands in the head of the silkworm. The liquid substance hardens immediately due to the combined action of air exposure, the stretch and pressure applied by the spinneret, and to acid that is secreted from still another gland. Under normal circumstances, the chrysalis inside the cocoon would develop into a moth within 2 weeks and would break through the top by excreting an alkaline liquid that dis￾solves the filament. Male and female moths would then mate within 3 days and the female would lay 400–500 eggs, after which time the moths would die. The life cycle is, however, generally interrupted after the cocoon is spun by applying hot air or boil￾ing water (called stoving or stifling) except in limited cases when egg production is desired. The filaments of 2–7 cocoons are then unwound (called reeling) in staggered sequence to obtain a ho￾mogeneous thread strength; see Plate 16.1. The usable length of the continuous filament is between 600 and 900 meters. Shorter pieces are utilized for spun silk. It takes 35,000 cocoons to yield 1 kg of silk. [Note in this context the silk fibers depicted in Fig￾ure 16.2(b)]. The raw silk is usually degummed to improve luster and soft￾ness by boiling it in soap and water, which reduces its weight by as much as 30%. (Sericin is soluble in water whereas fibroin is not.) The silk is subsequently treated with metallic salt solutions (e.g., stannic chloride), called weighting, which increases the mass (and profit) by about 11% and adds density. Excessive weighting beyond 11% causes the silk to discolor and decom￾pose. Likewise, dying adds about 10% weight. Silk fabric treated with polyurethane possesses excellent wet wrinkle recovery and dimensional stability during washing. Silk is more heat-resistant than wool (it decomposes at about 170°C); it is rarely attacked by mildew but degrades while exposed extensively to sunlight. Silk can adsorb large quantities of salts, for example during per- 16.2 • Production and Properties of Natural Fibers 335