SERICULTURE
Sericulture, the production of raw silk by
means of raising caterpillars (larvae), particularly those of the domesticated
silkworm (Bombyx mori).
The production of silk generally involves
two processes:
Care of the silkworm from the egg stage
through completion of the cocoon.
Production of mulberry trees that provide
leaves upon which the worms feed.
The silkworm caterpillar builds its cocoon
by producing and surrounding itself with a long, continuous fibre, or filament.
Liquid secretions from two large glands within the insect emerge from the
spinneret, a single exit tube in the head, hardening upon exposure to air and
forming twin filaments composed of fibroin, a protein material. A second pair
of glands secretes sericin, a gummy substance that cements the two filaments
together. Because an emerging moth would break the cocoon filament, the larva
is killed in the cocoon by steam or hot air at the chrysalis stage.
Silk is a continuous filament within each
cocoon, having a usable length of about 600 to 900 metres (2,000 to 3,000
feet). It is freed by softening the binding sericin and then locating the
filament end and unwinding, or reeling, the filaments from several cocoons at
the same time, sometimes with a slight twist, forming a single strand. Several
silk strands, each too thin for most uses, are twisted together to make
thicker, stronger yarn in the process called throwing, producing various yarns
differing according to the amount and direction of the twist imparted.
Silk containing sericin is called raw silk.
The gummy substance, affording protection during processing, is usually
retained until the yarn or fabric stage and is removed by boiling the silk in
soap and water, leaving it soft and lustrous, with weight reduced by as much as
30 percent. Spun silk is made from short lengths obtained from damaged cocoons
or broken off during processing, twisted together to make yarn. The thickness
of silk filament yarn is expressed in terms of denier, the number of grams of
weight per 9,000 metres (9,846 yards) of length. Silk is sometimes—in a process
called weighting—treated with a finishing substance, such as metallic salts, to
increase weight, add density, and improve draping quality.
The degumming process leaves silk lustrous
and semitransparent, with a smooth surface that does not readily retain soil.
Silk has good strength, resisting breakage when subjected to weights of about 4
grams (0.5 ounce) per denier. Wetting reduces strength by about 15–25 percent.
A silk filament can be stretched about 20 percent beyond its original length
before breaking but does not immediately resume its original length when
stretched more than about 2 percent. Silk, lower in density than such fibres as
cotton, wool, and rayon, is moisture-absorbent, retaining as much as a third of
its weight in moisture without feeling damp, and has excellent dyeing
properties. It is more heat-resistant than wool, decomposing at about 170° C
(340° F). Silk loses strength over a long period of time without appropriate
storage conditions and tends to decompose with extensive exposure to sunlight
but is rarely attacked by mildew. It is not harmed by mild alkaline solutions
and common dry-cleaning solvents. Friction imparts a static charge, especially
in low humidity. The rustling sound, or scroop, associated with crisp silk
fabrics is not a natural property of the fibre but is developed by processing
treatments, and it does not indicate quality, as is sometimes believed.
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