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INFORMATION
SILK History
The discovery of the product silk from the silkworm species Bombyx mori
occurred around 2700 BC. According to Chinese tradition the bride of Emperor
Huang Ti, a 14-year-old girl called Hsi Ling Shi, discovered the invention
of the first silk reel. Sericulture, the cultivation of the silkworm,
spread through China making silk a highly valued commodity much sought
after by other countries. In 139 BC the world's longest trade route was
opened stretching from Eastern China to the Mediterranean Sea. It was
named the Silk Road after its most valuable commodity. By 300 AD the secret
of silk production had reached India and Japan.
Silk manufacture eventually reached Europe and America. During the 18th
and 19th centuries Europeans produced several major advancements in silk
production. By the 18th century England led Europe in silk manufacturing
because of English innovations in the textiles industry. These innovations
included improved silk-weaving looms, power looms and roller printing.
Between 1855 and 1865 an epidemic called Pebrine disease, caused by a
small parasite, raged through the industry. It was the French scientist
Louis Pasteur who discovered that this could be prevented through simple
microscopic examination of adult silkmoths. Much research was carried
out on silkworms at this time, ultimately setting the stage for a more
scientific approach to silk production. Silk production today is a combination
of old and modern techniques.
Bombyx Mori
Silkworm is a common name for the silk-producing larvae of any of several
species of moths. The larva is not really a worm at all but a caterpillar.
There are several species of silkworm that are used in commercial silk
production, however Bombyx mori is the most common.
Bombyx mori is native to China and was introduced into Europe and western
Asia in the 6th century AD and into North America in the 18th century.
It feeds entirely on the leaves of the mulberry tree, so has flourished
only where conditions are suitable for large numbers of leaf-bearing mulberry
trees. Bombyx mori has been cultivated over many centuries and is no longer
known in the wild.
Silk - A Hardened Glandular Fluid
Silkworms possess a pair of specially modified salivary glands called
sericteries, which are used for the production of a clear, viscous, proteinaceous
fluid that is forced through openings called spinnerets on the mouthpart
of the larva. As the fluid comes into contact with the air it hardens.
The diameter of the spinneret determines the thickness of the silk thread,
which is produced as a long, continuous filament.
Typical Commercial Silkworm Production
The first stage of silk production is the hatching of the silkworm eggs,
in a controlled environment such as an aluminium box, which are then examined
to ensure they are free from disease. The female deposits 300 to 400 eggs
at a time. In an area the size of this page around 50 moths would deposit
more than 20,000 eggs, each about the size of a pinhead. The female dies
almost immediately after depositing the eggs and the male lives only a
short time after. The adult possesses rudimentary mouthparts and does
not eat during the short period of its mature existence.
These disease-tested eggs are raised in temperature and disease-controlled
conditions. They are fastened to a flat surface by a gummy substance secreted
by the female. The larvae hatch in about 10 days and are about 0.6cm long.
Once hatched, they are placed under a layer of gauze and fed huge amounts
of chopped mulberry leaves during which time they shed their skin four
times. The larvae may also feed on Osage orange or lettuce. Larvae fed
on mulberry leaves produce the very finest silk. The larva will eat 50,000
times its initial weight in plant material.
After it has achieved its maximum growth of 7.5cm at around 4 to 6 weeks,
it stops eating, changes colour and attaches itself to a compartmented
frame, twig, tree or shrub in a rearing house to spin a silk cocoon over
a 3 to 8 day period. This is period is termed pupating.
A Hard Day¡¯s Night
Steadily over the next four days the silkworm produces a fine thread by
making a figure of eight movement some 300,000 times, constructing a cocoon
in which it intends to spend the chrysalis stage where it is in a state
of sleep and casting off its skin. After this the pupae begin the sixteen
days that would normally result in the miracle of transformation to a
winged being - the moth. However, if the pupa (chrysalis) remains alive
it will begin to secrete an alkali, which eats its way through the cocoon,
ruining the silk threads. Therefore during the commercial production of
silk, only enough adult moths are allowed to emerge to ensure continuation
of the species. Most of the remainder of the silkworms are killed by heat,
e.g. immersion in boiling water, steaming or drying in an oven.
Hundreds Die
The amount of useable silk from each cocoon is small. One hectare of mulberry
trees yields 11.25 tonnes of leaves, producing around 200kg of cocoons,
but just 40kg of raw silk. The silk yield is many times smaller than this
in countries such as Thailand, where the silk is reeled by hand rather
than by machine. So it takes hundreds of tiny lives to produce just one
silk scarf or tie.
Stud Bank & Breeding Research
A limited number of pupae are allowed to complete their chrysalis stage,
the resulting silk moths being the stud bank that produces eggs to breed
future generations of silkworm.
Much research has been carried out on silkworm breeding or sericulture
worldwide over centuries and it continues today. Researchers are keen
to establish a certain type of silkworm variety typically for low cost
cocoons, disease resistance, high temperature resistance, polyphagy (ability
to utilise more than one type of food), and choice quality silk. As with
other types of animal farming industries, biotechnology is well established.
Thai Silk Production
The Thai silk moth is adapted to tropical conditions and is multivoltine,
producing at least ten batches of eggs each year. Silk from the Thai moth
is hand reeled from green cocoons. These are cocoons that still contain
the live pupae. These small cocoons do not have the pupae 'stifled' or
killed prior to the thread being unwound, as that would make it difficult
to reel. The green cocoons are placed in hot, nearly boiling water, which
loosens the end of the thread. With less than 10 days available before
the moths emerge and ruin the cocoon, the Thai workers may run out of
time, limiting the scale of the industry. Even experienced workers rarely
produce more than 300g per day. The pupae may be eaten by local workers.
Similar production techniques have been used in India, although increasingly
Indian silk is woven using machines.
Chinese & Japanese Silk Production
The moth favoured in China and Japan is univoltine or bivoltine, producing
one or two batches of eggs annually, which enter a diapausal state (suspended
development) and can then be treated to induce hatching at a commercially
convenient time. Cocoons are large and lend themselves to machine reeling,
offering a long, continuous filament. The adult moths retained for reproduction
purposes are too fat to fly, as the best fliers do not produce as much
silk. The stifled or dead pupae are usually composted to feed the mulberry
trees.
Whilst the tropical silkworm favoured by Thailand is a natural, hardy
creature quite capable of surviving in the wild state, the larvae farmed
by the industrialised mass production techniques of China and Japan are
selectively bred creatures aimed at maximum output for minimum input.
The Silk Thread
Silk is a continuous filament fibre consisting of fibroin protein secreted
from two salivary glands in the head of each larva and a gum called sericin
which cements the two filaments together. Silk must be reeled off the
cocoon quickly before the pupa begins to rot and taint the thread with
unpleasant smells. The cocoon is then softened in hot water to remove
the sericin, which frees the silk filament ends for reeling or filature.
Single filaments are drawn from cocoons in water bowls and combined to
form yarn. This yarn is drawn under tension through several guides and
eventually wound onto reels. The yarn is dried, packed according to quality
and is now raw silk ready for marketing. The worn and withered body of
what was intended to become the wondrous flying creature slips silently
away.
Fibre Properties
Silk is a natural protein fibre containing about 70-75% of actual fibre
fibroin and about 25-30% sericin. Silk filaments are very fine and long
- as much as 300 to 900 metres in length. Silk has a high natural lustre
and sheen of a white or cream colour; and is one of the strongest fibres
at 2.6 to 4.8 grams per denier. When it is dry the elongation (elastic
recovery) varies from 10-25% and when wet it will elongate as much as
33-35%. Silk has a relatively high standard moisture regain of 11%. At
saturation the regain is 25-35%. Silk can be dyed before or after it has
been woven into a cloth. It can be woven or knitted.
Types of Silk
Next, the raw silk is twisted into a strand sufficiently strong for weaving
or knitting. This procedure is called throwing, and prevents the thread
from splitting into its constituent fibres. Four different types of silk
thread may be produced from this procedure: crepe, tram, thrown singles
and organzine. Crepe is made by twisting individual threads of raw silk,
doubling two or more of these together, and then twisting them again.
Tram is made by twisting two or more threads in only one direction. Thrown
singles are individual threads that are twisted in only one direction.
Organzine is a thread made by giving the raw silk a preliminary twist
in one direction and then twisting two of these threads together in the
opposite direction. In general, organzine thread is used for the warp
threads of materials, tram threads for the weft or filling, crepe thread
for weaving crinkly fabrics and a single thread for sheer fabrics.
Broken or waste filaments and damaged cocoons are retained, treated to
remove the sericin, and combed. This is then processed into yarn, marketed
as spun silk, which is inferior in character to the reeled product and
much cheaper. Low grade silk is made from damaged cocoons that were spoiled
by emerging moths used for breeding stock. Filaments from the coarse outer
portion of the cocoon, which is removed by brushing before reeling, and
the inner portion of the cocoon, which remains after reeling the raw silk,
are mixed with silk from damaged cocoons to make low grade silk.
In common with all other animal production systems, nothing is wasted
if it can be sold.
After the silk is harvested from the cocoons it is brought to the weavers
for dyeing and preparation for weaving. Today most dyes are chemical although
a lac (insect) dye was once used as well as plant dyes.
Another product of sericulture is silkworm gut. Immediately before the
cocoon stage, pupae are killed by immersion in an acid bath. Their bodies
are opened and the fluid, which hardens upon contact with the air and
would otherwise be used to build the cocoon, is removed from their silk
glands. This 'gut' was once favoured by surgeons for stitching and by
anglers for lines, but has now been almost entirely replaced by nylon,
although it still figures in some surgical and contraceptive applications.
World Silk Production
World silk production has more than doubled during the last 30 years in
spite of the availability of man-made fibres. China and Japan have been
the main silk producers, together manufacturing more than half of the
world production each year. Chinese silk is highly prized throughout the
world. Since 1949 silk making methods have been modernized and silk is
of better quality.
World Raw Silk Production
Year Metric Tonnes
1938 56, 500
1970 41, 000
1980 55, 315
1985 59, 232
1990 69, 120
1992 80, 934
1993 89, 982
1994 95, 498
1995 91, 476
1996 83, 670
Say No to Silk
Silk is used for suits, coats, trousers, jackets, shirts, ties, lingerie,
hosiery, gloves, lace, curtains, linings and handbags. Synthetic fibres
such as nylon and polyester are stronger than silk and lower in price.
In common with western factory farming techniques, the main areas of silk
production are labour-intensive, automated and soul-less. The terminology
- stifled for killed and crop for pupae - echoes the denial that we are
dealing with living creatures which are awe inspiring when one considers
their metamorphic life-cycle. Plant fibres are capable of producing some
amazing fabrics. Fibres from the pineapple, for example, may be made into
fabrics as strong and lustrous as any silk. Synthetic fibres e.g. nylon
produced from minerals, polyester from petroleum spirit (Terylene, Dacron)
or acrylic from oil and coal (Courtelle, Orlon, Dralon) also have their
place.
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