SERICULTURE � SILKEN THREADS
In those days the knowledge of the Silk worm and its culture and processing were secrets as closely guarded as that of the origin of the spices. For silk is Queen of all the fibres that puts our crudely fashioned nylons and rayon�s with the accompanying pollution and waste firmly at the bottom of the table, for we are no match for Nature with our crudely fashioned molecules.
This short Monograph is purely an introduction to ancient knowledge of the natural world. It is compiled from 3 sources:
1.A Textbook of Pharmacognosy. T.C.Denston B. Pharm (Lond.) F.R.I.C. F.P.S.
2. Serriculture. H.F.Macmillan. F.L.S. A.H.R.H.S.
3. Enquire Within 113th Edition.
Towards the end of the larval or caterpillar stage of their life the silk moths, like most other moths, cease feeding, and each spins around itself a covering of filaments called a cocoon. The latter provides protection during the pupa stage, during which metamorphosis takes place � the biting mouth-parts of the caterpillar being converted to a sucking tube or proboscis, the wings developing, and many other remarkable changes taking place. In its final stage the chrysalis possesses spines or hooks by which it works its way out of the cocoon, emerging as the silk moth.
The larva possesses two, large, modified salivary glands, with a common exit from which the secretion emerges. On contact with air, this secretion solidifies as two solid threads, cemented together by a thin covering of sericin, or "silk-glue." Sericin causes silk to feel harsh and stiff but may be removed by boiling water.
The cocoon is dried in a warm oven to kill the chrysalis. The natural gum of the filaments is then softened and partly removed in hot water, and the filaments then wound off. A number of these are twisted together to form a single thread, and this raw silk is made up into hanks, for processing into fabrics.
Constituents : Raw silk consists of fibroin, about 65 per cent, sericin, about 22 per cent, moisture, about 11 per cent, with about 1 per cent of mineral and colouring substances. The sericin is removed by boiling in soap solution; the process is known as "degumming." "Degummed" silk consists mainly of the protein fibroin, distinguished from the protein of wool by the absence of sulphur.
MATERIALS REQUIRING MICROSCOPICAL EXAMINATION
Examine silk thread as follows �
(a) The fibres are mostly smooth; some exhibit fine longitudinal striations. Here and there fragments of the sericin sheath may be adherent.
(b) The fibres are solid, structureless rods, varying in thickness from about 5 to 40/4, according to the species and position in the cocoon.
2.Apply the tests described as for Cotton, and observe the distinctions from cotton and wool. Silk gives negative cellulose reactions, slowly dissolves in warm caustic soda solution, readily dissolves in hydrochloric and sulphuric acids, and is permanently stained with picric acid. It is coloured light-brown by iodine solution and dissolves in cuoxam.
3.Warm a little silk with Mercury Nitrate Solution (Millon's Reagent) and observe the red colour produced, due to the protein character of silk.
4.Ignite a few silk threads. The fibres burn relatively slowly with an objectionable odour and leave a large carbon ash. Repeat with artificial silk, which ignites rapidly and leaves little ash due to its cellulosic character.
Oiled Silk� Sericum Oleatum. This is a pure silk fabric, with a count of 120 by 85, treated with a suitable drying oil or oil-modified synthetic resins to render it completely waterproof. It may be coloured green with a suitable dye.
The silk of commerce, as is well known, is obtained from the cocoons formed by the " worms " (caterpillars) of certain moths, which in some countries, as in China, Japan, India, parts of S. Europe, etc., are raised in great numbers from " seed " (eggs). The worms are reared in light, airy sheds, on tiers of trays, and fed on the leaves of certain plants which they eat voraciously. There are numerous species or varieties of silk-producing worms, among the best known being :
(1)" Mulberry " silkworm (Bombyx mori), which feeds chiefly on the Mulberry leaves and is suited mostly to warm temperate countries ;
(2) " Eri " (Attacus ricini), a multivoltine worm (i.e. it undergoes from 6-8 generations in a year) which feeds on the Castor-oil plant and is adapted to the tropics;
(3) the Indian " Tasar " or " Tussar " (Antherea paphia), a bivoltine worm (yields 2 crops or generations a year) which feeds on various plants, as species of Cassia, Zizyphus, Lagerstroemia, Eugenia, Terminalia, etc.
The Chinese "Tasar" (Anthereae pernyi) is bivoltine and feeds on species of Oak. The worm of the " Atlas-moth " (Attacus atlas), which is common in Ceylon, also produces a good second-rate silk and is omnivorous. The "Mulberry" kind produces the finest silk, and is reared extensively in certain temperate countries as well as in India. Of this there are numerous varieties, all of which are univoltine (i.e. one generation in the year). The Eri and Indian Tasar worms are especially valued in India, while the Chinese Tasar is largely reared in China.
The silk-moth lays its eggs on a leaf or twig, to which they adhere by means of a gummy substance. The number of eggs laid by each moth varies from 250 to 600 or more, according to species or variety. These are hatched out on large trays of hessian, arranged as already stated. The caterpillars resulting are voracious feeders and require to be fed twice daily with leaves of trees on which they naturally feed. In 2-4 weeks, according to climate and variety of worm, they begin to spin. Before commencing to spin, they fast for 2 or 3 days, void their excrements, become soft and flaccid, and then proceed to construct their cocoons.
The spinning of the latter occupies from 3-5 days. The length of the silk-thread in a cocoon varies according to kind from 900 to 1,500 yards. or more, and cocoons weigh at the rate of about 200 � 300 or more to the pound, according to size, variety, etc. The process of spinning is effected by passing the thread through two small holes (spinnerets) in the head of the worm, which by a combined movement of the mouth and front legs unites the filament into one, binding it closely together by a gummy liquid. The cocoon finished, the worm undergoes metamorphosis, i.e. changes into the chrysalis or pupa state. In about 3 weeks, in the tropics, the moth under normal conditions emerges by forcing its way through the cocoon. The males appear first, then the females, the former being the more active in their movements; they proceed at once to pair, and both usually live but 4 or 5 days, the females meantime laying their eggs, thus completing the life-cycle.
In silk culture, however, the moth is not usually permitted to emerge from the cocoon, as in doing so the latter is seriously damaged, both by the act of piercing and by the dirt and stain left behind. Therefore to obviate this the pupae are destroyed by heating the cocoons in an oven or by steaming. Indispensable conditions for successful sericulture are cheap labour and a plentiful supply of suitable food for the worms. In Italy, Japan, China and other silk-producing countries, it is essentially a cottage industry. Efforts have been made from time to time to establish sericulture as an industry in Ceylon, it being considered an occupation adapted to the peasants, but these have hitherto failed, due partly to the necessity for " destroying life " in the cocoons, which constitutes an offence against the Buddhist religion.
Yield. Both the Mulberry (Morus indicus) and Castor-oil plant (Ricinus
cormmunis) flourish in the tropics at various elevations, the former
being readily propagated by cuttings, and the latter from seed and often
found in a wild state. Planted at about 4 x 5 ft., an acre should furnish
sufficient food for about 600,000 worms. The Eri worm will produce six
generations of 100,000 each in a year. Each of these, allowing for deaths,
will furnish about 450,000 cocoons, which should weigh about 300 lb. In 1928
these realised 3s.� 4s. or more per lb., or about �45-�50 per acre. The
value of the cocoons depends largely on colour, " whites " being
generally preferred. Italian silk is the highest priced in the European
market, the cocoons being sometimes worth as much as 18s. or more per lb.
3.Enquire Within (113th Edition.)
2232. SILKWORMS. � The silkworm is the larva of a moth which spins its silk in forming its cocoon, when about to pass from the state of the caterpillar into that of the chrysalis. It comes out of the egg about the latter part of May, and as the worms will confine themselves to those places whore food is provided for them, the rapid progress of their growth, their curious changes, and the production of their silk, afford a most interesting study. We recommend our readers to obtain from twenty to thirty silkworms' eggs. They are about the size of a pin's head, and are generally firmly attached to the paper upon which they were laid. A paper tray about twelve inches long by eight wide, should be made by turning up the edges of a piece of cardboard, or of stiff paper. Into the bottom of this the eggs should be placed, and when the time for hatching arrives, they should be watched from day to day, and some young lettuce leaves be provided for the young caterpillars.
These, when first hatched, do not exceed a quarter of an inch in length, and commence eating food immediately. Their growth is very rapid, and when they are about eight days old, their heads become considerably enlarged, they refuse food, and appear in a lethargic state for about three days. This arises from the pressure of their skins, which become too tight for the increased size of their bodies. As soon as they have east their skins, they will be re-invigorated, and eat a large amount of leaves. They cast their skins four times in the course of their growth. About the time of the second change of skin, they should be provided with mulberry leaves as well as lettuce leaves, and they will gradually discard the latter. The worm remains in the caterpillar state about six weeks.
When full grown it ceases to feed, and begins to form a loose envelope of silken pipes. It should then be taken from the paper tray, and each worm be placed in a cup of twisted paper, hung against the wall or in a warm aspect; when it will enclose itself in a ball of silk, called a cocoon, within which it passes into the chrysalis state. In about fifteen days it comes forth in the form of a moth. In escaping from the cocoon it destroys a portion of the silk, to prevent which the silk-dealers destroy the chrysalis, or unwind the silk of the cocoon before the chrysalis is broken by the moth.
2233. Silkworm gut. � From the full-grown larvae is also obtained the silkworm gut used for fishing casts, &c. The larvae are steeped in strong acetic acid, the silk glands are separated from the bodies and the clear, viscous fluid drawn out and pinned on a board, when, under exposure to sunlight, it hardens into gut.
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