The History and manufacturing of Silk
History of Silk
The history of silk is shrouded in the mists of time and commercial secrecy. Silk production started in China around 5000 years ago where the bombyx mori silk moth and the white mulberry, its food, coexisted. The most common story about the start of silk production is that told by Confucius of the Empress Xi Ling-Shi, the Lady of the Silkworms, who is reputed to have accidentally dropped a silk cocoon into a cup of hot tea in around 2640BC. When she retrieved it, out came a silken thread (history doesn't say what the tea tasted like!). She was so enamoured with its delicate beauty that she encouraged the development of silk as a textile.
Silk was used in China and exported along the Silk Road (the ancient trade route linking China and the Roman Empire). This trade brought China great wealth, but the Chinese did not give away the secret on how silk was formed.
The spread of silk and the mulberry tree is a fascinating story. Chinese silks were being sold in Japan and Asia, where the Romans first discovered this lustrous fabric, in the 3rd Century BC. Julius Caesar liked it so much that he restricted the wearing of silk to himself and his favoured officials. China maintained a monopoly for around 2500 years until the secrets of sericulture were reportedly smuggled out by Persian monks who also carried silkworm eggs in their hollow pilgrim canes. From there it travelled to Persia, Japan, Korea, India, Greece and much later, around the 10th century, to Spain and, in the 12th century, to Italy. By the 15th century the French, in the form of René, Duke of Provence, had imported from Italy the techniques and know how to produce silk. Lyons became an important centre for silk production and a succession of French Kings encouraged the industry.
Eventually silk production spread around Europe to Germany and England. James I was the first monarch to try and establish a silk industry in England. He had a good start in that the mulberry tree was already well established in England. James had seen how silk had helped create the prosperity of Florence, Venice and Lyons and invited French weavers to come to England. Production was centred mainly around London at Spitalfields, Bethnel Green, Whitechapel and Stepney.
Latterly, China produces around 50% of the raw silk world production of about 56000 tonnes (1985) with Japan, India, Russia, Korea and Brazil being major producers. Silk is still produced in England at Lullingstone Silk Farm in Dorset so some of the tradition goes on.
Silk has wonderful properties of movement, flow and luxurious sheen which come from its smooth, continuous, triangular fibres which reflect light. It is also a very strong, elastic fibre which can stretch 20-25% without breaking and will stretch back; that is, it has a memory. Once woven, it can have a wide variety of appearances and feel from delicate, translucent chiffon gauze to robust, opaque fabrics such as dupion. If you want to check whether a fabric is, in fact, silk you can burn a small piece. It should not burn easily and leaves a shiny black residue which crumbles to ash if you rub it between your fingers.
The weight of the yarn and the type of weave will naturally affect how silk dye flows and the techniques which can be used successfully. There are three main types of weave, plain, twill and satin.
The numbers seen after a description of a silk eg habutai 9 refer to the weight of the silk. It is only necessary to know that the higher the number the heavier the silk. So a pongee 4 is a light, fine silk and a crepe charmeuse 16 is a heavier weighted soft satin feel silk.
Silk therefore is the most luxurious, comfortable, absorbent fabric which is wonderful for draping and colouring. It has hypoallergenic qualities, offering resistance to mildew and moth attacks, and is by definition the strongest natural fabric in the world. It is naturally cool in summer and warm in winter by letting the skin breathe and adapting to changes in temperature. It contains about 18 kinds of amino acids, and is unrivaled by any other fibre.
At Sleeping Patterns our silk quilt products are constructed of the finest 100% first grade Mulberry silk floss, the highest quality silk available worldwide
Spider silk is incredibly tough and is stronger by weight than steel. Quantitatively, spider silk is five times stronger than steel of the same diameter. It has been suggested that a Boeing 747 could be stopped in flight by a single pencil-width strand and spider silk is almost as strong as Kevlar, the toughest man-made polymer. It is finer than the human hair (most threads are a few microns in diameter) and is able to keep its strength below -40°C. The toughest silk is the dragline silk from the Golden Orb-Weaving spider (Nephilia clavipes), so-called because it uses silk of a golden hue to make orb webs.
Spider silk is also very elastic and capture silk (sticky silk for catching prey) remains unbroken after being stretched 2-4 times its original length. Spider silk is tougher, more elastic and more waterproof than silkworm silk so it could have a much wider range of applications. It is simple to see why spider silk is of such interest to materials chemists since new ultra-strong fibres based on the silk could be developed.
How silk is made
Silk is produced in the wild by a variety of insects such as the Cecropia moth from North America, the Tussah, Muga and Eri moths from India, the Anaphe moth from Africa and indeed by some spiders! However, commercial production is dominated by the Mulberry Silk Moth, Bombyx Mori, literally the cocoon fed on mulberry.
The natural cycle of silkworm production follows the production of leaves by the mulberry, so the tiny pinhead sized eggs, left to hibernate by the previous year's moths, are warmed up gradually when the mulberry starts to bud in spring. They should then be ready to hatch into baby caterpillars 10 - 14 days later. Their lives are dominated by food; they eat ravenously, shedding their skins four times within a period of four weeks, moulting as they outgrow each skin. After the last moult the silkworm is 7 to 10cm long, fat, hairless and deeply unattractive!
At this point their appetites are prodigious with some 25g of eggs (around 36,000 worms) requiring as much as a ton of foliage.
After all this gluttony the silkworm stops eating and starts the production of its cocoon suspended on the twigs or straw provided. Two modified salivary glands, or sericteries, on the caterpillar's head produce a clear, sticky liquid which is then forced out through spinnerets and hardens on contact with the air to form a continuous filament. Working in a figure of eight the caterpillar constructs the cocoon which is held together with Sericin, a gummy substance, which literally sticks the threads together and hardens to form a distinctive shape.
During this period excessive heat or moisture have to be avoided or the hardening and therefore the quality of the silk being produced, can be adversely affected.
It takes around 10 days for the moth to metamorphose from the caterpillar. It then produces a liquid which dissolves the gum holding the threads of the cocoon together to allow the ash coloured moth to emerge, usually at dawn. The male and female moths now mate and the female lays her eggs, which these then need a period of cold for hibernation and the cycle starts again.
Unfortunately the very process of the moth pushing its way out of the cocoon destroys the continuity of the thread and makes the cocoon commercially useless except for the production of spun silk. So, most moths are killed before they emerge and only a small proportion are allowed to exit normally and mate to produce the eggs for the next round of production.
The producers now have completed cocoons which are ready for reeling. Each cocoon is normally made up of one extremely fine filament. The cocoons are placed in nearly boiling water to soften the gum and release the end of the filament which is then combined with the filaments from a number of other cocoons, depending on the thickness required for the final thread. For instance, a thread for weaving would typically be made from 8-10 filaments. This process of combining several continuous filaments to form a thread is called throwing from the Anglo Saxon 'thraw' meaning to whirl or spin. This distinguishes it from spinning which is the combining of short lengths of silk fibres by combing and then twisting them together in much the same way as wool.
Any remaining processes depend on the use to which the fibre is to be put but can include boiling off to remove the sericin, dying, finishing, weaving or knitting.
There have been many changes in the commercial production of silk over the years, to increase the number of cycles that can be achieved in a given period. The yield of leaves from a mulberry tree or bush has been greatly increased, the surplus being dried and converted into a sort of artificial silkworm food which allows silkworms to thrive when the mulberry tree is not in leaf. Methods of fooling the silkworms into thinking that they have had a period of hibernation have also been developed and production can now continue all year round.