Weighting of silk

7/28/2019 Weighting of silk

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Silk

16 06 2010

(http://oecotextiles.files.wordpress.com/2010/06/boudoir-360.jpg)Silk has set the standard in luxuryfabrics for several millennia. Silk is highly valued because it possesses many excellent properties. Not

only does it look lustrous and feel luxurious, but it is also lightweight, resilient, and extremelystrong the strongest natural fiber known to man, one filament of silk is stronger then a comparablefilament of steel! Although fabric manufacturers have created less costly alternatives to silk, such asnylon and polyester,silk is still in a class by itself.

The origins of silk date back to ancient China. Legend has it that a Chinese princess was sipping tea inher garden when a cocoon fell into her cup, and the hot tea loosened the long strand of silk. Ancientliterature, however, attributes the popularization of silk to the Chinese Empress Si-Ling, to around2600 B.C. Called the Goddess of the Silkworm, Si-Ling apparently raised silkworms and designed aloom for making silk fabrics. Silk was originally reserved exclusively for the use of the emperor;

gradually silk came into more general use. Silk, indeed, rapidly became one of the principal elementsof the Chinese economy. Silk was used for musical instruments, fishing-lines, bowstrings, bonds of allkinds, and even rag paper, the words first luxury paper. Eventually even the common people wereable to wear garments of silk.

During the Han Dynasty, silk ceased to be a mere industrial material and became an absolute value initself. Farmers paid their taxes in grain and silk. Silk began to be used for paying civil servants andrewarding subjects for outstanding services. Values were calculated in lengths of silk as they had beencalculated in pounds of gold. Before long it was to become a currency used in trade with foreigncountries.

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For more than two thousand years the Chinese kept the secret of silk altogether to themselves. It wasthe most zealously guarded secret in history. Indeed, the reigning powers decreed death by torture toanyone who divulged the secret of the silk-worm. Eventually, the mystery of the silk-making processwas smuggled into neighboring regions, reaching Japan about A.D. 300 and India around A.D. 400.

The first country to apply scientific techniques to raising silkworms was Japan, which produces someof the worlds finest silk fabrics. Other countries that also produce quality silks are China, Italy, India,Spain, and France. China was the largest exporter of raw silk in the early 1990s, accounting for about

85% of the worlds raw silk, worth about $800 million. Exports of Chinas finished silk products wereabout half of the worlds total at about $3 billion.

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Silk has a miniscule percentage of the global textile fibre marketless than 0.2%. This figure,however, is misleading, since the actual trading value of silk and silk products is much more

impressive. This is a multibillion dollar trade, with a unit price for raw silk roughly twenty times thatof raw cotton. (The precise global value is difficult to assess, since reliable data on finished silkproducts is lacking in most importing countries.)

With time and patience the mulberry leaf becomes a silk gown

Ancient Chinese Proverb.

The finest, most desirable silk comes from the mulberry silkworm, which is actually a caterpillar andnot a worm. Blind and flightless, it feeds solely on the leaves of mulberry trees. Known as the Bombyxmori, the mulberry silk worm is a fascinating but tragic bundle of insect life. Raised by professional

keepers in China on trays of mulberry leaves a thousand years before the Roman Empire when wildtribes were roaming Europe living in stick and mud huts, the mulberry silkworm has been totallydomesticated and can not live without humans for their care and feeding. There are no wildsilkworms or Bombyx mori moths that roam and feed in the wild.

The cultivation of silkworms for the purpose of producing silk is called sericulture. Over thecenturies, sericulture has been developed and refined to a precise science. Today, a hugely developedindustry has developed around the raising of silkworms for the production of silk. Silk worms are




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raised by large corporate silk worm farmers and hobbyists all over the world. Sericulture companiessell and ship all that the silk grower enthusiast needs from Bombyx mori ova (silkworm eggs) in anincubation dish to handling tools.

One acre of mulberry trees produces enough foliage to feed silkworms that create 178 pounds ofcocoons which can be unraveled into 35 pounds of raw silk. The mulberry leaves are a renewable andsustainable crop as the trees produce year after year. One mature mulberry tree will produce enoughfoliage for 100 silkworms. Generally, one cocoon produces between 1,000 and 2,000 feet of silk

filament, made essentially of two elements: a substance, called fibroin, makes up between 75 and 90%of the filament, and sericin, the gum secreted by the caterpillar to glue the fiber into a cocoon,comprises about 10-25%. Other elements include fats, salts, and wax. One silkworm produces verylittle useable silk. To make one yard of silk material, about 3,000 cocoons are used.

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The natural course in the cycle of worm to moth would be for the chrysalis to break through theprotective cocoon and emerge as a moth. But by breaking its way out, it cuts this fiber off in manyplaces, thus largely decreasing its value; So, sericulturists must destroy the chrysalis so that it doesnot break the silk filament. This is done by stoving, or stifling, the chrysalis with heat. The usualmethod is that of immersing the cocoons in steam for a few minutes. Another method, that of placingthe cocoons in boiling water, serves a double purpose. Not only does it kill the chrysalides, but it also

softens the seracin, the gum that sticks the threads together, so that they can be unreeled from thecocoon. Although silk is about 20% seracin, only about 1% is removed at this point.




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Reeling the filament

Reeling may be achieved manually or automatically. The cocoon is brushed to locate the end of the

fiber. The method is as simple as it is laborious. It is threaded through a porcelain eyelet, and thefiber is reeled onto a wheel. Meanwhile, diligent operators check for flaws in the filaments as theyare being reeled.

As each filament is nearly finished being reeled, a new fiber is twisted onto it, thereby forming onelong, continuous thread. Sericin contributes to the adhesion of the fibers to each other. s. Theaverage cocoon reels off about three hundred yards in a single thread.

Packaging the skeins

This is raw silk, just pure silk fibers without any chemicals or treatments added, although sometimesthe raw silk fibers will be soaked in a 1% hydrogen peroxide solution for a few hours to refine thecreamy color. Organic and sustainable certification organizations are working on standards fororganic silk but they have not yet been finalized and adopted.

Degumming

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The remaining sericin, or silk gum, must be removed from the yarn by soaking it in warm soapywater. This is called degumming, and it improves the sheen, color, hand and texture of the silk.Because the gum can serve as a protective layer, it is usually left on the silk until its ready to dye. Butthe degumming process, which enables the silk to accept dyes readily and whichcontributes to itshigh gloss, also causes the silk to lose about 25% of its weight and not a little strength. If the scouringand bleaching are not well and carefully done, the reduction in strength may be serious indeed.

Finishing silk fabrics

After degumming, the silk yarn is a creamy white color. It may next be dyed as yarn, or after theyarn has been woven into fabric. After dyeing, the skeins are again dried, run through anequalizing machine similar to a stretcher, and then rewound into the form in which they arewanted by consumers and the trade, such as spools, bobbins, skeins, etc.

This completes the process of silk throwing. The silk is now ready for the weaver, the knitter, the

lace maker, or the embroidery maker.

After the raw silk has been reeled into skeins or hanks, the most laborious parts of silk production arecompleted; that is, most of the work done on the fiber thereafter is done by machine processes insteadof by hand. The amount of hand labor that it takes to produce raw silk is almost incredible, and theamount of labor taken after the machine processes begin is no less than for other textiles. It has beensaid that it takes more human labor to produce a ladys silk dress, from the mulberry leaves into thefinished product ready for wear, than it takes to produce and build a locomotive out of the raw ores inthe ground. More hours are expended, and more people have something to do with the work. If thelaborers employed in the production of silk were paid as high wages as are commonly paid in the ironand steel industry the silk dress would cost almost as much as that locomotive. As it is, raw silk

production is carried on chiefly in countries where wages are very low. At the present prices of silk,the most efficient workmen doing their very best could not earn more than fifteen cents per day at thiskind of work.

WEIGHTED SILK:

Silk is sold by weight. Weighting is a textile manufacturing practice peculiar to silk manufacturingand involves the application of metallic salts to add body, luster and physical weight to silk fabric.The reason for adding metals to silk fabric is to increase the weight of the fabric and, because silkfabric sells by the pound, the extra weight increases the selling price of the fabric. Generally, only the

finer and more expensive reeled silks are weighted rather than the less costly spun silks By means ofweighting the manufacturer can increase the weight of silk by 3 to 4 times.

Weighting is done by immersing the silk in a solution rich in tannin, then transferred to iron or tinbaths, then washed. Weighting causes the fabric to lose its strength as soon as the weighting isapplied. Heavily weighted silk must be made into garments as soon as it is made. Spots develop in thedyes. Saltwater, perspiration and tears cause spots to be formed which seems as if the silk is eaten byacids. Sunlight also attacks weighted silk and can cause silk to fall to pieces.




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The silk industry makes a distinction between pure-dye silk and weighted silk. In the pure-dyeprocess, the silk is colored with dye, and may be finished with water-soluble substances such asstarch, glue, sugar, or gelatin. But it is not weighted. If weighting is not executed properly, it candecrease the longevity of the fabric by causing it to lose much of its strength and durability, so pure-dye silk is considered the superior product. Also, the metallic salts used to weight silk can causehealth risks and problems for some people.

After dyeing, silk fabric may be finished by additional processes, such as bleaching, embossing,

steaming, or stiffening.

WILD SILKS:

The wild silks are gathered principally in Japan, China; and India. There are several varieties of wildsilk cocoons, each with qualities somewhat different from the rest. The principal variety of Japan is theYamai-mai, and the chief varieties of India are the tusser, or tussah, and the ailanthus. Most of thesesilks are much darker in color than the domesticated silk, the Bombyx mori, probably because of thedifference in feed. Wild silkworms do not always have mulberry leaves to eat. Great numbers feed onoak leaves and in some cases on other plants.

In general, it may be said that wild silks are in most respects of poorer quality than domesticatedsilk. They are harder to bleach, and do not take dyes so well. They are generally very uneven intexture, but when made up into fabrics are often more durable than common silks. Wild silks are usedprincipally in the manufacture of pile fabrics such as velvet, plush, and imitation sealskin, and inheavy or rough cloths such as pongees and shantungs. While the silkworms of the wild varieties takecare of themselves, and therefore do not require the constant labor that must be given to domesticatedsilk, the expense of gathering is nevertheless high. The wild cocoons must be hunted, trees must beclimbed to gather them, and much time may be consumed in collecting comparatively few. On thewhole, however, because of the poorer qualities, wild silks are worth considerably less than tamesilks.

CHARACTERISTICS OF SILK:

Silk, a protein fiber like wool, with a smooth hand, is very lustrous and retains its shape well. Silk cantake on many different appearances. A raw silk fabric may fool you into thinking that it is cotton orsynthetic. The more refined the silk and the smaller the yarn, the more it resembles the look and feelthat we know as silky.

Silk is the strongest natural fiber and is very strong in terms of tensile strength, meaning it canwithstand a lot of pulling type pressure without breaking. This should not, however, be confusedwith wear ability or abrasion resistance. Silk will not stand up to the heavy wear that other fibers will.

Because of its good absorbency, fabrics made from silk are comfortable in summer and warm inwinter.

Silk creases and wrinkles easily, especially when damp or wet. Some silk clothing manufacturersapply softeners, elastomers, and synthetic resins such as EPSIA a silicone-containing epoxycrosslinking agent to increase the dry and wet anti-wrinkling and crease-resistance performance ofsilk garments. With the family of silicone epoxy crosslinking agents (EPSIA, EPSIB and EPTA) thiscrease resistance occurs because chemical cross links occur between the silk fibroin strand and theepoxy groups. Research by Zaisheng Cai and Yiping Qiu in the Textile Research Journal




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Weighting of silk

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