Natural silk – sourcing, usage, and application
Animal protein fiber, its source, usage, properties, fabric care, and application
Silk is a natural protein fiber. It is similar to wool in that it is composed of amino acids arranged in a polypeptide chain. Silk is produced by the larvae of a moth, while wool is produced by animals. All protein fibres have some general characteristics in common.
Natural Silk is an animal protein fiber produced by certain insects to build their cocoons and webs. Silk is commonly considered to be the queen of all fabrics, yet many enchanting and interesting facts about silk are absent from the silk information in possession of the ordinary user of silk fabric
Many different types of silk are produced by a huge variety of different types of insects other than moth caterpillars. Yet none of these have been exploited for commercial purposes, though there has been basic research into the structures of such silks. Silk is most commonly produced by larvae, and thus largely limited to insects with complete metamorphosis. Silk culture has been practiced for at least 5000 years in China
Silk is a natural protein fiber. It is similar to wool in that it is composed of amino acids arranged in a polypeptide chain. Silk is produced by the larvae of a month, while wool is produced by animals. All protein fibers have some general characteristics in common.
The commercial process of silk making is highly complex and labor intensive. The following will provide basic information on how silk is made.
Cultivation of the silkworm is known as sericulture. Although many insects produce silk, only the filament produced by Bombyx mori, the mulberry silk moth and a few others in the same genus, is used by the commercial silk industry.
The “silkworm” is, technically, not a worm but a moth pupa. For the sake of simplicity and consistency, however, we will use the term silkworm throughout this writing.
Raw or Mulberry silkworm is cultivated over Mulberry plants under protected and controlled environment.
Production and Hatching the Eggs
The first stage of silk production is the laying of silkworm eggs, in a controlled environment such as an aluminum box, which is then examined to ensure they are free from disease. The female deposits 300 to 400 eggs at a time. Eggs are then incubated under a very controlled temperature and humidity for one week to ten days.
The Feeding Period
Once hatched, the larvae are placed under a fine 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.
For about six weeks the silkworm eats almost continually. After growing to its maximum size of about 3 inches at around 6 weeks, it stops eating, changes color, and is about 10,000 times heavier than when it hatched.
The silkworm is now ready to spin a silk cocoon.
Collecting the cocoons
Handpicked cocoons are collected in baskets made of wooden sticks with pupa still inside the cocoon. After collecting them at one place, workers sit and separate healthy and clean cocoons from the bad ones. To get a continuous, long thread, it is important to collect it before the pupa cuts the cocoon and comes out to enter the next stage of its lifecycle — the moth.
Spinning the Cocoon
The silkworm 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 period is termed pupating.
Silkworms possess a pair of specially modified salivary glands called sericteries, which are used for the production of fibroin – a clear, viscous, proteinaceous fluid that is forced through openings called spinnerets on the mouthpart of the larva.
Liquid secretions from the two large glands in the insect emerge from the spinneret, a single exit tube in the head. The diameter of the spinneret determines the thickness of the silk thread, which is produced as a long, continuous filament. The secretions harden on exposure to the air and form twin filaments composed of fibroin, a protein material. The second pair of glands secretes a gummy binding fluid called sericin which bonds the two filaments together.
Steadily over the next four days, the silkworm rotates its body in a figure-8 movement some 300,000 times, constructing a cocoon and producing about a kilometer of silk filament.
Reeling the Filament
At this stage, the cocoon is treated with hot air, steam, or boiling water. The silk is then unbound from the cocoon by softening the sericin and then delicately and carefully unwinding, or ‘reeling’ the filaments from 4 – 8 cocoons at once, sometimes with a slight twist, to create a single strand.
As the sericin protects the silk fiber during processing, this is often left in until the yarn or even woven fabric stage. Raw silk is silk that still contains sericin. Once this is washed out (in soap and boiling water), the fabric is left soft, lustrous, and up to 30% lighter. The amount of usable silk in each cocoon is small, and about 2500 silkworms are required to produce a pound of raw silk.
Silk thread that has been reeled from cocoons and is still in its natural state. It consists mainly of fibroin (the filament) with about 10-25% sericin (a gluey secretion). Raw silk is golden yellow in color and somewhat stiff.
The process of unwinding raw silk filaments from cocoons to produce a raw silk thread.
A traditional process for dyeing textiles with patterns. Various methods, including wax, paste, tying, stitching, and blocks, can be used to “resist” or prevent the dye from reaching all the fabric. This creates a pattern and ground. In Thailand, mudmee is created by tying off parts of the fabric with waterproof material to prevent the dye from entering the material.
Silk that is colored with dye and to which metallic substances have been added during the dying process. This adds back weight which is lost during de-gumming and also adds body to the fabric. If weighting is not done properly, it reduces the life of the fabric. Pure-dye silk is considered superior.
Types of Silk
Raw silk is twisted into a strand sufficiently strong for weaving or knitting. This process of creating the silk yarn is called “throwing,” and prevents the thread from splitting into its constituent fibers.
Saving the seeds for next cultivation
It is important to let a good number of pupa become the moth by cutting the cocoon and coming out of hibernation. This new generation of pupa would turn into a healthy moth after metamorphosis. The weaker ones will die, of course at some phase of their life. The healthy couples would mate and lay eggs to give birth to a new generation of the silkworm.
Packaging the skeins
The end product, the raw silk filaments, are reeled into skeins. These skeins are packaged into bundles weighing 5-10 pounds (2-4 kg), called books. The books are further packaged into bales of 133 pounds (60 kg) and transported to manufacturing centers.
Forming silk yarn
Silk thread, also called yarn, is formed by throwing, or twisting, the reeled silk. First, the skeins of raw silk are categorized by color, size, and quantity. Next, they are soaked in warm water mixed with oil or soap to soften the sericin. The silk is then dried.
As the silk filaments are reeled onto bobbins, they are twisted in a particular manner to achieve a certain texture of the yarn. For instance, “singles” consist of several filaments which are twisted together in one direction. They are turned tightly for sheer fabrics and loosely for thicker fabrics. Combinations of singles and untwisted fibers may be twisted together in certain patterns to achieve desired textures of fabrics such as crepe de chine, voile, or tram. Fibers may also be manufactured in different patterns for use in the nap of fabrics, for the outside, or for the inside of the fabric.
The silk yarn is put through rollers to make the width more uniform. The yarn is inspected, weighed, and packaged. Finally, the yarn is shipped to fabric manufacturers.
Degumming thrown yarn
To achieve the distinctive softness and shine of silk, the remaining sericin must be removed from the yarn by soaking it in warm soapy water. Degumming decreases the weight of the yarn by as much as 25%.
Finishing silk fabrics
After degumming, the silk yarn is a creamy white color. It may next be dyed as yarn, or after the yarn has been woven into a fabric. The silk industry makes a distinction between pure-dye silk and what is called weighted silk. In the pure-dye process, the silk is colored with dye and may be finished with water-soluble substances such as starch, glue, sugar, or gelatin. To produce weighted silk, metallic substances are added to the fabric during the dying process. This is done to increase the weight lost during degumming and to add body to the fabric. If weighting is not executed properly, it can decrease the longevity of the fabric, so pure-dye silk is considered the superior product. After dyeing, silk fabric may be finished by additional processes, such as bleaching, embossing, steaming, or stiffening.
Not all of the silk filament is usable for reeled silk. The leftover silk may include the brushed ends or broken cocoons. This shorter staple silk may be used for spinning silk in a manner of fabrics like cotton and linen. The quality of the spun silk is slightly inferior to reeled silk in that it is a bit weaker and it tends to become fuzzy. The waste material from the spun silk can also be used for making “waste silk” or “silk noil.” This course material is commonly used for draperies and upholstery.
Summary of the performance of silk in apparel fabrics
- Aesthetic luster – Variable, beautiful and soft
- Durability, abrasion resistance, and tenacity – high, moderate and high for natural fibers.
- Elongation – moderate.
- Comfort absorbency and thermal retention – High, high and good.
- Appearance retention, resistance, dimensional stability and elastic recovery – Moderate, moderate, high and moderate.
- Recommended care – Dry clean.
- Physical structure – Silk is a natural continuous-filament fiber. It is a solid fiber, smooth but irregular in diameter along its shaft. The filaments are triangularine cross-section with round corners. Silk fibers are very fine —1.25 denier/filament.
- Chemical composition and molecular structure – The protein in silk is fibroin, which contains 15 amino acids in polypeptide chains. Silk has re-active amino and carboxyl groups. Silk has no cross-linkages and no bulky chains. The molecular chains are not folded as in wool but are almost fully extended and packed closely together. Thus, silk is highly oriented, which gives the fiber its strength. As with all fibers, there are some amorphous areas between the crystalline areas, giving silk its elasticity.
- Properties – Silk can be dyed and printed in brilliant colours. It is adaptable to a variety of fabrication methods. Thus it is available in a wide variety of fabric types. Because of cultivated silks smooth but slightly irregular surface and triangular cross section. The lustre of this fiber is soft with an occasional sparkle. It is this cluster that has been the model for many man-made fibers. Fabrics made from cultivated silk usually have a smooth appearance and a luxurious hand. Silk is one of the strongest natural fibres, with a tenacity ranging from 3.5 – 5.0 g/d dry. It may lose up to 20 percent of its strength when wet. When silk is stretched a small amount, it does not return to its original length.
Silk has good absorbency with a moisture regain of 11%. Silk is a poor conductor of electricity; thus, problems may develop with static cling. Silk fabrics are comfortable in summer in skin contact apparel.