Silk is an animal fiber. It is a secretion given out by the silkworm in the form of a sticky fluid which dries up by exposure to air and forms long continuous thread. It is long, smooth, and strong and has a natural lustier, which makes it the most attractive of all textiles. So, it is considered the ‘Queen of Fabrics’. The preparation of the silk fiber is preceded by what is known as ‘sericulture’ which is carried out in stages before the silk is reeled off from the cocoons. The silk worm lives on mulberry leaves.
This fiber is known to have been used in China for the first time more than 2500 years age. Silk is largely produced in countries like China, India, Japan and France. There are two types of silk.
1. Cultivated silk:
Cultivated silk is a narrow fiber with no markings. It is generally superior, having the qualities of elasticity and durability.
2. Wild silk:
Wild silk is coarse, thick form, which appears flattened. It is a broader fiber with fine waves, longitudinal lines running across its surface, giving it a dark hue under the microscope. Examples of wild silk are Muga, Tassar and Eri silk.
Silk is the only natural filament. It is a solid fiber. The filaments are 300-1800 yards long. Silk fiber has a double rod-like structure, covered with lumps of gum. Wild silk fiber is very irregular and resembles flattened, wavy ribbons with longitudinal markings. Cultivated silk is smooth, cylindrical and generally uniform in thickness, like glass rods.
Longitudinal view of fiber:
1. Rod like structure
3. High luster
4. Gum present on the surface
5. No crimp.
The chief constituents of silk are ‘fibroin’, the protein substance, consisting of two filaments, each of which is called a ‘bring’ held together by ‘sericin’ a gummy substance that gives the bake (cultivated cocoon) a rather uneven surface.
Silk is the strongest natural fiber. The continuous length of the filaments in thrown yarns provides a factor of strength above what is possible with short natural fibers.
Silk is an elastic fiber; however its elasticity varies, as expected of a natural fiber. It may be stretched from 1/7 to 1/5 its original length before breaking.
Silk fabrics retain their shape and resist wrinkling rather well.
5. Heat conductivity:
Silk is a non-conductor of heat. Because it prevents body heat from radiating outward. Silk has a pliability and suppleness that give an excellent capability.
Silk fiber has the good absorptive properties, which can generally absorb about 11 percent of its weight in moisture, but the range varies as much as 30 percent.
7. Effect of Moisture and Friction:
This fiber is not affected by moisture. It does not shrink or stretch when wet. Friction may spoil the smooth, soft texture of the fiber and therefore, washing silk fabrics should be avoided.
8. Reaction to Bleaches:
Strong bleaches containing hypochlorite i.e. javelin water has a harmful effect on silk. Other mild oxidizing bleaches may be used with normal caution.
Smooth-surfaced silk fabrics have only a normal shrinkage, which is easily restored by ironing.
10. Effect of Heat:
Silk is somewhat sensitive to heat.
11. Effect of light:
Continuous exposure to light weakens silk faster than either cotton or wool.
12. Reaction to Alkalis:
This fiber is not very much sensitive to alkalis, but can be damaged if the concentration and temperature are high enough. Weak alkalis, such as borax or ammonia can be safely used, when necessary, in steeping or stain removal.
13. Reaction to Acids:
Organic acids do not harm silk, but concentrated mineral acids will dissolve silk fiber.
14. Resistance to perspiration:
Silk fabrics are damaged by perspiration.
15. Affinity to Dyes:
Silk fiber has a very good affinity for dyes. It readily absorbs basic, acid and direct dyes. Hence, prints on silk fabrics are taken so well that the colour on the back of the fabric often differs only slightly from the face.