Paragraph 70. Silk is the most valuable and the most wonderful of all the textile fibers. It was first used by the Chinese, probably as early as 1700 B. C. The origin of silk was kept a secret among the Chinese until about 550 A. D. About that time it became known in Europe. Silk is by far the strongest of the textile fibers, being almost equal in tensile strength to an iron wire of the same diameter. This great strength is due to the nature of its fibers, being composed not of short interwoven fibers, as is the case with wool or cotton, but consisting rather of one long and continuous filament, ranging from four to thirteen hundred yards in length. Each of these filaments is the product raveled from a single cocoon. These cocoons are made by what is commonly known as the silk worm. It is a peculiar kind of caterpillar which spends about three days wrapping itself in a silk covering or cocoon. The material from which the caterpillar spins its thread is an excretion from its body.
After the caterpillar is surrounded by its cocoon, it undergoes a very remarkable change, and would in due time break open the cocoon and emerge in the form of a butterfly. This would rend the cocoon and practically destroy the silk fibers, therefore men engaged in the culture of silk, kill the pupa by subjecting it to heat or steam. It is then possible, by skillful handling, to unravel the silk fiber which has formed the covering of the caterpillar. These are the fibers which are spun into thread.
Silk fiber has another peculiar property which is distinct from all other textile fibers. That is, its very high luster. Just why silk has this property is not fully understood. It is this property which makes possible the beautiful sheen of the fine silks and satins.
Silk may be dyed with various colors, in a manner similar to wool. Silk also acts under alkali tests about the same as wool; however, it will stand a very much higher temperature. It is not so sensitive to alkali as wool, neither is it so quickly destroyed by acids.
On account of the many attractive properties of silk, also on account of its scarcity, it is quite frequently adulterated by the addition of other fibers such as mercerized cotton or linen. A great many tests have been devised by which one may determine whether a so-called silk fiber is adulterated. Most of these tests are too complicated to be introduced here. You can, however, generally recognize silk by its very soft pliable nature. If you are in doubt, a few threads may be drawn and examined separately. If examined under the microscope these threads will reveal the perfectly smooth surface of the fiber, showing that it is made up of the long filaments already described rather than of short fibers like cotton or linen. If the threads are given the burning test, the peculiar odor similar to burning feathers will be evidence of genuine silk. While silk is not a common fabric in everyday wearing apparel, yet it is sufficiently common that its general properties should be understood.
The following are the most common kinds of silk fabrics.