A ripening cotton boll can contain as many as 5,000 separate cotton fibers, each fiber growing from a tiny seed and formed as a hollow cylindrical sheath of as many as thirty layers of almost pure cellulose. Cotton fiber is mercerized by being stretched under controlled tension at room temperature while being treated with a 21%-23% solution of caustic soda (NaOH). The effect is to swell the fiber and make its surface much more reflective, thus dramatically increasing its luster (and also its tensile strength). After the chemical treatment, cotton yarn is often singed to remove whatever small amount of fuzz remains on the surface of the fibers. Sometimes cotton is calendered by being passed between heated rollers. The effect is to increase the luster and sheen of the fiber still more. However it is treated, cotton remains cellulose: C6H10O5.
Like cotton, rayon is made of almost pure celulose, but rather than being grown, rayon is produced by first dissolving cellulose (obtained from cotton or woodpulp) to produce a thick yellow liquid called viscose. The viscose is extruded through tiny holes into a chemical bath that produces long filaments which can be spun into thread and yarn. Viscouse rayon was the first man-made fiber. In 1920, DuPont bought from the French the technology for making viscose rayon. DuPont first called the material "artificial silk", and formed a company (The DuPont Fibersilk Company) to manufacture it. Other artificial fibers would follow quickly: acetate (also derived from cellulose) in 1924, nylon, (commonly, adipic acid reacted with hexamethylene diamine) in 1939, acrylic (from acrylonitrile, a petrochemical) in 1950, polyester in 1953, and triacetate in 1954.
First of all, pay attention to whatever clues the dealer--or the rug--gives
you. For instance, we have seen many artificial silk Kayseri rugs (and
some Hereke rugs), both Turkish types. In Turkey, a real silk Kayseri is
an ipek Kayseri: ipek is "silk" in Turkish. An
artificial silk Kayseri is a flos Kayseri ( a yun Kayseri has a
wool pile). The dealer might be accurately describing the piece to you as a flos
rug, but by not explaining the difference between flos and ipek,
he lets you jump to the intended assumption, and you unwittingly buy an
artificial silk rug.
Indian rug dealers are seldom as delicately circumspect as some of their
Turkish counterparts. Artificial silk rugs in India are often blatantly sold as
real silk, complete with certificates of authenticity and written guarantees.
For many years Kashmir in northern India has been the major source for
both real and artificial silk Indian rugs. Look carefully at the
"silk" rug: it should be tightly woven (with more than 200 knots per
sq. in., and often with 500 or more knots), intricately detailed, closely
clipped, and it should have real silk fringe that is clearly an extension of the
rug's structure, not sewn on or sewn into the ends of the rug. Artificial silk
rugs often have only medium weaves (less than 250 knots per sq. in., and
sometimes less than 150 knots per in.), and often have cotton fringe. Good
quality real silk rugs always have real silk fringe. In Pakistan we often
see rugs called jaldars. These wool pile rugs often have "silk
touch," meaning that there is artificial silk inlay in the pile (often
outlining part of the design). This artificial silk is almost invariably ivory
in color, and is made of mercerized cotton.
Rub it! It is sometimes claimed that you can tell real silk from
artificial silk by vigorously rubbing the pile with your open palm. The real
silk rug feels warm, the artificial silk rug stays cool to the touch. We
sometimes think we have felt this difference. Of course, it helps to have a real
silk rug with you so that you can compare a known quantity!
Burn it! This test is at least good theatre, and actually can be
helpful. Clip off a small piece of the fringe, or pull a knot out of the rug
from the back (why should the owner object?). Burn it. Look at the ash and smell
the smoke. If the material was cellulose (rayon), the ash should be soft and
chalky, and the smell should be like burning paper (most paper is made of
cellulose). If the sample is real silk, the burning sample should ball to a
black, crispy ash, and the smell should be of burning hair (you're burning
protein, the same stuff your hair is made of). You've got to be a little careful
with this test to avoid smelling the smoke from the match (and to avoid igniting
yourself or the rug dealer's shop).
Dissolve it! The most accurate test is one that chemically differentiates protein from cellulose or petrochemicals. One such test: at room temperature, mix a solution of 16 g copper sulfate (CuSO4) in 150 cc of water. Add 8-10 g glycerine, then caustic soda (sodium hydroxide: NaOH) until a clear solution is obtained. This solution will dissolve a small sample of natural silk, but will leave cotton, rayon, and nylon unchanged.