Is Your Silk Rug Really Silk
We don't mean to be alarmist, but we sometimes see "silk" Oriental rugs that are made of something other than genuine, natural silk. This is not a problem if you know that the rug you are considering is made of artificial silk, but sometimes rug dealers neglect to pass on this information! The problem happens most often with just a few rug types sold in tourist markets in Turkey and India (and sometimes in Pakistan).
Real silk is produced as the cocoon covering of the silkworm, the pupal form of the Asian or mulberry silk moth, bombyx mori. The cocoon is spun by the silk moth caterpillar of a single silk fiber that can be up to several thousand feet in length. To harvest the silk, completed cocoons are boiled or heated to kill the silkworms, then laboriously unwound into single fibers which are plied together and spun into thread or silk yarn.
Natural silk is a fibrous protein composed of a number of amino acids: glycine (44.5%), alanine (29.3%), serine (12.1%), valine (2.2%), tyrosine (5.2%), glutamic acid (1%), others less than 1% each. Chemically, natural silk is C15H23O6N5 (we give the formula in case you want to whip up a batch of your own). Silk is extremely high in tensile strength, exceeding that of nylon. It has been estimated that if a single silk fiber with the diameter of a pencil could be produced, the fiber could lift a 747 aircraft (who figures these things out, anyway?). Silk is used to make Oriental rugs because dyed silk is a fiber with rich, saturated colors, and a distinctive, almost translucent luster.
Artificial silk is everything billed as silk that doesn't come from the silkworm cocoon. Most often this means mercerized cotton; sometimes it means a manufactured fiber like rayon or a blend of chemically altered and/or manufactured fibers. It's not that artificial silk is intrinsically evil, it's just that the whole point of using artificial silk in a rug is to save the cost of real silk. It is not nice when this cheaper, artificial silk rug is misrepresented and sold for the price of a real silk rug.
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.
How to Identify A Real Silk Oriental Rug
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.
Tests for Silk
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.