Originally published in Installation & Cleaning Specialist magazine. 07-01-2000.

Synonymous with luxury and splendor, silk has reigned as the undisputed queen of luxury textiles over the centuries, and has been woven into tapestries, rugs, fine fabrics and accessories for over 4,000 years. Sericulture (the cultivation of silk-worms for silk) has remained virtually unchanged over the centuries.

Silk was discovered around 2,600 BC in China. According to legend, an empress accidentally dropped a cocoon into a cup of hot tea. The heat melted the gum holding the cocoon together and she discovered the cocoon unraveled into one continuous fiber. Chinese silks found their way to Rome and Greece, travelling over the 6,000 mile "Silk Road." Despite China's attempt to keep the secret of silk production to itself, it spread to Japan in the 3rd century AD and into India in the 4th century AD. Marco Polo is credited with introducing silk production to Venice. To this day, Italian silks are renowned for their impeccable quality and workmanship.

Silkworms, which are actually caterpillars, belong to two families: Bombycidae (the commercial silkworm) and Saturniidae (the wild silkworm). The most common source of silk, Bombyx mori, is raised domestically, but only where there are mulberry leaves to satisfy its finicky appetite. More than 500 species of wild silkworms fend for themselves, eating oak and other leaves. When they become moths, they are bigger than commercial Bombyx. More robust than their domesticated cousins, wild silkworms produce a tougher, rougher silk, not as easily bleached and dyed as the mulberry silkworm. The leaves they eat are full of tannin, thus giving the silk a beige look. Wild silk is usually called tussah silk, however, there are many kinds of wild silks, each with its own characteristics. Wild silk is usually less expensive than cultivated white silk.

Two glands in the silkworm produce liquid silk that's made up of giant protein molecules called fibroin. The fiber is coated with a sticky substance called sericin and holds the cocoon together. The strand can be up to a mile long and is extruded through a narrow aperture on the worm called the spinneret.

• Hygroscopicity - The ability to absorb moisture without feeling wet, holding up to 30% of its dry weight in water.
• Low Specific Gravity - It has a low density. The molecular structure resembles a long string of ladders. Between the "steps" is air space, which act as insulation and allow the fiber to breath.
• Strength in Fineness - Silk's extraordinary fineness can be used to weave fine fabrics and rugs with up to 3,000 knots per square inch. Its strength, given equal diameters, is stronger than any other natural fiber including nylon or steel wire. Also, silk will not weaken when wet like rayon and some synthetics. As strong as it is, silk is subject to wear through abrasion. As a result, silk carpets are for reclining and touching, but not for walking - except maybe barefoot.

Although resistant to various molds, mildew and dry rot, silk is damaged by long periods in direct sunlight, as are all fibers. The wool moth does not attack silk, but silverfish and some carpet beetles do.

Silk as it comes from the cocoon is coated with a protective layer of sericin or silk gum. This coating may be any color: white, yellow, brown, beige; its color is not related to the color of the silk beneath it. Silk gum is dull and stiff, so it's usual to remove it to reveal the pure fiber luster. Silk with all its gum is called raw silk. This should not be confused with the fashionable "raw silk" fabric woven from noil silk, which is made from very short fibers and little tangle-balls of fiber.

We have two basic types of silk yarns, the thrown and the spun. They correspond to the two basic forms of the fiber, reeled (continuous) and cut (staple).

Reeling silk was one of the great Chinese discoveries. First, the cultivated silk cocoons are exposed to steam to kill the living pupae. Then, the cocoons float in a basin of hot water to soften the gum allowing the thread ends to come loose. They are often plied together - five to eight filaments at a time - then fed into the reeling machine and put onto a spool. "Throwing" is the process of twisting (plying) the unspun filaments of reeled silk. It's almost always made from cultivated silk.

Spun silk is made from wild cocoons. The silkworm exudes a brown liquid enzyme that softens the cocoon, enabling it to push outside. This ruins the cocoon for reeling, and now becomes a stable fiber. After degumming the cocoon, it's carded and combed, twisted into singles, and then plied to make the yarns. The process is similar to making cotton or wool yarns.

Wild silk traditionally has been used for pile fabrics and rugs. Most Chinese rugs seem to be made from spun wild silk. The finest silk rugs, such as the ones from Hereke, Turkey, are made from reeled silk. The fine threads made from the continuous fibers are used for the foundation and rug face allowing the knot count to exceed 3,000 per square inch.

Wet-cleaning moderate to lower priced silks is generally plagued by pile distortion and fugitive dyes. A possible explanation is the silk used for these problem rugs may be noil silk. Noil has a reputation of wearing out quickly as the very short fibers disintegrate more rapidly than do intact long fibers.

Another factor in pile distortion on silk rugs can be heat. Silk fibers can withstand high temperatures (330F or 166C) but due to the minimal twist found in plush staple silk yarns, temperatures should be limited.

Unbleached, cultivated silk noil usually has small black flecks appearing throughout. These flecks are the crumbled remains of the chrysalis, that horn-like envelope that encloses the transforming silkworm inside the cocoon. Noil often is bleached and the dyed well.

Reprocessed silk is made from silk waste from any source including mill floor sweepings. The yarn is pulled apart and carded and spun like noil. The fibers appear non-uniform and may have bits of fine yarn mixed with it. The final yarn may be over-dyed by the spinner.

Finishes are sometimes applied to silk fabrics. Sizing, and accessory to finishing and weaving, is often used on some fabrics as a strengthening effect or they will fray or fall apart. Sizing has also been used on cheap manufactured silk fabrics to give them more body. This is put on rather heavily, dried then "broken in," which breaks the glue between the yarns and softens the fabric so that it passes for a soft silk that feel more substantial than it really is. Sizing is not commonly applied to the face yarns of rugs.

The weighting of silk is an old practice, one that is objectionable. Fortunately, it's hardly done anymore. Weighting means the addition of metal salts like iron, tin or lead to the dye process to increase the finished weight of the fabric and defraud the buyer. However, excess metal salts would cause the fabric to disintegrate after a short time.

Unfortunately, rayon and mercerized cotton are sold to consumers as silk. Even to the professional cleaner, rayon can have a sheen that first appears to be silk. Reading a label on a rug can also be misleading. The terms "Art Silk" (used for artificial silk) and "Faux Soie," a French term for fake silk, both define rayon.

The only way to be certain is to perform a chemical or burn test. The burn test is a quick and accurate. (The chemical test is to put some of the fibers in chlorine bleach - it will dissolve completely). Silk has a slight burning hair smell and a distinct ash - it generally forms a round bead (see figure 1). Rayon and cotton, both cellulose fibers, have a very fine ash and smell like burning paper.