Legend has it that silk was discovered by a Chinese empress, when a cocoon fell from a tree into her tea, and extruded its fine filament. Thus were the beginnings of this brilliant fiber that has been wooed, prompted thievery, and created the longest road in China, the ancient trade route between China and Rome. Fast forward to today, silk is raised under controlled conditions called “sericulture” (reared in captivity), an expensive and labour intensive process, and sells at a premium. Because sericulture is for the most part a manual process, production is common in countries with low cost labour forces. China and India are the top producers.
Where Silk Comes From
A variety of silkworms produce silk. Silk is obtained from the cocoons of their larvae. The most common species of silkworm is the Bombyx mori. Their larvae eat only mulberry leaves, rendering the fiber a shiny, pure white.
Another common silk fiber, Tussah, comes from a wild or semi-cultivated silkworm. They feed on many different types of leaves, most of which contain tannins that impart colour to the fiber. The resulting fiber ranges in colour from off white to light brown. Tussah fibers are coarser with an irregular surface, less lustrous, and stronger than Bombyx silk. Tussah silk is a less expensive fiber.
The cocoons are subjected to heat to unwind the fine, lustrous filament in one continuous strand. Silk is the only natural fiber that produces a filament fiber, rather than staple or short fibers. Silk doesn’t grow like other natural fibers, so the fiber is smooth like a rod, with no cellular structure.
Silk Production in Sericulture
The silkworm begins its journey to a filament by indulging on chopped leaves, growing 10,000 times its original size, the size of a peanut shell. When the worm stops eating, it’s ready to spin its cocoon.
At this point, the worm secretes fibroin, with some of this fibroin secreted in one continuous filament. The filament is coated with a gummy substance, sericin to protect the cocoon.
Once the cocoons are complete, the workers have approximately two weeks to gather and “stifle” further growth so the moths don’t hatch. If a moth works its way out, the filament is broken, lessening the value of the silk.
Cocoons at this stage are soaked in warm, soapy water to degum the sericin coating. Then the silk filament is reeled or unwound. Reeling is a labour intensive step of the process.
There are three grades of silk. Reeled silk is the finest quality used for woven fabrics. Reeled silk is not common in hand knitting yarns, because of the high cost, but also because it’s very slippery. Spun silk is the second and most common type found in hand knitting yarns. The silk fibers are cut into shorter lengths, carded and spun. Often the waste silk from reeling is used to produce spun silk. The third grade is noil silk yarns. These are processed a step further than spun silk, by combing to remove the shorter fibers.
Silk is one of the strongest natural fibers. It’s an excellent insulator to keep you warm. For this reason, long silk underwear are perfect in the winter time. It dyes well in bright colours. It has similar moisture absorbing properties as wool, absorbing and releasing moisture away from the body.
Although silk is a strong fiber, it has other characteristics which make it less durable and not as versatile as wool. Silk has little elasticity, making it not as resilient or flexible as wool. It has a tendency to stretch with wear, and is slow to recover. After washing silk, it usually goes back to its original shape. Silk tends to fade more readily with each cleaning, and over time may develop a fuzzy surface. It doesn’t have the durability or longevity of wool.
One unusual characteristic that you may have experienced, is an odour when silk is wet. This usually occurs with Tussah and lower grade silk fibers. The smell sometimes disappears after the first few washings. If the silk has not been processed properly, the smell may be very strong indicating a low quality fiber. However, it’s just the nature of wet silk.
Characteristics in Yarn
Pure silk is very smooth and slippery, lacks body, and forms a relaxed, drapey fabric suitable for lace wraps or drapey tops. Silk performs best when blended with more elastic and durable fibers. Common blends are silk with wool, and silk with mohair. Silk adds its shimmer and drape to the blend.
You’ll also notice some silk yarns, like Tussah, have a nubby, irregular surface, with more of a matte finish or no shimmer.
You’ll probably have to experiment with different types of needles, to get the right amount of slippage. Needles with duller tips are better so they don’t catch and snag the yarn. Rib patterns aren’t the best choice for silk, because of its low elasticity. Silk is not the best yarn choice for socks, because you’ll constantly be pulling them up.
Some silk blended yarns that I have used include Elsebeth Lavold Silky Wool (featured in the above image), a classic that has been produced for quite some time; Rowan Kidsilk Haze, a blend of kid mohair and silk; and Shibui Silk Cloud, another silk and mohair blend with a beautiful halo.
For any project, there is a suitable yarn; the same goes for silk. If you’ve never tried silk, a blended yarn is probably your best bet, to maintain the beautiful shimmer and drape of silk, yet add the durability of other fibers.