Chemists think of silk as a liquid crystal protein. In its natural form it is two proteins. A core filament forms the fiber that we know. It is encased in a very thin layer called sericin, also a protein. More about sericin later.

Everyone has heard of the silkworm. In fact it is an immature moth, a larva. It makes a silk cocoon as a place for the final transformation to a winged flying creature. The silk filament is made from a liquid produced in the silkworm’s head.

The filament is pulled as a viscous liquid as the head moves in a continuous figure eight pattern (or if you prefer, the mathematical symbol for infinity). The coating on the liquid silk filament, sericin, serves as a lubricant to help the silk move though the tiny hole in the larva’s head; as a stabilizer while the filament is stretched (stretching orients the molecules and gives it strength); and as adhesive to glue the filaments together. Sericin is considered to be waste in the silk fiber industry, but it is absolutely essential for the production of the fiber itself.

It takes two to three days of continuous activity to produce a closed and complete cocoon. Sometimes two cocoons will overlap during construction — these are called doupionni. The cross section of the silk filament is said to be triangular, but this is an oversimplification. Shape and size actually vary a great deal from one variety of larva to another. In general wild silks of the Tussah (tassar) family are heaviest and flattest. Silks native to Northern Thailand and Cambodia are smaller in section but most perfectly triangular. This gives them the highest reflectivity to light (hence the best optical sheen) and the best strength/weight ratio.

As liquid silk is pulled by the larva’s head motion molecules within the liquid silk filament stretch along the lengthwise axis, compacting and interlocking. Each filament will have many internal ‘cables’ of molecules within it, much in the way that a bridge suspension cable will be made up of smaller ones. This is what gives the fiber such incredible strength — only exceeded in nature by spider web fiber, which is produced in the same way but pulled further.

The thickness of a silk fiber varies according to the pulling motion and the point in cocoon construction where it is needed. Most silk fibers have thin points that can become fracture planes. Also, the innermost layers in the cocoon require less strength and are thinner. In commercial silk recovery the thin places may break off. Due to fragility and difficulty of recovery they are termed silk waste, but they are not really defective. Silk waste is incorporated into less expensive grades of silk called noil, which tend to feel coarse and have lower sheen.

Silk cultivation originated in China but has spread far beyond Asia. It tends to be labor intensive, so in high wage areas like Japan it has disappeared. Brazil has an important silk cultivation industry founded by Japanese emigrants who relocated to South America after WWII. Some of that industry has now spread to Argentina. Korea, Vietnam, Cambodia, Thailand, India, Bangladesh, parts of Central Asia, and Greece are restoring and expanding traditional silk industries.

A great deal of research is under way in technical developments related to silk. Sericin recovery processes are reclaiming silk protein, formerly discarded as waste, for use in cosmetics and food, mainly for the Japanese market. Biotech modifications of silk are producing new applications in several medical fields. Silk protein can be expanded and stuffed with other molecules that radically alter its properties. It is not a stretch to say that future generations could be driving composite cars based on modified silk, or wearing silk garments grown to custom sizes on engineered plant/ insect hybrids.

this is the first in a Yarntasting series on fibers and spinning

Tags: fiber, silk

This entry was posted on Friday, August 28th, 2009 at 12:45 pm and is filed under fiber. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.