Where Does Silk Come From? Adventures of a Sericulturist

Coleen’s Bombyx mori cocoon crop from 2020.

Polyphemus caterpillar found in the wild on Quadra Island, BC.

Silk is organic, renewable and biodegradable. It requires less water and fewer chemicals to produce than many other fibres.

Each spring, when mulberry trees have leaves, I run a herd of silk caterpillars. I feed them three to four times a day, clean their trays, monitor their environment for temperature and humidity, and generally fuss over them. In the words of a Chinese proverb: “With patience a mulberry leaf becomes a silk gown.”

As a former student and instructor at the Olds College Master Spinner Programme, I have worked with silk fibre for many years. To teach silk reeling, I sought opportunities to expand my knowledge by first attending a workshop with Michael Cook, and then studying at Mulberries Organic Silk Farm in northern Laos. In Laos, my interest shifted from silk fibre to the caterpillars that produce it, thus beginning my journey as a sericulturist, walking in the footsteps of those who have practiced sericulture for millennia.

Over 99% of silk produced in the world is from the Bombyx mori moth. Through selective breeding, it has become the world’s only truly domesticated insect totally dependent on human care. Bombyx mori silkworms eat only the leaves of the mulberry tree (Morus alba, Morus nigra), which has been planted around the globe to provide raw material for them.

To this day, the origins of silk remain clouded in myth and mystery. The discovery of silk proteins, as well as bone needles and weaving tools, in tombs in Jiahu, Nenan Province, suggest sericulture was well established in China as long as 8,500 years ago. Regardless of how its mysteries might unravel, silk has captured the imagination of artisans for thousands of years.

A male Bombyx mori moth, just emerged from its cocoon.

Every spring I raise Bombyx mori caterpillars, and when I am able to find wild moth eggs, I also raise native Polyphemus caterpillars. Like an expectant mother, I am presently awaiting a Polyphemus moth to emerge from a cocoon I harvested in the forest last fall. If the moth is female, I will tether it with sewing thread in the hopes of attracting male moths to fertilize her eggs. If the moth is male, I will release it to find a mate.

There are hundreds of varieties of silk moths in the wild. These varieties are primarily from the family Saturniidae, the giant silk moths, of which the Polyphemus is one. The caterpillar species and the type of vegetation it eats determine the colour and quality of the cocoon. Wild silks tend to be more textured than Bombyx and range in color from a pale tan to a dark golden brown.  The caterpillars live and eat in a variety of trees, and the colour of their cocoons is determined by their diet.

Silkworms are the caterpillar, or larval stage, of a silk moth. Silk is the glandular secretion the caterpillar uses to create a secure cocoon, protecting itself from the elements while transforming from larva into a moth.

The lifecycle of a silkworm lasts twenty-eight to thirty-two days, depending on temperature. I raised my first Bombyx mori caterpillars from eggs I brought from Laos. Moths were allowed to mate and produce eggs for the following season. Bombyx mori eggs go into diapause, similar to hibernation, and can be kept in a refrigerator until the next season. I have also purchased eggs in Canada from Silkworms.ca.

Once eggs are removed from cold storage, they usually hatch within seven to fourteen days, and they thrive in temperatures of 25 C to 31 C. The eggs are the size of the tiniest pin dot and are hatched in a small incubator to ensure a consistent temperature.

The larvae, called Kego or Keiko in Japanese, hatch by eating a hole in the soft covering of their eggs. Once hatched, they remain in the incubator until they are about two weeks old. Kego must eat within four hours of hatching or they will die of dehydration. They are fed fresh leaves up to three to four times per day.

Silkworms molt four times during their lifecycle, with each new skin allowing space for additional growth. The period before and after molts are called instars.  When a silkworm is ready to molt, it stops eating and remains motionless for one day. Over its lifecycle, a silkworm increases to 10,000 times its original size. As the silkworms increase in size, it is necessary to relocate them to new accommodations. At the farm in Laos, bamboo trays were used to house the caterpillars. I use sandwich-type trays with lids, which are perfect for housing them and are stackable as caterpillar condos.

One of my small pleasures is hearing the sounds caterpillars make, similar to the sound of falling rain. This sound is attributed to their mandibles munching on the leaves and their feet moving over the leaves. Not quite so romantic is the sound of their frass (poop) dropping!

During the fifth instar, silkworms gorge themselves in preparation for pupation. The worms have insatiable appetites. By the next feeding, a mere four hours later, all the leaves are eaten to twigs and the worms are looking for more food. Larvae change from primarily increasing their body mass to mainly forming silk proteins.  As the digestive tract empties, it causes the caterpillar to become somewhat translucent, and they begin to wander in search of a place to spin their cocoon.

The best cocooning practice is to provide worms with individual compartments, allowing  them to produce a perfect cocoon. Egg cartons work well, as do toilet rolls. The caterpillars locate a suitable spot and commence their silk hammocks.  This silk is called flossing, keba or blaze. It is coarse, irregular and not always a continuous strand. Once the hammock is built, serious cocooning begins.

Cocoons are made from protein filament extruded by the silkworm through a spinneret located near its lower lip. The filament is made of two separately excreted threads of fibroin (brin) glued together by a protein substance called sericin. Silk is in liquid form inside the silkworm’s body, but solidifies when it comes in contact with air. As the silkworm produces silk, it moves its head in a figure-eight pattern. Three to four days are usually required for the caterpillar  to complete its cocoon. The caterpillar does not break the filament through the entire process.   This allows the cocoon to be unwound, or reeled, into a fine continuous thread.

After two weeks, a mature moth exudes a brown liquid enzyme that softens the cocoon, allowing it to emerge. If moths are allowed to emerge from their cocoons, the filament is no longer continuous and is unsuitable for reeling. Moths are flightless and have no mouths to eat or drink, and usually die within five days. Their sole function is to mate. In my practice, I allow a small number of moths to mature and emerge from their cocoons to produce eggs. The remaining cocoons are harvested within a week of spinning and are reeled fresh.    If this is not possible, the pupae are stifled (killed) with heat and the cocoons stored for future reeling. 

Polyphemus cocoons, pupation from caterpillar to moth happening inside.

Cocoons vary in length from 600 to 1,500 yards. Fully intact cocoons may be reeled, while damaged cocoons, where the filament is no longer continuous, may be stretched into mawata and spun. Waste from the reeling process is degummed, cut and carded for hand spinning.

My preferred method of working with raw silk is to reel it. Silk was first reeled by holding a cocoon in one’s mouth to soften the silk fibres, and then the filament was wound onto fingers or a stick. I prefer to soak the cocoons in hot water.

Using a brush, 20-25 cocoons are groped to find the filament ends. Once the ends are secured, the cocoons are unwound. Some cultures lay the unwound silk in a figure-eight pattern in a basket, while other cultures use silk reels. While I have used both methods, I like to reel silk through a croissure onto an antique Japanese reel or a modern Clemes & Clemes reel. The croissure serves to squeeze the filaments together into one filament and removes any excess water.

Twist is added to the filaments at the spinning wheel, and filaments are then plied together to make a variety of yarns, ranging from sewing thread to knitting yarns.  Yarns are dyed with both natural and acid dyes and used in knitted lace, knotted pile weaving, tablet-woven bands and embroidery.

My path down the silk road has been an incredible journey. Working with caterpillars, harvesting cocoons and reeling silk is very satisfying and truly makes my heart sing. As an artist, I dream of transforming my harvest into everything from knitted lace underwear to cut-pile carpets. And just like farmers everywhere, my thoughts are always on next year’s crop.

Click through the photogallery below to enjoy Coleen’s photographs of her sericulture practice and process, and the yarn that results.

All images courtesy Coleen Nimitz.