For the cotton-centered sewist, silk tends to be the most mysterious and daunting of fibers. I’ll try to demystify silk for you in a series of posts this week. Like previous series on rayon and linen, I’ll talk about how the fabric is made, different weaves and knits, and sources for buying yardage.
Today: all about silkworms and the process of silk harvesting.
The silkworm is the larva (caterpillar) of the moth Bombyx mori, a domesticated moth that no longer occurs in the wild and, in fact, cannot live outside the controlled environment provided by humans. They are descended from Bombyx mandarina, which still exist in the wild. Wild silk is harvested and made into textiles as well. Many sources use the term “tussah” to refer to any wild silk, but it is actually silk from a particular species of moth, Antheraea pernyi. More about tussah here.
The female adult lays eggs, which take 10-14 days to hatch. The larvae eat the leaves of the white mulberry tree, a native of China. Over the next two weeks, they eat day in and day out, multiplying in size many times over and molting four times in the process.
The silkworms spin a cocoon from their salivary glands — this is silk in its raw form. The silkworm inside is killed by heating, usually by steaming. If the moth is allowed to hatch, it damages the thread. One cocoon contains 900-1,500 meters (2,900-4,900 feet) of ultrafine raw silk filament. Dipping the cocoons into hot water starts the unraveling process.
These filaments are spun together to make silk yarn, which can then be woven or knit to make cloth. It takes at least a couple thousand cocoons to produce one pound of silk.
People for the Ethical Treatment of Animals has a stance against silk. What do you think? Are you upset that insects have been bred so much that they can no longer live apart from humans? About the massive amount of insect death that must occur to harvest the silk?
You might be asking about the environmental impact of silk production. According to author Kate Fletcher in the book Sustainable Fashion and Textiles: Design Journeys, cultivation of the mulberry trees requires far fewer chemical fertilizers and pesticides than conventional cotton, since the worms are very sensitive to these chemicals. Wastewater from degumming the fibers is a minor pollutant. Overall it’s relatively eco-friendly, but that’s not including the dyeing, printing, and post-processing methods that have varying degrees of environmental impact.
Did you know you could grow silk at home (or in the classroom)? Cultivating silkworms for their silk is called sericulture, and there is even a little amateur sericulturalist community on the web. Wormspit.com is is not only just about the best name for a website ever, it shows the process step-by-step with tons of pictures. You can get a good idea of the size of the worms and their cocoons. There’s also an interesting blog. Below, a segment from the HGTV show That’s Clever featuring wormspit.com proprietor Michael Cook harvesting silk and weaving a bookmark from the yarn.
Sources: Silk Road Foundation; Wormspit.com; Wikipedia: silk, silkworms, sericulture; books: All About Silk: A Fabric Dictionary & Swatchbook (Fabric Reference Series, Volume 1) by Julie Parker; Sustainable Fashion and Textiles: Design Journeys by Kate Fletcher.
Big thanks to Michael Cook for helping me correct several errors in the first draft of this post.