As you know, I explore hidden parts of the world to find and create textiles that are socially and environmentally beneficial (… officially the best job ever). I’ve just come back from a visit to our weavers in Asia, and today I wanted to give you a little peek into how our silk fabric comes to life. How is silk fabric made??
Where does it start?
It all starts with these guys. Silk is an animal protein fibre produced by certain insects, like worms and spiders, to build their cocoons and webs. Ugly to some, these little silk worms are the very beginning of the incredible journey to making some of the most stunning fabrics out there – just check out the silk range on Offset Warehouse if you don’t believe me!
The “silkworm” is technically not a worm but a moth pupa. They are always referred to as worms, however, and I’m happy to go with the majority on that one! These particular worms are called Bombyx Mori, the mulberry silk moth, so-called because they feed on mulberry leaves. They are a breed of silk worm that relies on human intervention to survive – they are domesticated.
This practice of breeding the silkworm for the production of silk is known as sericulture. It’s unsure as to when sericulture first began, but it’s certainly been at least 5000 years, which is when it was taking place in China. From there, it spread to Korea and Japan, and later to India and the West. Over millennia, the silkworm was slowly domesticated from the wild silk moth, Bombyx Mandarina.
As I mentioned, many insects produce silk, but only the filament produced by this Bombyx mori and a few others in the same species is used by the commercial silk industry.
Eggs take about 14 days to hatch into larvae, which eat continuously – literally bushels and bushels of mulberry leaves! The worm droppings are black (why you’d want to know that, I’ve no idea, but I thought it was important). There are lots of phases of the larvae, as they hatch from tiny pin head size and grow into these big old worms (well, 30 day old worms to be precise). When the colour of their heads turns darker, you know that they are about to “moult“. After the first moult, the instar phase of the silkworm begins.
How does the worm turn into a cocoon?
After they have moulted four times, their bodies become slightly yellow and the skin becomes tighter. The silkworm attaches itself to a compartmented frame (above), twig, tree or shrub in a rearing house to spin a silk cocoon over a three to eight day period. The larvae then prepare to enter the pupal phase of their life-cycle and enclose themselves in a cocoon made up of raw silk produced by the salivary glands. Steadily over four days, the silkworm rotates its body in a figure of eight movement approximately 300,000 times, constructing a cocoon. The final moult from larva to pupa takes place inside the cocoon, which provides a vital layer of protection during this phase.
These are the cocoons that the larvae produce – aren’t they incredible? The cocoon is made of a single thread of raw silk which is usually between 300 and 900 metres long (that’s 1,000 to 3,000 feet). The fibres are very fine and lustrous, about 10 μm (0.0004 in) in diameter – if you don’t know about microns, check out this article. Approximately 2,000 to 3,000 cocoons are required to make a pound or 400 grams of silk. Interesting bit of trivia – at least 70 million pounds of raw silk are produced each year, requiring nearly 10 billion pounds of cocoons!
But what exactly is this fibre made of?
Silkworms have salivary glands called sericteries, which are used for the production of fibroin – a clear, viscous, protein fluid that is forced through an opening in their heads called the spinneret. The diameter of the spinneret determines the thickness of the silk thread (or micron), which is produced as a long, continuous filament. A second pair of glands secretes a gummy binding fluid called sericin which binds the two filaments together. Why did I tell you all that? Well, the sericin is tricky to get off and this forms part of the production process later on!
The cocoons are then put into a shallow, woven basket made of bamboo, like the one below. You may find a wood engraving of a cat on the basket – particularly in superstitious areas. This is because mice often climb to the shelves of silkworms for a tasty treat, and cats make excellent deterrents! This is a bit like putting a fake heron next to our ponds in the UK… although I can’t imagine a picture of a cat does much deterring to a mouse. In any case, that’s why you might find that not only do the sericulturists continue to put pictures of cats in their baskets, but they also nearly always raise cats!
There are lots of superstitions surrounding sericulture – it’s quite fascinating! In the past, when sericulture was a home business, there were many taboos surrounding the raising of silkworms. Women weren’t allowed to see their friends and men weren’t allowed to be shirtless around hatching silkworms. Any loud noises were completely forbidden – including shouting or crying children and knocking on doors or windows (how they managed the former, I’d like to know!). Liquor, vinegar and anything smelling of fish or mutton wasn’t allowed, and it you weren’t allowed to dig, cut grass, husk rice with a mortar and pestle or burn fur and hair around a sericulturist’s home. My favourite of the traditions, is when a couple got married in China, the girl’s parents would send the couple two young mulberry bushes, two round shallow baskets of silkworms, silk clothes and silk bedding as a dowry. This gesture was a symbol of the hope that the bride would bring much business! … a girl after my own heart.
How does the cocoon turn into a fibre?
At this point, I was so distracted by the process that I completely forgot to take a photo! But I did take a video, so please forgive the rather poor quality of the screen grabs. The cocoons are submerged into boiling water. The silk is then unreeled from the cocoon by softening the sericin and then delicately and carefully unwinding the filaments. You can see here that this skilled silk worker, Rose, is ‘reeling’ many many fibres – the norm is between four to eight cocoons at once.
As you can probably see, a single thread filament is too thin to use on its own, so many fibres are combined to produce a thicker, usable fibre. This is done by hand-reeling the threads onto a wooden spindle to produce a uniform strand of raw silk. As the fibres are being drawn through the small hole of the wooden panel above the pot, Rose periodically stirs the pot, which twists the fibres together – more on why we want twisted fibres here!
The twisted fibres, now a thread, then feeds over the round barrel above this plank of wood, and then onto the spindle that is constantly being turned by hand. It takes Rose nearly 40 hours to produce a half kilogram of silk.
As the sericin protects the silk fibre during processing, this is often left on the fibres until the yarn or even woven fabric stage. Raw silk is silk that still contains sericin. Once this is washed out (in soap and boiling water), the fabric is left soft, lustrous, and up to 30% lighter.
What happens to the moth inside the cocoon?
If the moth is allowed to survive once it’s spun its cocoon, it will eventually emerge as an adult moth. To emerge, it releases proteolytic enzymes to make a hole in the cocoon. These enzymes are destructive to the silk and can cause the silk fibers to break, reducing a fibre over a mile in length to segments of random length. This seriously reduces the value of the silk threads, unless the cocoons can be used as “stuffing”, as they are in China and elsewhere for doonas and jackets, and so on. To prevent this, silkworms are usually not allowed to hatch from their cocoons, and so are boiled or sun dried. The heat kills the silkworms and the water makes the cocoons easier to unravel.
That means that silk is not a vegetarian fabric!?
Yes. And no.
In this instance, yes, the silkworm has been boiled. The difference here, however, is that the silkworm itself is eaten and so the silk is a by-product. Just as you might eat beef, lamb or pork and use the hide to make leather.
And when the moths emerge?
Unlike its predecessor, the wild Bombyx mandarina, as the moths have been bred for domesticity for thousands of years, they never fly and the wings are essentially vestigial. The males will buzz their wings sometimes and stomp around, but none of them can manage even just a short flight. The females are so big with eggs, they can barely walk. You can actually see yellow eggs through the skin of the female between the furred stripes. Silk moths have a wingspan of three to five centimetres and a white, hairy body. Females are about two to three times bulkier than males, but are similarly colored. Adult Bombycidae have reduced mouth parts and do not feed, though a human caretaker can feed them.
The rather unattractive brown stains around the moths is meconium. It’s the fluid that moths use to inflate their limp wings after they emerge; once the wings have hardened, the fluid is secreted as waste.
As soon as the female moths emerge they almost immediately secret a pheromone, a scent. This drives the males crazy and they flap and waddle around to find them. Usually, moths will wait until dark to do this – but not these moths! Once he finally finds her, the male grasps her by the abdomen, flap their wings in short bursts for a while, and then stop. They remain paired like this for twelve to twenty-four hours. They can be safely separated after three hours, because the male will have inseminated the female. You can manually separate them, but you must be careful not to damage the female’s ovipositor and cause her trouble laying eggs. A single male can be reused for an additional one to two matings.
Once they’ve done their thing, the female will begin to lay her eggs. Most females lay between 150 and 300 eggs over two to three days. Some moths can lay as many as a thousand! The eggs start out lemon yellow, and over a day or two will fade through tan to dark gray or black. You can see that they are still lemon yellow in the photo above. If they don’t change colour, the eggs are most likely infertile.
After they change to dark gray, they can be placed into cold storage, which will keep them safe until needed. If the eggs are not refrigerated, sometimes they will hatch, and the process starts over again!
Stay tuned for my next post, where I’ll show you the next steps on how these amazing fibres get woven by hand into the fabric that we sell!
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