Silk – the Basics and a Little Beyond

Humans have been fascinated with silk for thousands of years.  The oldest piece of silk cloth found to date is just a tiny fragment,  found near Shanghai.  It dates back to about 2750 B. C. and has 53 warp threads and 59 weft threads to the centimeter!  That would be equivalent to a 250 thread count sheet.

There are many moths and spiders that spin silk, but the one that has been easiest for humans to manage is Bombyx mori, the silk moth. (Since spiders are carnivorous, finding them a food supply while raising them might be problematic.)

In their natural life cycle, a moth lays about 500 eggs, which then stay dormant throughout the cold months of winter.  When the caterpillars hatch, they begin eating the leaves of the mulberry tree, growing until they need to split their tough outer skin.  The new skin underneath is soft and can accommodate more growth.  The caterpillar molts like this about 4 times, until it has grown to 8000 times its initial weight , and then begins to spin its cocoon.  It will take 2 or 3 days to spin, turning and tossing its head, about 200,000 times.  Inside the cocoon it will metamorphose into a pupa or chrysalis, and eventually into a moth.  (Butterflies, by the way, can spin a few threads of silk to anchor themselves when they pupate, but do not spin true cocoons.  Children’s books that show them doing so drive me crazy!)

silk moth life cycle

Silk moth life cycle, from 1915 Textiles

After ten days, the now-developed moth releases a liquid enzyme that creates a hole in the cocoon, emerges, lets the blood pump into its wings, and reaches its full size of 2 inches.  The moth has no mouth parts because it will only live about three days in that stage.  It will immediately mate, and females will go on to lay their eggs, moving less than 3 inches in those days.

Of course humans have been managing this cycle for thousands of years, trying to reap the most silk possible.  Over the last few decades technology has made all the conditions of silk management more consistent.  The eggs are inspected for quality, and then kept at warm temperatures and 75% humidity for about 12 days, and then sold in boxes of 20,000.  Then the eggs are kept in cold storage for four or five months to simulate winter, then they are warmed up until they hatch.  (Traditionally the eggs were sometimes warmed by being carried about on the body of the silkworm worker until they hatched.  I know I would have smooshed several of them. )

Humans raise the mulberry trees to provide food for the worms, who after centuries of domestication, have lost the ability to forage for themselves.  Different species of the trees are raised, so that the leaves will bud out at different times, making food always available for the caterpillars.  It would take 2 1/2 acres devoted to mulberry trees, to provide food for 400,000 silkworms, to end up with 1540 pounds of cocoons, for 220 pounds of silk!  The caterpillars are continually fed with bits of fresh leaves.  My hundred-year-old textile books say that it would take one to two months for the caterpillars to reach the stage of spinning their cocoons – my modern book says one month, so I would guess a little selective breeding has gone on.

The object of all this effort is the filament of the silk worm’s cocoon.  It is made of two strands of protein called fibroin, glued together with irregular patches of another protein called sericin.  The fibroin strands are smooth and triangular, reflecting light at different angles.  There are pages of information about the domains, peptides, molecular weight, etc. of these protein families in the book Silk by Mary Schoeser [2007, Yale University Press], but I have an old-fashioned kind of mind, so I will show you instead a diagram from 1915.

raw silk diagram

Fbroin filaments with sericin wrapped around them, from 1915 Textiles

One cocoon filament can be 1200 meters (1300 yards) long.  Obviously if the moth is allowed to emerge on its own, that long filament is broken into very short pieces, so most silk worms are killed with heat before they would emerge. (They don’t go to waste – they are used for animal food or even as snacks for humans.)

Even when the cocoon is intact, the outer layers are too tough to be used, and the innermost filament is too thin, so only about 30 % of the cocoon can be used for the highest quality, reeled silk.  The rest is set aside and saved for other uses.  It can be spun just like wool or cotton.

Reeling combines the thin cocoon filaments into one glossy thread.  As I understand it, the cocoons are put in almost boiling water to soften the sericin.  Traditionally, a worker would brush the cocoons to find the end of the filament, but even 100 years ago, a mechanical rotating brush was in use.  The brush sweeps against the cocoons forward and back several times, then lifts up.  Some loose ends will be caught and those cocoons will lift up.  A worker takes about six of these filaments (depending on the desired thickness of the thread) and runs them through an eyelet, and then attaches them to the reel, a simple frame cylinder that keeps winding the silk into a skein.  As these filaments cool and dry out, their sericin hardens, sticking all the fibers together into one smooth thread.

Workers keep constant watch on the threads, to add in another filament when one cocoon runs out.  Also, the very innermost parts of the cocoon are too thin to use and must be added to the waste pile.

I checked out a few dozen YouTube videos, and I think these illustrate the process best.  First we have the very traditional handreeling, still done in Thailand.  Verónica at Agujas has a lovely post about this too.

Then we have a mechanized plant in China.

And finally we have an impressive state-of-the-art silk plant in India.

The video is 9 minutes, but if you just watch a little bit, you will be amazed by how huge and complicated it is.  I would love to tour it someday!

While we are on the subject of silkworms, I first learned from Karen at The Sweaty Knitter, that some people protest against silk’s use, because of the fact that the silkworms die in order for us to have silk.  I found out that this issue has been around for about 1500 years, since Buddhists along the Silk Road had to decide whether monks and temples could accept donations of bolts, skeins, and robes of silk, at a time when cotton was rarer and more expensive than silk.  One philosopher’s take was that the good intention of the donor outweighed the suffering of the worms.

PETA’s statement against the use of silk is here – I can see their point, but it seems to me the damage was done centuries ago when the moths were domesticated to the point that they can no longer fend for themselves.  Also, it seems a little ironic that PETA suggests using petroleum products such as nylon and polyester as substitutes.  Doesn’t it always come back to carbon footprint?  Anyway, for those who are concerned about it, there is ahimsa silk, which is made by using the broken cocoons of moths who are allowed to emerge naturally.

I myself could probably get along just fine without silk – it tends to catch on chicken wire, dewberry brambles, and fallen trees.  But I would not want to do without my treasured books which show a myriad of  inspiring silk creations from the world over!