How Silk Is Made: From Mulberry Leaf to Cloth

A silkworm grows to 10,000 times its hatching weight in about four weeks. A 7-pound newborn on the same schedule would reach 35 tons before the end of its first month, on nothing but mulberry leaves.

Then the worm stops eating, settles into a frame the farmer has set out for it, and spends two or three days making the one thing the whole trade depends on. What follows is part farm and part factory, and harder on the silkworm than most descriptions admit.

For the basics of what silk is, read what is silk and where does it come from? This is the step-by-step, with the numbers.

What do silkworms eat, and how fast do they grow?

Silkworms eat mulberry leaves, almost without stopping, for about four weeks. They shed their skin four times on the way up, and by the end the silk glands alone make up about a quarter of the worm's body weight, according to London's Horniman Museum.

Estimates of the leaves behind one pound of raw silk run from about 90 pounds to about 200, depending on the breed, the source and the century. Whichever figure you take, a silk farm is mostly a mulberry farm.

How does a silkworm spin a cocoon?

A silkworm spins by pushing liquid silk from two glands out through its head, looping its head in a figure-eight for two or three days until it has walled itself in. Two strands of a protein called fibroin emerge side by side, glued together by a second protein called sericin, and they harden in the air.

The result is one continuous filament. The UN Food and Agriculture Organization's silk reeling manual puts its total length at about 650 to 1,640 yards (600 to 1,500 meters), depending on the cocoon, with about 80 percent of that able to be reeled. The filament averages 15 to 20 microns across, which is finer than most human hair.

What happens to the silkworm inside the cocoon?

Most silkworms don't survive. Left alone, the pupa would become a moth and break its way out, which would cut that long filament into short pieces. So producers kill most pupae with heat before that can happen, usually by steaming or roasting. One schedule in the FAO manual dries cocoons in hot air starting near 208°F (98°C) and stepping down to 140°F (60°C).

Only the cocoons needed for breeding are left to hatch. The female lays 300 to 400 eggs and dies soon after, followed by the male. This is the part of the story silk marketing tends to skip, and it's why conventional silk isn't vegan. Is silk cruelty-free? goes through that in detail, along with peace silk, where the moth emerges first.

What is silk reeling?

Reeling is unwinding the filaments from several cocoons at once into a single thread, and it's why silk is described as reeled where cotton and wool are spun. The cocoons are first cooked in water and steam to soften the sericin. When the cooking is right, the FAO manual says, the filament unwinds by itself the moment its end is picked up.

One filament is far too fine to use alone, so about eight to ten are drawn together, and the softened sericin glues them into one thread. As a cocoon runs out, another joins, which keeps the thread an even thickness.

You'll read that all this is still done by hand. Some of it is. The FAO manual also describes automatic reeling machines with 400 ends to a set, which find and feed the filament ends themselves, with people correcting problems in the thread. What comes off the reel is raw silk, still stiff with gum. The part of the filament that can't be reeled is collected as waste, and waste silk can be spun into yarn much as wool is.

What is degumming?

Degumming is washing the sericin out of raw silk, usually in a hot soap bath, and it's the step that makes silk feel like silk. Sericin is 19 to 28 percent of the cocoon's silk by the FAO's figures, so the thread loses roughly a quarter of its weight here. It goes in stiff and comes out soft. It also shines, because the fibroin filament is roughly triangular in cross-section, and its flat faces reflect light at many angles.

Degumming can happen to the yarn before weaving or to the cloth afterward. In the 19th and early 20th centuries, some makers put the lost weight back with metal salts, with results you can still see in shredding antique silk, as why silk deteriorates explains.

How does silk thread become cloth?

The thread is twisted as needed, dyed either as yarn or as finished cloth, and woven on a loom. Twist and weave decide what the cloth becomes. Long floats of smooth thread make the glossy face of a satin weave, the kind often used for pillowcases. High-twist yarn in a plain weave makes the dry, sheer cloth called chiffon. The finished weight is stated in momme, which this guide explains.

How many cocoons does it take to make silk?

Estimates run from about 1,400 to 2,500 cocoons per pound of raw silk (roughly 3,000 to 5,500 per kilogram). The spread comes partly from the breed. A cocoon is mostly pupa by weight, and raw silk comes to only about 16 percent of it.

Stage	The number
Growth before spinning	10,000 times hatching weight, in about 4 weeks
Spinning	2 to 3 days
Filament in one cocoon	About 650 to 1,640 yards (600 to 1,500 m), about 80% reelable
Filaments in one raw silk thread	About 8 to 10
Raw silk as a share of cocoon weight	About 16%
Weight lost in degumming	Roughly a quarter

Run those numbers forward to something you might own. A 20 × 30 in pillowcase at 22 momme holds about 2.6 ounces of finished silk, which is about 3.5 ounces of raw silk before degumming. By our arithmetic that's roughly 300 to 550 cocoons, before anything lost in weaving and sewing.

How long have people been making silk this way?

Silkworms have been bred for more than 5,000 years, and the basic sequence of leaf, cocoon, hot water and reel is very old. The craft began in China, and who invented silk? sorts the archaeology from the legend.

See 22-momme, 100% mulberry silk pillowcases from LS Silk →

Four weeks of leaves, two or three days of spinning. Everything after that is people trying not to break what the worm made.

Frequently asked questions

How long is the silk thread from one cocoon?

The FAO's silk reeling manual gives a total of about 650 to 1,640 yards (600 to 1,500 meters), depending on the cocoon, and says about 80 percent can be reeled. Britannica's figure for the usable length is 2,000 to 3,000 feet (600 to 900 meters). It's one unbroken filament, though it's too fine to use alone.

Are silkworms killed to make silk?

In conventional silk, yes. Most pupae are killed with heat before the moth can emerge, because a moth breaking out would cut the long filament into short pieces. A small number are left to hatch for breeding. Peace silk lets the moth emerge first, which gives a shorter, coarser fiber.

How many silkworms does it take to make a silk pillowcase?

By our arithmetic, roughly 300 to 550 for a 20 × 30 in pillowcase at 22 momme, before anything lost in weaving and sewing. That comes from about 3.5 ounces of raw silk and estimates of about 1,400 to 2,500 cocoons per pound. Treat it as an estimate, since cocoon size varies by breed.

What is the difference between raw silk and degummed silk?

Raw silk still carries sericin, the natural gum that holds a cocoon together, so it feels stiff and looks dull. Degummed silk has had the sericin washed out, which removes roughly a quarter of its weight and leaves the soft, shiny fibroin. Most silk cloth you'd wear or sleep on is degummed.

Is silk spun or reeled?

The long filament silk used for smooth cloth is reeled, meaning several cocoon filaments are unwound together into one thread. Spinning is what's done to short fibers such as cotton and wool. Broken filaments and waste silk can also be spun, which gives a duller yarn than reeled silk.

Related reading: Who Invented Silk? · What Is Momme in Silk?