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Secrets of Silk

An ancient mystery reaches the end of the road.

By Jacqueline Adams | October 26, 2009
Courtesy American Museum of Natural History.
Courtesy American Museum of Natural History.

The people of ancient Rome faced a mystery: Traders returned from the East with a fine, shimmering cloth like nothing else they had ever seen. It was silk. This strong but soft material kept wearers cool in hot weather and warm in cold weather.

Roman citizens, from the lowest classes to the Emperor himself, were soon hooked on silk, paying high prices for the luxurious material while they tried to figure out how to produce it themselves. Because they made other cloths from plants, such as flax and cotton, they mistakenly thought silk came from a plant too. Read on to see why silk-making was so mysterious and how the secret finally got out.

An Ancient Mystery

Romans didn’t have direct contact with the people who manufactured silk. The glistening material came from China via the Silk Road, a network of trading pathways stretching from Asia through the Middle East and Europe. On these pathways, camel caravans carried goods across terrain that included harsh deserts and high mountains. But merchants didn’t travel the whole route. Goods moved through a series of middlemen. “People would take their goods a couple hundred miles; they’d trade it for other things, and then they’d go back in the other direction,” says Mark Norell, curator of the exhibition Traveling the Silk Road at the American Museum of Natural History in New York City. “So not only did the people at the ends of the Silk Road not know what the people at the other end looked like, they didn’t even know where the goods came from.”

China’s rulers guarded the secret of sericulture, or silk production, with extremely harsh penalties. Norell says, “They had a virtual monopoly on making silk during this time period, so much so that it was a penalty of death if you shared silk-making secrets with foreigners.” When Romans looked to plants for the answer, they were barking up the wrong tree. At the other end of the Silk Road, tiny creatures were spinning the mysterious fabric.

Crafty Caterpillars

The Chinese discovered the secret—that caterpillars spin cocoons of silky filaments—at least 5,000 years ago. Through selective breeding of moths with the desired traits, they developed a domestic species of silk moth (Bombyx mori) whose caterpillars produce smoother, finer filaments. These moths are blind and flightless, and can survive only under the care of silk farmers. Each female moth lays up to 500 pinpoint-size eggs before dying.

Caring for the creatures was a time-consuming project for China’s women, who handled most of the sericulture. They placed the eggs in shaded trays to keep them at the right temperature. Ten to 12 days later, the larvae, in the form of tiny caterpillars commonly called silkworms, crawled out. To feed these picky eaters, who munch around the clock, the women chopped the silkworms’ favorite food: mulberry leaves. The caterpillars molted, or shed their skin, as their weight multiplied 10,000 times in one month. Workers carefully protected these fragile creatures from drafts, loud noises, and even strong odors.

After several caterpillar molts, farmers set up frames of twigs. Each caterpillar climbed a twig and secreted a silk thread from its spinneret (tube in its mouth). The thread is made of two strands of fibroin. Fibroin is a protein (substance made of chains of chemicals called amino acids) that is produced by a pair of silk glands. These glands also produce another protein, sericin, that glues the two threads together as the silk hardens in the air. The result: a white, puffy cocoon made of a single filament 600 to 900 meters (1,970 to 2,950 feet) long.

End of the Line

Normally, the pupae inside the cocoons would transform into moths. But silk farmers couldn’t let that happen. Norell explains: “If it changes into a moth, the threads are broken as the animal climbs out.” So the farmers killed the pupae by baking or steaming the cocoons, or soaking them with salt water. Then they dumped the cocoons into boiling water to remove the sericin and loosen the filament.

Workers wound filaments from roughly five to eight cocoons on a wheel to make a single silk thread, then rewound them to make them finer. They wove the finished threads into cloth. To get enough silk for one robe, farmers had to successfully raise 2,500 caterpillars.

Timeless Technology

The Silk Road reached its peak during China’s Tang dynasty (618 to 906 A.D.) as a trading network not only for silk but also for many other goods, technologies, and ideas. “Just about everything that could be traded was traded,” Norell says.The practice of sericulture gradually spread to other lands. In 552 A.D. the secret reached Europe through two monks who, as the story goes, smuggled silk moth eggs inside hollow bamboo walking sticks. But the highest-quality silk still came from China.

After thousands of years, the steps for producing silk haven’t changed. “Quite a bit of it has been mechanized, but the basic principles of it are still identical: You’re growing the silkworms and then boiling the cocoons, and then pulling the threads off the cocoons and spinning those into more usable, consistent threads,” says Norell. When you wear a silk shirt, camp with a silk-lined sleeping bag, or put silk sheets on your bed, you’re benefiting from an ancient technology that is popular even in today’s modern world.

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