Silk

• 1. Introduction

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  Silk came up from a shipwreck in 1840 looking whole, pulled from a wreck that had gone down in 1782. Divers recovered a pair of black satin breeches and a satin waistcoat, the silk itself undamaged though the lining had rotted away. One account from the time called it flatly: "the most durable article found has been silk." No woollen clothing from that same wreck was ever recovered.

  That kind of durability starts somewhere far less impressive: inside the cocoon of an insect larva. This program asks what actually makes that thread so strong. It asks why one civilization guarded the process behind it as a closely held secret for so long. And it asks why something spun by a caterpillar still turns up in parachutes, surgical stitches, and centuries of religious argument.

• 2. The Anatomy Of Shimmer

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  Fibroin makes up 75 to 80 percent of a silk fiber, with a thinner coating of sericin covering the rest. Sericin accounts for the remaining 20 to 25 percent, acting as a natural glue holding the strands together. The fiber itself is a natural protein, produced by insect larvae as they build a cocoon around themselves. Its cross section is shaped like a triangular prism. That geometry splits incoming light at different angles, producing silk's natural shimmer.

  The best known source is Bombyx mori, the mulberry silkworm, reared in captivity through a practice called sericulture. Most moth caterpillars can produce silk, but only a handful of species have ever been used commercially for textiles. Insects far beyond moths make silk too: webspinners and raspy crickets spin it throughout their entire lives, not just as larvae. Bees, wasps, ants, silverfish, caddisflies, mayflies, thrips, leafhoppers, beetles, lacewings, fleas, flies, and midges all produce silk in some form. So do arachnids such as spiders.

  The word silk traces back to sioloc and the Latin sericum, itself from the Greek σηρικός, meaning "silken." Both ultimately point to the Chinese word sī, echoed in the Mongolian sirkek. That linguistic trail across three continents hints at how far silk travelled before anyone outside China ever wove it themselves.

• 3. China's Guarded Secret

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  At a Neolithic site called Jiahu, in Henan, soil from two tombs contained traces of fibroin dating back about 8,500 years. That is the earliest known evidence of silk anywhere. The oldest surviving silk fabric dates from around 3630 BC. It was used to wrap the body of a child at a Yangshao culture site called Qingtaicun, near Xingyang, also in Henan.

  Legend credits a Chinese empress named Leizu, also called Hsi-Ling-Shih or Lei-Tzu, with developing silk in the first place. Silk was originally reserved only for emperors, used for their own clothing and as gifts, before it spread through Chinese society and trade. By the Warring States period, between 475 and 221 BCE, silk was even used as a writing surface. It survived the Yangtze region's damp climate and absorbed ink well.

  China kept its production process confined within its own borders until the Silk Road opened sometime in the latter part of the 1st millennium BC. Even after that, the monopoly held for another thousand years, as emperors worked to keep the secret of sericulture from leaking out. The secret finally broke around AD 550, when it reached Europe through the Byzantine Empire. According to contemporary accounts, monks working for the emperor Justinian I smuggled silkworm eggs out of China inside hollow canes, carrying them to Constantinople.

  Inside Constantinople, every top-quality loom and weaver was kept within the Great Palace complex. The cloth produced there went into imperial robes or diplomatic gifts, with the rest sold at very high prices.

• 4. A Trade That Reshaped Nations

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  Silk turned up in the hair of an Egyptian mummy from the 21st dynasty, dated to around 1070 BC. It is the earliest known evidence of long-distance silk trade. From there the trade reached the Indian subcontinent, the Middle East, Europe, and North Africa. That network of routes eventually took the name the Silk Road. Sericulture itself spread more slowly. It reached Korea around 200 BC with Chinese technical help, the ancient Kingdom of Khotan by AD 50, and India by AD 140.

  Between 1850 and 1930, raw silk was the leading export for both Japan and China. It made up 20 to 40 percent of Japan's total exports, and 20 to 30 percent of China's. Japanese silk exports then quadrupled between the 1890s and the 1930s, making Japan the world's largest silk exporter. Economic reforms during the Meiji period drove the rise, as the Qing dynasty's industries stagnated in China. World War II embargoes against Japan pushed manufacturers toward synthetics like nylon, ending Japan's run as the top silk exporter. China now exports the largest volume of raw silk in the world.

  India ranks as the second largest silk producer after China. About 97 percent of its raw mulberry silk comes from six states: Andhra Pradesh, Karnataka, Jammu and Kashmir, Tamil Nadu, Bihar, and West Bengal. In Europe, Italy became the medieval era's most important silk producer, starting with the city of Catanzaro in Calabria during the 11th century. Catanzaro's silk supplied nearly all of Europe and sold at a market fair in the port of Reggio Calabria to Spanish, Venetian, Genovese, and Dutch merchants. Since the 15th century, France has centered its silk industry in Lyon. In northern Italy's Piedmont region, water-powered machines took over the work of silk throwing.

  In England, James I once planted 100,000 mulberry trees, some near Hampton Court Palace, hoping to start a silk industry there. The trees turned out to be the wrong species for silkworms, and the attempt failed.

• 5. Faith And Forbidden Cloth

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  In the Torah, a scarlet cloth called sheni tola'at in Hebrew, literally "crimson of the worm," appears in purification rituals. Leviticus 14 lists it alongside cedar wood and hyssop. The medieval translator Rabbi Saadia Gaon rendered that same phrase explicitly as "crimson silk" in his Arabic translation of Jewish scripture.

  Islamic teachings forbid Muslim men from wearing silk, a restriction many religious jurists trace to concerns about clothing seen as feminine or extravagant. Scholars still dispute how much silk a garment can contain and remain lawful for men to wear. One debated example is a small decorative silk patch on an otherwise cotton caftan. Modern dress raises the same question in a new form: is a silk necktie, a masculine article of clothing, permissible under that same rule?

  In the Odyssey, Odysseus, disguised as someone else, describes a husband's shirt as "gleaming like the skin of a dried onion." Some readers take that line as an early reference to silk's shine. Aristotle separately wrote about Coa vestis, a wild silk textile that came from the island of Kos. Roman emperor Tiberius passed sumptuary laws banning men from wearing silk, though the laws proved ineffective. According to the Historia Augusta, the third-century emperor Elagabalus was the first Roman to wear garments of pure silk. Before him, custom called for blends of silk and cotton or silk and linen.

  Sea silk from certain large sea shells also carried real value in the ancient Mediterranean, a fiber gathered from mollusks rather than insects at all.

• 6. The Chemistry Of Strength

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  Silk fibers from the Bombyx mori silkworm have a triangular cross section with rounded corners, measuring 5 to 10 micrometers wide. That fibroin-heavy chain is built mostly from beta sheets, arising from a 59-amino-acid repeat sequence with some variation. Other silkworm species produce different shapes entirely: a crescent-like cross section in Anaphe, and an elongated wedge in tussah silk. Each caterpillar extrudes silk from two glands as a pair of filaments called brin, glued together by sericin into a single strand called a bave. In tussah silk, that bave can reach 65 micrometers across.

  Silk feels smooth and soft, without the slipperiness common to many synthetic fibers. It ranks among the strongest natural fibers, though it loses up to 20 percent of its strength once wet. Its moisture regain sits at a healthy 11 percent, but its elasticity is only moderate to poor, meaning a stretched fiber tends to stay stretched. Too much sunlight can weaken the fiber over time, and dirty silk becomes more vulnerable to insect damage.

  Silk is a poor conductor of electricity, which makes it prone to static cling. It also has a high emissivity for infrared light, a property that makes silk feel cool against the skin. Unwashed silk chiffon can shrink by as much as 8 percent as its fiber structure relaxes. Dry cleaning alone can still shrink it by up to 4 percent. Gentle steaming with a press cloth can sometimes reverse that shrinkage, though the fabric shows almost no further shrinkage at the molecular level after that.

  Silkworm silk was actually used as the historical standard for the denier, a unit measuring linear density in fibers. Its own density works out to about 1.1 dtex. By comparison, a spider's silk, from Argiope aurantia, measures only about 0.14 dtex, a fraction of the diameter of Bombyx mori's fiber.

  Chemically, silk's fibroin has an amino acid sequence of Gly-Ser-Gly-Ala-Gly-Ala, folding into beta pleated sheets held together by hydrogen bonds between the chains. Roughly half of that sequence, by proportion, is glycine, an amino acid with no side chain, which allows the protein strands to pack tightly together. Alanine and serine add strength and resistance to breaking, since stretching the fiber spreads the load across many hydrogen bonds rather than snapping any single one. Silk resists most mineral acids, with one notable exception: sulfuric acid dissolves it outright. Perspiration yellows the fabric over time, and chlorine bleach destroys silk fibers entirely.

  Chemically dissolving a silkworm cocoon while keeping its molecular structure intact produces a material called regenerated silk fiber. That material is reportedly twice as stiff as ordinary silk.

• 7. From Cocoon To Cure

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  After hatching, silkworm caterpillars are fed only fresh mulberry leaves for about 35 days, through four rounds of molting. By the end of that period, they weigh 10,000 times more than they did at hatching and are ready to spin a cocoon. Two glands push liquid silk through openings on the head called spinnerets, and the thread solidifies into a sericin-coated strand the instant it touches air. Within two to three days, a single caterpillar spins about a mile of filament and is completely sealed inside its cocoon.

  Farmers then heat the cocoons to kill the pupae inside, though some are set aside to become moths and breed the next generation. Boiling water softens the sericin holding each cocoon together, letting workers unwind it into one continuous thread. Because a single filament is too fine and fragile on its own, three to ten strands are twisted together into one usable thread of silk.

  Harvesting silk kills the larvae by boiling them, a fact that has drawn criticism from animal welfare groups. People for the Ethical Treatment of Animals, among others, urges people not to buy silk. Mahatma Gandhi objected to silk production on similar grounds, guided by his philosophy of Ahimsa, or non-violence. His stance helped promote both cotton and a substitute called Ahimsa silk, made from the cocoons of wild and semi-wild silk moths.

  Silk has served as a biomedical suture material since as early as the second century CE. Manufacturers strip away the outer sericin coating to make the fiber usable as non-absorbable surgical thread. Researchers have spent the past 30 years studying silk fibroin as a biomaterial. It is valued for its mechanical strength, biocompatibility, and its ability to be shaped into films, gels, particles, and scaffolds.

  Scientists have also begun genetically modifying domesticated silkworms, altering the silk they produce. That work could open the door to entirely new proteins made inside a caterpillar's own glands.