Silk originates from the cocoons of silkworms, primarily the domesticated moth species Bombyx mori, which has been raised in China for at least 5,000 years. The fiber is a protein that the caterpillar produces from specialized glands and spins into a protective shell before it transforms into a moth. From that single biological source in ancient China, silk production eventually spread across Asia, the Middle East, and Europe to become one of the most prized textiles in human history.
How Silkworms Produce Silk
A silkworm is not actually a worm. It’s the larval stage of a moth, and it spends its short life doing essentially one thing: eating mulberry leaves and converting them into silk protein. The caterpillar is monophagous, meaning it eats only one food source. Mulberry leaf proteins are directly responsible for over 70% of the silk a worm produces, and the finished fiber is composed mainly of three amino acids: alanine, glycine, and serine.
The silk itself is made in a pair of glands that run along the length of the caterpillar’s body. These glands produce a liquid protein called fibroin, which forms the core of the silk strand. As the fibroin moves through the glands toward the worm’s mouth, it gets coated in a second protein, sericin, which acts as a natural glue. When the caterpillar is ready to pupate (usually after about a month of feeding), it begins moving its head in a figure-eight pattern, extruding a single continuous filament and binding it into a tight cocoon. One cocoon can contain a filament roughly 300 to 900 meters long.
The Oldest Evidence of Silk Use
The earliest physical evidence of silk in China comes from Neolithic archaeological sites. At Xiyin Cun, a site from the Yangshao culture in northern China’s Shanxi Province (roughly 5000 to 3000 BC), excavators found what appeared to be an artificially cut cocoon. Later analysis identified it as a wild silk species native to China rather than the domesticated Bombyx mori, but it still points to very early human interest in silk-producing insects.
The oldest confirmed evidence of domesticated silk comes from Qianshanyang, a Liangzhu culture site in Zhejiang Province in southern China, dating to approximately 2750 BC. Remains there include Bombyx mori silk woven in a tabby weave, the simplest interlacing pattern. This tells us that by roughly 4,700 years ago, people in China were not just collecting cocoons but actively farming silkworms and weaving their fiber into fabric.
Chinese legend credits the discovery of silk to Empress Leizu, wife of the mythical Yellow Emperor. The most popular version of the story says a silkworm cocoon fell into her cup of hot water while she sat in the palace gardens. When she pulled it out, the heat had dissolved the sericin binding the cocoon together, and a single delicate thread unraveled in her hands. Whether or not the legend reflects a real event, it neatly describes the actual chemistry: hot water softens the gummy sericin coating and frees the continuous filament for reeling.
From Cocoon to Fabric
Turning a cocoon into usable thread is called reeling, and the basic principle hasn’t changed much since antiquity. Cocoons are first placed in hot water or an alkaline solution to soften the sericin glue that holds the filament in place. Workers then find the true end of the continuous strand and draw it out, combining filaments from several cocoons together to create a single thread strong enough for weaving. A single cocoon filament is extremely fine, so anywhere from three to ten filaments are typically twisted together.
In traditional practice, this was done entirely by hand. Modern production uses pedal-driven or motorized reeling machines, but small-scale producers in parts of India still use older methods. After reeling, the raw silk thread can be degummed (removing the remaining sericin) to reveal the smooth, lustrous fiber most people associate with silk fabric.
Types of Silk and Their Sources
Mulberry silk from Bombyx mori accounts for the vast majority of commercial silk. These worms are entirely domesticated. They’ve been selectively bred for so many generations that they can no longer survive in the wild, and they feed exclusively on mulberry leaves grown on purpose-planted farms. The result is a fine, uniform, white fiber that takes dye exceptionally well.
Wild silks come from different moth species that live in forests rather than on farms. The most notable types include tussah (also called tasar) and muga silk, both produced by moths in the Antheraea genus. These caterpillars feed on oak, castor, or asan tree leaves instead of mulberry. Wild silk fibers tend to be coarser, shorter, and darker in color, with a more textured appearance. India, China, and parts of Africa produce most of the world’s wild silk.
How Silk Left China
For centuries, China guarded the secrets of silk production fiercely. Silk fabric traveled west along what would become known as the Silk Road, but the knowledge of how to raise silkworms and reel their cocoons remained tightly controlled. The most famous account of that secret escaping comes from the Byzantine Empire. In 553 or 554 AD, two monks reportedly smuggled silkworm eggs into Constantinople after an audience with Emperor Justinian. They had learned silk production methods abroad and returned with eggs concealed (according to tradition) inside hollow bamboo canes. A competing account from the Byzantine historian Theophanes credits a Persian man with bringing the eggs from “the land of Serer,” a reference to China.
From Byzantium, sericulture gradually spread further west. Evidence suggests mulberry cultivation and silk production moved from Syria to the Iberian Peninsula after 726 AD. By the medieval period, Italy had become a major European silk producer, and the fiber was no longer an exclusively Asian commodity.
Why Silk Is Remarkably Strong
Silk’s reputation as a luxury fabric can obscure the fact that it’s also an engineering material. Cocoon silk fibers from muga moths have a tensile strength of around 535 megapascals, and when minor structural flaws are accounted for, that number rises to over 600 MPa. Eri silk fibers reach about 450 MPa under similar conditions. For context, these values are comparable to naturally spun spider dragline silk, which is often held up as nature’s benchmark for high-performance fibers at around 574 MPa.
What makes silk especially impressive is that it combines strength with stretch. Most silk fibers can elongate more than 30% before breaking, which gives them remarkable toughness, the ability to absorb energy without snapping. Muga silk’s toughness reaches 136 megajoules per cubic meter, right in the range of spider silk. This combination of strength and flexibility is why silk has historically been used not just for clothing but for parachutes, surgical sutures, and body armor linings.