Silk is a natural protein fiber historically prized for its distinct luster, smooth texture, and impressive strength. Its origin lies not in a plant, but in the biological process of the domesticated silkworm. Traditional silk, known as mulberry silk, is produced almost entirely by the larvae of the silk moth, Bombyx mori. The cultivation of this insect for textile purposes is a practice called sericulture. This complex material, spun by the insect to create a protective cocoon, is a remarkable example of a natural biopolymer utilized for millennia.
The Creature Behind the Fiber
The source of the world’s most common and commercially relevant silk is the larva of the silk moth, Bombyx mori, an insect entirely dependent on human care for survival. The term “silkworm” refers to the caterpillar stage of the moth’s life cycle, a period characterized by voracious feeding on mulberry leaves. During this larval phase, which lasts for about 27 days, the caterpillar molts four times and increases dramatically in size.
The silk itself is produced in a pair of large, modified salivary glands that run nearly the entire length of the larva’s body. These glands secrete a thick, liquid protein mixture through a single exit tube called the spinneret, located near the mouth. As the liquid stream of protein is pushed out and exposed to the air, it hardens instantly into a solid filament.
The silkworm continuously wraps this single, unbroken filament around itself, forming the protective structure known as the cocoon. This cocoon is spun to shield the developing pupa inside as it undergoes metamorphosis. The completed cocoon is the raw biological unit from which textile silk is harvested.
The Protein Structure of Silk
Chemically, silk is a protein fiber, making it fundamentally different from plant-based fibers like cotton. The raw silk filament is primarily composed of two distinct proteins: fibroin and sericin. These two components are secreted simultaneously by the silkworm and make up the fiber’s internal structure and external coating.
Fibroin is the core structural protein, accounting for approximately 70–80% of the silk filament’s total weight. This protein forms the continuous, strong inner thread that gives silk its tensile strength. Fibroin’s strength comes from its unique structure, which consists of repeating sequences of small amino acids like glycine and alanine, allowing the protein chains to pack tightly into stable beta-sheets.
The second protein, sericin, is a sticky, water-soluble glycoprotein that surrounds the two fibroin filaments, acting as a natural adhesive. Sericin typically constitutes 20–30% of the raw silk’s weight and functions to bind the two fibroin strands together and attach the thread layers to form the cocoon structure. While the fibroin core provides strength, the presence of sericin makes the raw silk filament feel stiff and dull.
From Cocoon to Thread
The transformation of the biological cocoon into a usable textile thread requires a careful, multi-step process. The process begins with the harvesting of the cocoons approximately 7 to 8 days after the silkworms begin spinning. To preserve the long, continuous silk filament, the pupa inside the cocoon must be prevented from emerging, which would break the thread.
This is accomplished by a process called stifling, where the cocoons are subjected to heat, often through steam or hot air, to kill the pupa inside. The stifled cocoons are then prepared for reeling by being immersed in hot water. This boiling process serves a dual purpose: it kills the pupa and softens the gummy sericin coating.
Softening the sericin allows the continuous fibroin filament to be unwound, or reeled, from the cocoon. Specialized machinery or manual methods gently brush the cocoons to locate the loose outer end of the filament, which is then carefully drawn out. Since a single fibroin filament is too fine for textile use, the filaments from several cocoons, typically between five and ten, are combined and twisted together to form a single strand of raw silk thread.
The raw silk thread still retains the sericin coating, which must be removed to reveal the material’s signature softness and luster. This final step, called degumming, involves washing the thread in hot, mildly alkaline soap and water solutions to dissolve the sericin. Once the sericin is washed away, the thread becomes the lustrous, smooth fibroin fiber ready for dyeing and weaving into fabric.