Silk is prized for its natural protein structure, which gives the material its signature luster, strength, and soft drape. While produced by certain insect larvae forming cocoons, the vast majority of commercial silk originates from the domesticated silk moth, Bombyx mori, often called the mulberry silkworm. Its cultivation in a controlled environment, known as sericulture, ensures a consistent, high-quality fiber.
The Silkworm and Silk’s Composition
Silk is made from a pair of structural proteins secreted by the silkworm larva as it enters its pupal stage. The silkworm feeds exclusively on mulberry leaves for 25 to 30 days before spinning its protective cocoon. This single, continuous filament is composed of two distinct protein types: fibroin and sericin.
Fibroin is the core structural protein, making up 70% to 80% of the raw silk fiber’s total mass. It is characterized by a high concentration of the amino acids glycine and alanine, arranged in a tightly packed beta-sheet structure that provides the fiber’s tensile strength and durability. The remaining 20% to 30% of the raw silk is sericin, a gummy protein that coats the fibroin and acts as the natural adhesive.
The silkworm secretes these two proteins through specialized silk glands. The two filaments, coated in sericin, exit the spinneret and harden upon contact with the air, forming a double strand of silk. Sericin fuses the two fibroin strands together, allowing the larva to spin a cohesive, dense cocoon, which can contain a single thread up to 1,500 meters long.
Transforming Cocoon into Usable Thread
Transforming the raw cocoon into usable textile fiber requires a multi-step industrial process called sericulture. The process begins with stifling, where the cocoons are subjected to heat (typically steam or hot air) to kill the pupa inside. This prevents the emerging moth from breaking the continuous silk filament as it chews its way out, which would render the cocoon useless for high-quality reeling.
The next step is softening, where the stifled cocoons are immersed in hot water or steam to begin dissolving the sericin gum. The heat softens the sericin, allowing the fibroin filaments to be separated and located. Softening is followed by reeling, the process of gently unwinding the continuous filament from the cocoon. Since a single fibroin filament is too fine for commercial use, filaments from several cocoons (often four to eight) are combined and twisted together to form a single, stronger thread.
The combined strands are wound onto reels, creating raw silk, which still retains some sericin to help the filaments adhere during reeling. This raw silk then undergoes degumming, often involving boiling in a soap solution, to remove the remaining sericin and reveal the smooth, lustrous fibroin core. The resulting degummed threads are twisted, or “thrown,” into multi-ply yarns ready for weaving.
Other Forms of Natural and Synthetic Silk
While the domesticated Bombyx mori produces the finest and most uniform fiber, other natural silks exist, notably those harvested from wild silkworms. These are often referred to as wild silks, or Tussah silk, which comes from species like the Tussah moth. Unlike the cultivated mulberry silkworm, wild silkworms feed on various leaves, resulting in a silk that is coarser, less lustrous, and naturally beige or honey-colored.
Several materials are manufactured to mimic the feel and appearance of silk, though they lack the protein structure of true silk. Rayon, a semi-synthetic fiber, was historically developed as an artificial silk substitute. It is made by chemically processing plant-based cellulose from wood pulp, providing a soft, drapey, and more affordable alternative, but it is not a protein fiber like silk.
Polyester is another common synthetic substitute, a man-made fiber derived from petroleum-based polymers. While polyester can replicate silk’s sheen and lightweight feel, it does not share the natural breathability or protein composition of silk. The development of synthetic spider silk, which focuses on bio-engineering a silk-like protein structure, represents an emerging field attempting to replicate natural silk’s strength without the traditional silkworm process.