Silk, a natural fiber prized across centuries, is renowned for its unique combination of strength, softness, and shimmering appearance. It is the only continuous filament fiber produced in nature, lending itself perfectly to fine textiles. The distinctive luster of silk fabric results from its triangular prism-like fiber structure, which refracts incoming light at multiple angles. Understanding silk requires examining the biological process and molecular composition that defines this highly valued material.
The Primary Biological Source
The source of nearly all commercially available silk is the domesticated silkworm, the larva of the moth species Bombyx mori. These insects have been selectively bred for thousands of years, making them dependent on human care, a process known as sericulture. The larva feeds exclusively on mulberry leaves, accumulating energy reserves for its pupal stage.
Once fully grown, the larva begins spinning its cocoon, a protective casing for its transformation. The silk is secreted as a liquid from two large glands, exiting through a single opening called the spinneret. Upon exposure to air, this fluid hardens into two continuous filaments that the silkworm wraps around itself. A single cocoon yields one long, unbroken thread, typically ranging from 300 to 900 meters in length.
The Molecular Building Blocks
At a chemical level, silk is a natural protein fiber composed of two primary proteins: Fibroin and Sericin. Fibroin forms the structural core, making up approximately 70–80% of the fiber mass, while Sericin is the gummy coating that encases the twin fibroin strands. Fibroin is responsible for silk’s high tensile strength.
This strength is due to the Fibroin molecule’s structure, which consists largely of repeating sequences of the amino acids glycine and alanine. These sequences fold into antiparallel beta-sheets, allowing the protein chains to pack tightly together in a crystalline structure. Sericin, a water-soluble glycoprotein, functions as a protective glue, cementing the two fibroin filaments together.
From Cocoon to Usable Thread
Transforming the cocoon into a refined textile thread requires careful harvesting and extraction. Cocoons are harvested approximately 7 to 8 days after spinning to prevent the pupa from maturing and breaking the continuous silk filament upon emergence. The cocoons are then subjected to heat, often by immersion in boiling water or steam, which kills the pupa and softens the Sericin gum.
The next step is reeling, where the softened Sericin allows the long, continuous filament to be gently unwound. Multiple filaments, typically six to seven, are twisted together to form a single, strong raw silk thread. Finally, the Sericin is largely removed through degumming, utilizing hot water and mild alkaline solutions, transforming the coarse, gum-coated raw silk into the smooth, lustrous finished fiber.
Natural Silk Variations
While most commercial silk comes from the cultivated Bombyx mori silkworm, wild silks offer alternative variations in texture and color. Wild silks are produced by non-domesticated species, such as the Tussah silkworm (Antheraea pernyi), which feeds on oak and juniper leaves. Tussah silk is coarser, less uniform, and possesses a distinct honey-colored tint because the moths are usually allowed to emerge, breaking the continuous filament.
Another variation is Eri silk, sometimes called “peace silk,” because the moth completes its life cycle before harvesting. This results in shorter, staple-length fibers that produce a wooly, matte texture rather than the high luster of reeled silk. Superior in strength is spider silk, known for its tensile strength and elasticity. However, challenges in commercially farming spiders mean this material is not currently a viable competitor in the textile industry.