Silk is a natural protein fiber known for its luster and softness. This specialized form of agriculture, known as sericulture, involves the cultivation of the domesticated silkworm, the larva of the Bombyx mori moth, to produce usable cocoons. The transformation from a tiny egg to a finished textile is a highly controlled sequence of steps refined over thousands of years. The entire process relies on managing the silkworm’s life cycle to harvest the single, long filament it spins.
Rearing the Silkworms
Rearing the silkworms begins with the hatching of larvae. These larvae, the caterpillar form of the Bombyx mori moth, are entirely dependent on humans due to millennia of domestication. Their diet consists almost exclusively of fresh white mulberry leaves, which provide the necessary nutrients for rapid growth.
The larval period lasts 20 to 30 days, during which the silkworms molt four times and increase their body mass greatly. They consume an enormous quantity of leaves, with the final larval stage, known as the fifth instar, accounting for the majority of their total food intake. Maintaining precise environmental conditions is important, typically requiring temperatures between 23°C and 28°C and controlled humidity for healthy development.
When conditions are managed successfully, the full-grown larva reaches about 7 to 8 centimeters in length and becomes translucent and yellowish. This change signals that the caterpillar is ready to transition to the spinning stage. The silkworm ceases eating and searches for a suitable structure, often a straw frame called a mountage, to attach itself and begin spinning its protective shell.
Cocoon Formation and Preparation
Upon finding a site, the silkworm excretes a liquid protein from two specialized glands in its head, moving its head in a figure-eight pattern. This secretion consists of two components: fibroin, the structural fiber, and sericin, a sticky protein that coats the fibroin and binds the structure together. When this combined secretion meets the air, it hardens into a single, continuous filament, creating the cocoon over three to four days.
The cocoon serves as the protective casing for the pupal stage, where the larva undergoes metamorphosis into a moth. For the silk filament to be unwound in one piece, this natural cycle must be interrupted before the moth emerges. If the moth breaks through the cocoon, it severs the long thread into short, unusable segments.
To prevent this damage, the cocoons are subjected to stifling, which involves applying heat to kill the pupa inside. This is typically achieved using hot air drying (around 60–70°C) or steam treatment for several hours. This preparation preserves the integrity of the single silk filament, which can measure between 600 and 900 meters in length, allowing for efficient extraction during reeling.
Reeling the Raw Silk
Once the cocoons are stifled, they are transported to a filature for the extraction of the raw silk thread. The cocoons are submerged in vats of hot water (typically 85°C to 95°C). This controlled application of heat softens the sericin gum, making it pliable and allowing the filaments to separate without breaking.
After the sericin is softened, the outer, tangled layer of fiber, known as the floss, is brushed away to locate the beginning of the continuous filament. Workers then gather the ends of multiple filaments, usually drawing together four to eight strands from different cocoons. This combining is necessary because the individual thread from a single cocoon is too fine for commercial textile production.
The combined filaments are passed through a small guide hole and twisted slightly as they are unwound onto a swift, or reel, under precise control. This twisting action creates a single strand of raw silk thread, ensuring cohesion and providing the necessary thickness and strength for weaving. The finished raw silk is then wound into skeins before post-processing.
Post-Processing and Finishing
The raw silk skeins contain sericin gum, which makes the thread stiff, rough, and dull. The next step is degumming, which removes the sericin by boiling the silk in a solution of warm water and mild alkaline agents, such as soap and soda ash. This process can result in a weight loss of 20 to 25 percent of the thread, but it reveals the natural sheen and soft texture of the fibroin core.
Following degumming, the now-softened silk is often subjected to throwing, which involves adding a specific amount of twist to the yarn to increase its tensile strength and elasticity. The degree of twist applied determines the final texture and drape of the resulting fabric, differentiating materials like crepe from smooth satins. After the yarn structure is finalized, the silk thread is ready for dyeing, where it readily absorbs color due to its protein structure.
The finished silk thread is woven or knitted into the wide variety of fabrics seen in the market. The control required at every stage, from the cultivation of the mulberry leaves to the final finishing, underscores why silk remains a highly valued and distinct textile fiber.