Silk, a natural protein fiber known for its luxurious sheen, is produced through sericulture, focusing on the domesticated silkworm, Bombyx mori. The silkworm is the larva of a moth, and its product is a single, continuous filament that it spins to form a protective cocoon. The manufacturing process begins with the rearing of these insects, moving through the extraction and processing of the thread.
Cultivating the Silkworms
The process starts with eggs laid by the female silk moth, which hatch into larvae under controlled conditions. These larvae, the silkworms, are placed in rearing trays and begin their intensive growth phase. The silkworms are monophagous, meaning they feed almost exclusively on the leaves of the white mulberry tree (Morus alba) for the nutrients necessary for silk production.
The larval stage lasts for approximately 25 to 30 days, during which the silkworms shed their skin four times as they rapidly increase in size. This feeding period is important, as the quality and quantity of silk produced are directly linked to the larva’s diet and health. Once the silkworm reaches its final, fifth instar, it stops feeding and begins spinning its cocoon. The larva secretes a liquid protein called fibroin, which is coated in a sticky protein gum called sericin, producing the silk filament.
Harvesting and Preparing the Cocoon
Once the silkworm has completed its cocoon (usually taking about three days), the cocoons are ready for harvest. Timing is important, as the pupa inside must be prevented from maturing into a moth, which would chew a hole in the cocoon and break the continuous silk filament.
To prevent this damage, the cocoons undergo a heat treatment known as “stifling,” which kills the pupa inside. Stifling methods include exposure to hot air, steam, or sun drying. After this thermal treatment, the cocoons are sorted based on factors like size, shape, color, and quality. Only cocoons of similar characteristics are batched together to ensure the final reeled thread maintains a uniform thickness and texture.
Extracting the Silk Thread
The extraction of the silk thread begins with softening the cocoon. The stifled cocoons are immersed in hot water (typically between 85 and 95°C) to soften the sericin, the natural gum that binds the fibroin filaments together. This step allows the single filament to be unwound without breaking while preserving the integrity of the fibroin. Precise temperature control is necessary to soften the sericin without degrading the silk protein itself.
The next step is “reeling,” where the loose outer layer of silk floss is brushed away to locate the continuous filament end. Once found, this filament is drawn up and unwound from the cocoon. Because a single filament is too fine for commercial use, several filaments (usually between four and eighteen) are combined as they are simultaneously reeled. The softened sericin acts as a temporary adhesive, cementing these individual filaments into one strong strand of “raw silk.”
This combined strand is then twisted slightly and wound onto a rotating reel. The raw silk thread can measure from 600 to 1,200 meters in length. The resulting raw silk retains a percentage of the sticky sericin, which provides a protective coating during the subsequent handling.
Turning Thread into Fabric
The raw silk thread undergoes “throwing,” which involves twisting the threads together to increase their strength and texture. The amount and direction of the twist determine the final yarn type, whether it is organzine, tram, or crepe, each suited for different weaving applications. This twisting adds durability and bulk to the fine raw silk.
Following throwing, the thread or the woven fabric must be “degummed,” a process that removes the remaining sericin. This is typically achieved by boiling the silk in a solution of mild soap and water, often with an alkaline agent. Degumming removes the brownish, stiff sericin, which can account for up to 25% of the raw silk’s weight, revealing the characteristic soft handle and lustrous sheen of the fibroin core.
The soft and lustrous thread is ready for the final stages of textile manufacturing. If a color is desired, the thread is dyed before weaving or knitting. Finally, the finished silk yarn is loaded onto looms, where it is interlaced to create the finished silk fabric. The complex journey from a tiny larva to a finished textile relies on this sequence of biological cultivation, thermal preparation, and mechanical processing.