The core secret of silk production lies in a tiny creature called the silkworm, a caterpillar of the silk moth (Bombyx mori) that feeds exclusively on the leaves of the mulberry tree. Here's how silk is made from cocoon to thread:
1. Silkworm Breeding
Selection and Reproduction
Only the healthiest moths are used for breeding. Their eggs are sorted, graded and meticulously tested for infection, unhealthy eggs are burned and the healthiest eggs are placed in cold storage until they are ready to hatch. It usually takes seven days for the eggs to hatch, and normally the eggs hatch once every spring when the mulberry trees begin to produce leaves, but with the intervention of silkworm farmers, they can breed up to three times a year.
Feeding and Growth
Silkworms feed exclusively on the leaves of the mulberry tree. Mulberry leaves are chopped up and fed to the greedy silkworms every few hours, a process that lasts 20 - 35 days. During this period, the silkworm also sheds its skin or molt four times and changes color from gray to translucent pink.
2. Cocoon Formation
Preparing to Spin Silk
When a silkworm begins to get agitated and shakes its head back and forth, it is preparing to spin a cocoon. The caterpillar will attach itself to a branch or stand for support.
Spinning silk to make a cocoon
As the worm twists its head, it spins a double strand of fiber in a figure-eight pattern and builds a symmetrical wall around it. The filaments are secreted from each of two glands called spinnerets located below the silkworm's jaws. Insoluble protein-like fibers called fibrin are bound together by sericin, a soluble gum secreted by the worm that hardens once exposed to air, resulting in a raw silk fiber called bave. The caterpillar spins the silk until it completely envelops itself, forming a cocoon, after which it can safely enter the pupal stage.
3. From cocoon to silk thread
Handling silkworm pupae
In nature, silkworm pupae break through the protective cocoon to become moths, but in order to obtain intact silk, silkworm farmers must prevent this process, which can be achieved by heating or inactivating the pupae so that the precious silk remains intact.
Sorting and Softening of Cocoons
During the reeling process, the cocoons are sorted based on various characteristics, including color and size, to ensure uniform quality of the final product. The cocoons are then soaked in hot water to loosen the sericin, and while silk contains about 20% sericin, only 1% is removed at this stage.
Winding the Filament
The winding can be done manually or automatically. The cocoons are first brushed to locate the ends of the fibers, which are then passed through the porcelain holes and then rolled onto the wheel. At the same time, the operator inspects the filaments for defects as they are reeled in. When each filament is almost finished winding, a new fiber is twisted onto it, creating a long, continuous thread, with the sericin helping the fibers adhere to each other.
Making and Packing the Filament
The final product (filament) is rolled into hanks. These hanks are packaged into bundles weighing 5-10 pounds (2-4 kilograms), called books. These books are further packaged into 133-pound (60-kilogram) bales and shipped to manufacturing centers.
Making Silk Thread (Yarn)
Silk thread (also called yarn) is created by throwing or twisting bobbins of raw silk. First, the skeins of raw silk are sorted by color, size, and amount, then they are soaked in warm water mixed with oil or soap to soften the sericin, after which the silk is dried. As the silk threads are wound onto the bobbins, they are twisted in a specific way to achieve a certain yarn texture. For example, a "single" consists of several filaments twisted together in one direction, and a twisted one can be used for sheer fabrics, while a loose one can be used for thicker fabrics. The silk yarn is finally passed through rollers to make it more uniform in width, and the yarn is inspected, weighed, and packaged for shipment to fabric manufacturers.
Degumming
In order to obtain the unique softness and sheen of silk, it must be soaked in warm soapy water to remove the remaining sericin. Degumming reduces the weight of the yarn by up to 25%. After degumming, the silk is a creamy white color, after which it can be dyed as yarn, or dyed after the yarn is woven into fabric. To produce weighted silk, metallic substances are added to the fabric during the dyeing process