One of the finest fibers in the world that is the subject of traditions, cultures and class.
A legendary aura surrounds the discovery of silk and its diffusion from the Far East to Europe.
Sericulture, or the breeding of silkworms for the production of raw silk, is certainly a Chinese invention dating back to around 4500 years ago.
For the Western world, China in itself has often been synonymous with Silk, evident in the name “Silk Road”, the road that led from China to the Western world. It is on this road that, already two thousand years ago, silk, perfumes, spices and precious stones were brought from Beijing to the Mediterranean through Hami, bordering the terrible Taklamakan desert and passing through the caravan cities of Aksum and Kashgar.
In the Heavenly Kingdom during the Tang period (618-907), Hangzhou was a silk production center and known as “the Silk House” and further developed during the Song (960-1279) and Ming (1368-1644) dynasties.
Various legends are told about the origin of silk. One narrates the story of the Chinese Empress S-Ling-Chi who, thanks to a book by Confucius, discovered how to breed silkworms and weave the burr, a good 2600 years before Christ. In return, the people deified and worshipped her as the “Silk Goddess”.
Another legend narrates the story of a cunning Chinese princess who when sent to be married to the king of Turkestan, in order not to give up her silk clothes hid some precious silkworm eggs in her hair.
In the times of Emperor Augustus, in the first century after Christ, Rome came into contact with the splendor of the East and silk became the preferred fabric for women’s clothing in the west too.
In the markets of Asia Minor, the Romans learned that silk came from China but they did not know what material it was made of. It was only later that worm breeding was introduced to the Mediterranean countries.
It is said that, when the western Roman empire was, by then, extinguished under the wave of the barbarian invasions, two monks brought to Byzantium some silkworm cocoons hidden in the reeds of their walking sticks. And so, the first silkworm farms were organized in Greece, Persia and in the Italian cities of Byzantine influence.
The Arabs brought new development to sericulture in Persia, Sicily and especially in Spain. China, however, remained the largest producer.
It can be said that the spread of sericulture in other European countries was mainly due to the Italians. The Genoese were the first to transfer the silk trade to Avignon and again the first worm breeders in England and Switzerland were Italian.
This shouldn’t come as a surprise, moreover, considering that in Dante’s Florence, so rich with industries and trade and with the neighbouring countries just on the other side of the Alps, a “Silk Guild” already existed to protect the rights of silk workers.
Silkworms and Sericulture
Silk is a very prestigious textile filament that derives from the secretion of an insect called silk moth, “cavalier” in local dialect or simply silkworm.
The best-known silkworm is the bombyx, the larva of the Bombyx mori moth, that is, the mulberry bombyx that belongs to the order of the Lepidoptera and Bombycidae family. It is originally from China and the only moth able to produce the precious cocoon.
The Life Cycle
The complete life cycle of the insect lasts approximately 45 to 50 days and passes through the following stages: hatching, growth of the larvae in five growth stages called instars, spinning of the cocoon, emergence of the butterflies, mating and laying.
Over the course of a year, silkworms appear in four different forms: embryo or seed, larva or worm, chrysalis and butterfly. By continuously moving its head, the worm facilitates the extraction of the liquid silk from the reservoir. The liquid solidifies when it comes in contact with air, giving origin to two parallel burrs of fibroin, glued together by sericin.
Embryo or Seed
During the winter, when the mulberry is in the vegetative state of rest, the silkworm embryo remains inside the egg in a state of winter dormancy called “diapause”.
In spring, the eggs hatch as the mulberry tree leaves begin to reach the ideal stage of maturity for the newborn larvae to feed on, which is between the end of April and the beginning of May. Upon hatching, the seeds whiten and the larvae are born.
Larva or worm
When the eggs hatch, the breeder transfers the small worms to horizontal racks covered with mulberry leaves. The larvae are small and dark and after ten days they acquire a whitish color.
Not all seeds produce the cocoon; on average 10% is lost due to death of the embryo or larva. The larvae are insatiable and feed voraciously on the leaves of the white mulberry “Morus alba“, an arboreal plant native to the Asian regions around the Himalayas. The growth rate is impressive: the worm increases 40 times its initial length and 8000 times its weight.
During development, the larva changes its skin four times and each change constitutes a moult. During moulting, the worm does not eat and breathes badly, while during the instars it feeds day and night.
After about 6 weeks, the caterpillar is fully developed and starts building its cocoon. In the beginning, it sticks some tracts of burr to the branches, making up the outer shell of the cocoon. Then, by moving its head in all directions, it performs the spinning process, extracting a sticky filamentous secretion that gets glued together floss by floss.
During this phase, which lasts 3 to 4 days, the caterpillar ceases to feed and decreases in weight and volume.
In silkworm farms, the reproductive process is interrupted at the chrysalis stage, for if it were to leave the cocoon, the moth would produce lacerations which would reduce the commercial value of the silk. Therefore, most of the worms are suppressed. The cocoons are immersed in boiling water or dried in ovens. Only the moths strictly necessary for breeding are left to emerge.
Once the cocoon is finished, the worm stays “tucked inside” to transform itself into a leathery, motionless, brown insect, making up the chrysalis. After about two weeks the chrysalis turns into a butterfly and, thanks to a small gland that emits a special liquid, it opens a gap between the filaments that form the cocoon and comes out. It will mate and give origin to a new life cycle.
The adult moth is usually yellow or yellowish white in color, with a stocky and hairy body. During its short life (10 days) it does not feed and it dies immediately after laying the yellow colored eggs, which it secures on a surface by means of a rubbery self-produced substance.
Usually, a moth lays 300 to 500 eggs which, if fertilized, turn brown shortly afterwards and hatch within ten days. The seeds are very small, lenticular in shape and brown. Special institutes collect them, preserve them and then distribute them to farmers in gauze frames.
It is mostly the women of the farmer families who take care of the worms and follow the various stages of breeding. This begins by placing the seeds of the worms, well covered and fed with dry, fresh, clean and crushed mulberry leaves, on wicker straw mats.
The first worm that emerges is called the head cavalier. As they grow, the cavaliers’ appetite increases and they grow rapidly by eating mulberry leaves. When the worms eat continuously, the work on behalf of the farmer family becomes very demanding. For this reason, it is necessary that the breeding of silkworms take place near mulberry plantations.
After about forty days from hatching, four skin changes and an x10,000 increase in body weight, the worms stop feeding. The time has come to spin the cocoon.
The farmer family prepares the “forest” with intertwined dry twigs so that the worms can climb on it and make the cocoon by attaching themselves to the twigs, secreting a damp filament that solidifies as soon as it comes into contact with air.
Once the cocoon is completed (about 4 days), it is detached from the “forest” and processed immediately, before the metamorphosis of the worm occurs and the chrysalis inside turns into a butterfly that would exit and break the precious filament.
For centuries, silkworm breeding has been the basic economy of farmer families in rural areas of Zhejiang and Jiangsu, and even today mulberry farming represents a primary source of income for them.
The Manufacturing Process
Reeling and Twisting
Collection of the silk thread is a very delicate process, technically called reeling, and begins with the drying out of the cocoons. This is then followed by an accurate sorting to classify the regularity of the silk and obtain homogeneous yarns.
The different types of cocoons are divided before starting the activity and then they are treated separately, in order to guarantee uniformity of the thread. The operations that follow are stripping, brushing and reeling.
The cocoon is covered with a fluff, called ‘strip,’ which must be removed in order to unravel the thread after finding the leader. To remove this fluff, cocoons are rubbed with sorghum brushes. Once the leader thread gets tied to a reel, the cocoons are rotated in boiling water to eliminate the rubbery layer that covers them. The thread obtained is called raw silk and is normally composed of 48 single filaments. In order to obtain a consistent semi-finished product, four subsequent operations are necessary: winding, rewinding, doubling and twisting.
The skeins are unraveled on a winder, which allows the passage of the threads from skeins to spools. The rewinder instead transfers the thread from one spool to the other, making it pass through a very thin slot (clearer) the task of which is to clear it from any knots and eliminate tangles. The threads are doubled through the winding machine in order to prepare the yarn for the subsequent twisting operation that will make it more consistent. Thus, through the twisting machine, the spools, that quickly rotate vertically while the reel slowly turns horizontally, give the filaments a twist, which produces different types of yarn.