The origin of silk | The silkworm | Morphology | Quality and typesProperties | The main processes of working | Weighted silk


The origins of silk – historical outline

According to tradition, the birth of silkworm breeding is due to the Chinese empress Xi Ling Shi, wife of the Yellow Emperor, who lived during 27th century BC. Also known as the Lady of the Silkworms, it was she who discovered the special characteristics of this insect. The Chinese were able to keep this secret until 420 DC when a princess who, sent to marry a prince of Turkestan, stole eggs, silkworms and mulberry seeds, thereby starting the spread of silk, which became together with spices, one of the most sought after prime materials. Its distribution expanded throughout the known world, firstly in Japan, India and Korea and then in Europe where its commercial traffic intensified so as to give birth to the name of the most important communication route between the Far East and Europe: The Silk Road. All of this occurred in spite of the severe restrictions enforced by the Chinese emperors who prohibited the sale of silk outside the empire. Clothes made of silk, initially the prerogative of only the Chinese emperors, thanks to the merchants, came to be a part of the wardrobe of the well-off as a much sought after luxury item. The Romans, who were willing to pay up to a pound of gold for a pound of silk, only began producing it in 550 DC, during the Byzantine Empire. Officially, silk was introduced to Italy in 1146 through Sicily, due to the domination of the Norman and the presence of a few Corinthian prisoners, specialized in silkworm breeding and expert silk manufacturers. It was from here that began the actual production and expansion of the fabric throughout the country, which gave impetus to a tradition of production that still today generates work and wealth.


The silkworm

The silkworm (Bombyx mori) is a little more than 2 millimetres at the time of hatching. It is extremely voracious and continuously feeds on mulberry leaves. Their voracity for food stops as they begin the cycle of moulting four times, as they grow through their five different stages of larval life. After about 30 days, they reach a size of approx. 6-10 cm. The breeder ensures that they have twigs and straw to allow the silkworms to anchor the cocoon in which it will enclose itself. The tangle of silk with which the bug fixes the cocoon is called silk filaments or silk floss. The silk, a fibroin (animal protein) is extruded through a fissure located under the lower lip of the silkworm and fed by two parallel glands (chambers or cavities) in the silkworm containing the silk in the viscous state. The filaments that emerge are coated with a rubbery substance, sericin. This thin froth solidifies on contact with air. Moving its head in a continuous figure-eight pattern, the silkworm creates a cocoon composed of approx. 20 to 30 concentric layers of a single unbroken thread or filament which is on average about 1,200 meters long. The cocoon is then soaked in boiling water to prevent the completion of the metamorphosis and to unravel and loosen the silk filaments, partially dissolving the protective layer of sericin that covers it. The transformation from chrysalis or pupa to butterfly takes about two weeks. The butterfly pierces the wall of the cocoon, using its legs and secreting a liquid that is intended to soften the end of the cocoon in order to slip out. After mating, the female lays her eggs. 50,000 silkworms produce approximately 100 kilos of cocoons from which about 120 meters of raw silk is extracted. The wild silk is a quality that is generated by other kinds of silkworms, and not that of the mulberry tree, the most important of these is the tussah, not cultivated in Europe, which lives on oak tress.


Morphology of silk

Raw silk is composed of two proteins. Fibroin, which makes up about 75-77% of the total, permits silk to respond both to basic and acidic substances. The remaining part is composed of sericin (silk gum), which is soluble in hot water, through a process called degumming. The remaining fatty and waxy substances and mineral salts are eliminated in a low temperature bath. The raw silk, which contains most of the gum and for this reason is quite rough and dull, lends itself poorly to dyeing. The raw silk is subject to a very light washing, which reduces its gumminess, dullness and coarseness. Finally, in the cooked silk, the rubber is completely eliminated and the fibre is particularly bright. Its colour can be white, quite or less intense, greenish, yellowish, yellow and sometimes red. The microscopic analysis of the longitudinal section of the degummed silk, characterised by two parallel filaments held together by a cementing substance, appears formed by thin cylindrical filaments. This cross section is mostly triangular. The diameter of filament varies from about 10 to 22 microns Both during the selection of the products and the treatment of the silk, and in the different reeling and twisting operations, silk waste is produced, pierced cocoons from which the moths have been allowed to emerge for reproduction, floss or blaze (the first silk filament), damaged and incomplete cocoons and unravelled cocoons. The silk waste, which constitutes some 50% of raw silk is made up of non-continuous filaments commonly called schappe silk (silk noil), and is used in knitwear after being treated with machines similar to those used to prepare and work wool.


Quality and types of silk

The raw silk or reeled silk, is obtained from unpierced cocoons, which contain the complete amount of filament that can be unravelled, which corresponds to approximately half of the total length (approx. 2,400 – 3,000 meters). This quality of silk produces a fabric that is very smooth and glossy, very even without any fibre protrusions. It is warm, soft and extremely fine to the touch. This reeled silk is considered as first quality silk. The schappe silk or floret silk is obtained from pierced cocoons, from silk waste and scraps generated by the unravelling and the remnants of the membrane. The length of these varies between 4 to 15 centimetres (generally twisted). The resulting fabric is semi-glossy but regular with minor fibre protrusions. It is very fine, warm and softer than raw silk to the touch. This silk is considered as second quality silk. The bourette or noil silk is obtained by processing the fibre resulting from the schappe silk, with a length that is less than 4 centimetres. With these fibres, one obtains a fabric that is irregular, not very shiny and quite dull, with knots that create notable bulges. It is resistant and quite warm to the touch. This silk is considered as third quality silk.


Properties of silk

The sheen or gloss is one of the characteristics of silk, an aspect that is very much influenced by the type of silkworm breeding, the choice of cocoons and their processing. The evenness is not regular in silk; in fact, the final layers of the cocoon tend to be thicker than the initial ones. During the unravelling phase, one can notice that the fibre is not regular. Each silkworm can produce more than 1,000 meters of continuous filament; therefore, the length is also considered a part of the characteristics of the fibre. The tenacity of silk is greater than that of wool and close to that of cotton. Its wear resistance is good and, if rumpled it easily returns to its original state, given that that the folds have not been produced in a warm and humid environment. Silk has very good elasticity and can stretch by up to 20%. When subjected to heat silk tends to turn yellow, a phenomenon however that does not occur at temperatures below 130 C°. Sweat, sprays and perfumes also weaken the fibres, causes yellowing, and possible discoloration. Its power of absorbing moisture from the air and sweat leaves on the body a very pleasant feeling of warmth and prevents the fabric form charging with static electricity.


The main processes of working silk

During the spinning, the cocoons are separated from the floss (the first silk filament). The successive phase, called sorting (selection) is done to separate, based on size and quality, the cocoons, eliminating those that are damaged. The categories that are the result of this selection must be as homogeneous as possible, taking also into account the criteria of colour and texture. The best selection of cocoons is called royal silk. This operation is done with the help of adjustable sieves that let the cocoons of different sizes pass through, thereby determining its denier count. Subsequently the silk passes through a phase of softening, pulping and carding (brushing): The first layers of the cortex of the cocoons are softened in boiling water and rubbed by a rotating brush. The surface of each cocoon is lightly scraped to eliminate the burrs of silk found on the surface (silk waste) and then brushed to find the end of the filament. This process is not done more than absolutely necessary, in order to avoid the loss of weight (and the resulting loss of income). In the next step called throwing, the raw silk is fixed to a throwster or reel that continually spins and twists the strands together and its water mass is reduced by up to 70%. Given the fineness or evenness of silk, between four to eight cocoons are twisted together according to the type of thread or yarn that is desired. The different fine silk strands are held together by the sericin to form a single filament. This process improves the size and evenness of the raw silk. Approximately half of the 3,000 meters of filament contained in a cocoon is unravelled. In the next phase of reeling the silk filaments are put onto reels using porcelain rollers and then dried by air circulation at a temperature of 35 ° C and wrapped and reeled into skeins, which are packed in bales and bundles. The operation of rewinding serves to remove anything from the yarn that contrasts with its cleanliness and purity. After this cycle, a number of yarns are doubled to form one single yarn. The twisting prevents the silk from tangling and gives the yarn greater strength, and prepares it for the next operation called degumming, which eliminates the sericin (silk gum) from the silk filaments. Twisting, which affects the shininess of the threads, is determined by the number of spirals, and can be used to achieve different levels of shades from shiny to dull. The silk is then marketed and sold as skeins or hanks. The hanks are previously soaked in a bath to make the yarn softer and to customize the quality, before being wound on to reels. Before dying and weaving, the yarn is purified a clearing process by which defects and irregularities are eliminated from the yarn, after which it is measured, subject to anti-static lubricating oils and cross winding (laying the yarn on the spools so that the threads cross each other).


Weighted silk

The degumming (elimination of the sericin from the silk by bathing in salt minerals, soap or chemical agents) causes a notable reduction in weight (approx. 25%). To compensate for this loss, the silk is treated with vegetable (tannin) and/or mineral substances (stannic chloride, phosphate and sodium silicate), which penetrate and adhere to the fibres to increase its weight. This can be done several times to arrive at the original weight of the fibre (par weight) or even to exceed it (up to a maximum of 30% is permitted). However, in this last case the wear resistance of the fibre is compromised.