Lecture 3: Silk
- May 08, 2023
- By Stephen Cole
- In Lecture Notes
Sericulture and the treatment of cultivated and wild silk from the main silk producing countries. The properties of silk that determined its use.
The images in the right-hand margin (indicated by red numbers within the text) are the slides Karen displayed while she was delivering the lecture. We have retained Karen’s original numbering, so where slides are missing due to copyright issues some numbers will be non-sequential.
China is the home of silk, the most desirable of all textile fibres. From the beginning it was surrounded by myths and legends and through trade and conquests, the designs created for the weaving of silk have spread to give inspiration for the designs of other textile materials and techniques.
The words for silkworm, Mulberry and silk itself have been found in many ancient texts and inscriptions on bone and tortoiseshell going back to the 12th century B.C.
Before this time there is further evidence in the form of a cocoon found in 1926 among objects that support the legend of HSI-LING-SHIH, the wife of Emperor HUANG-TI who is known to have ruled in 2640 B.C.
The legend tells that Hsi-Ling-Shih saw a silkworm spin its thread and this gave her the idea of keeping silkworms for the purpose of unreeling their spin and making it into fabric. However, another legendary emperor 200 years earlier is believed to have invented a musical instrument with silken cords and made the word “silk” part of the Chinese character for “Musical Instrument”.
This character is used as one of the 3 elements which make up the name HSI-LING~SHIH which are Woman, Field and Silk.
The name may be contrived, but the idea of a beautiful Empress inventing silk became venerated by the Chinese people.
As far as it is possible to tell from ancient geography the discovery of silk took place in the North of China in SHANTUNG. History tells us that Huang-Ti did indeed push the Eastern borders to the sea of the province of Shantung and we know that wild silk is still produced in this province.
Chinese empresses and their ladies were often depicted occupied with silk processes. From these depictions and from ancient documents we are able to piece together some of the development of the silk industry and learn how it affected the history and economy first of China and eventually much farther afield.
One early reference tells us that there used to be, in the neighbourhood of the Imperial Palace in Peking, a street with a name which means “the street which leads to the place for the rearing of silkworms for the amusement of Queens and Empresses”.
The SHU-CHING or book of annals is a collection of ancient documents relating to the history of China. It was compiled and rewritten by CONFUCIUS in the 5th century B.C. and in it there is a chapter about taxes.
This is known as THE TRIBUTE OF YU and concerns 6 provinces where silk was produced.
One province is mentioned as producing “mountain silk” which might be a kind of wild silk.
The “tribute” consisted of woven silks and these included “silks, blue, white and of mixed colours, flowered silk tissues, imitating the veins of precious shells from YANG CHOU, lengths of silk of blue or red, and silk with warp of black and weft of white”. End of quote.
Other materials than silk were also mentioned in the Tribute of Yu – cloth made of vegetable fibres such as hemp, “flowered tissues” whose composition is unknown. “Garments from the isles”, mentioning those with birds feathers woven in and “tissues made from the hair of foxes, bears and wild cats”.
All these last mentioned fabrics appear to be valued as highly as silk fabrics though silk was from the beginning reserved for the use of the ruler.
Whether as tunic, belt, parasol, banner or winding sheet, it was permitted only to the emperor, his close relatives and the very highest of his dignitaries.
When the empire was split into smaller states, silk continued to be reserved for the courts, which now consisted of a great many more people.
Production figures for raw silk soared to meet the demand and as they grew, more rapid methods of weaving were introduced and perfected until eventually the demands of the courts were satisfied.
As a consequence economics dictated the need for a larger market which brought silk into more general use though only the richest members of the aristocracy could afford the brocaded silks.
A surplus of raw silk now came to be produced for which uses had to be found and these included such mundane kinds as fishing lines, bow-strings, ropes and ties of all kinds. By weaving and doubling silk the Chinese could make waterproof containers for the transport of liquids and there were even cups made from silk and lacquer.
Silk waste, like wool, was used for padding winter garments and for making paper, which was first mentioned in 105 A.D. Printing is mentioned in 175 A.D.
It was not until the HAN Dynasty from 205 B.C. to 220 A.D. which reached its height at about the same time as the Roman Empire, that silk began to become a value in itself. It was used as currency, to pay civil servants, and to reward subjects for outstanding service to the state.
Silk was hoarded like gold and it became the currency of trade with foreign countries.
In every silk producing province daughters, mothers and grandmothers in every home devoted a large part of their day for 6 months of every year to the feeding, tending and supervision of silk worms and the unreeling, throwing, spinning, weaving, dyeing and embroidering of silk.
The entire silk industry appears to have been in the hands of women and it was a family affair. The steadfast work of the Chinese woman over many centuries defines Chinese silk and gives it its essential character.
This responsibility of the Chinese woman is symbolised by the legendary empress herself attending to a silkworm rearing house and by the name of the silk spinning species of silk moth – BOMBYX – which is said to mean “a caterpillar which forms a web from which is woven material for making women’s garments”.
Sericulture means controlling and directing the development of the silkworm through its breeding and feeding to produce yarns that are fine, strong, elastic and white and this took centuries of patient work involving selective breeding of both moth and their food, because uniformity depends not only on the kind of silkworm being bred but also on its food and surroundings. It has to be protected from loud noises, draughts and strong smells, such as those of fish and meat or even the odour of sweat.
When sericulture spread to other countries, the basic techniques of necessity stayed the same in their essential forms.
18th century illustrations of processes outlined in a 13th century Chinese textbook may be seen in the Museum of Art and Industry in Lyons, France.
One set of illustrations begin by showing silk moths laying eggs and eggs stored in folds of paper and hung on lines till they are wanted and go on to show that the eggs have to be warmed before they can hatch. In most silk producing areas, this was apparently done on the human body. After hatching the silk worms were kept in constant gentle heat to stimulate the best formation of the cocoons in order to produce even-textured thread.
Another set of illustrations shows first a woman chopping mulberry leaves. Another woman is feeding the worms.
Yet other illustrations show mulberry pickers 5 6 of different dates 2300-1695 B.C. and baskets filled with silkworms being scrutinised 7 8 before being placed on screens prior to spinning their cocoons.
The screens were covered with cloth, so that the silk worms might continue their spin in relative darkness.
After spinning the chrysalids would be stifled with smoke from burning paper or killed in some other fashion.
The next step would be to place the cocoons in boiling water to soften the sericin and to start the unreeling. For this the water was gently beaten with twigs, which picked up the ends of the silk threads.
Several cocoons were usually unreeled together 11 into a single strand and wound onto bobbins.
For thousands of years these processes changed very little and were closely guarded secrets to be kept inside China.
It was forbidden under pain of death to take eggs or cocoons out of the Chinese provinces. The secret of sericulture was kept inside China for a very long time, possibly for several hundred years longer than any other secret in history, and this despite the fact that raw silk and woven stuff were exported all over Asia along perilous routes by land and sea.
Eventually the knowledge of sericulture did spread. According to legend the secret was given to her new husband by a Chinese Princess, who married the sovereign of what is now SIN-KIANG. She is supposed to have smuggled silkworm eggs hidden in her hair-do to her new home in KHOTAN.
The knowledge of silk weaving in its most sophisticated forms also came from China. The knowledge of when and where is lost in the mist of time – there are legends and myths, but very few hard facts, perhaps because it must have been connected with the concept of industrial espionage.
An ancient Japanese book NI-HONGI describes how in 300 A.D. a party of Koreans were sent out from Japan to China to engage people skilled in the weaving and finishing of silk cloth.
Four Chinese girls were brought to Japan and they instructed the court in plain and figured weaving and the Japanese erected a temple at SETTSU in their honour.
In the West the existence of silk was known as early as the 5th century B.C. when Herodotus wrote of a fabric called MEDIAN which had the characteristics of silk.
In 1978 a piece of purple silk with gold embroidery was found in Macedonia covering the bones of PHILIP the father of Alexander the Great who reigned in the 4th Century B.C.. Alexander’s teacher Aristotle related that silk was woven on the island of KOS where it was known as BOMBYKIA or BOMBYXINE. He tells us that Bombykia was made from unravelled and rewoven silk fabric by the ladies of Kos led by PAMPHILA, who is celebrated for having invented this process.
She may have, but there is also a native wild silk moth called Cos bombyx.
The Romans are said to have seen silk for the first time in 53 B.C. at the battle of CARRHAE, which they lost when the PARTHIANS unfurled their gleaming silk banners and won the battle in the ensuing amazement.
The Romans called this fabric for SER or SERIC. Ser is the Mongolian for thread and the people who produced silk were called Seres after their product. China was known as Serinda.
At this time there were four strong empires – the Romans in Europe, the HAN Dynasty in China and next to China the CUSHAN Empire which included Afghanistan and Northern India.
In the middle were the PARTHIANS.
All these empires had a consistent trade policy and as a result some of the trade routes shifted from sea routes to overland routes.
Travelling by sea route from Italy to India could take 3 and a half months and the travellers would arrive at ports on the COROMANDEL coast in October and leave with the monsoon the following April with goods of all descriptions.
Some of these goods came from China via 3 distinct routes. One was the BACTRIA-TAXILA road which crossed the Himalayas and was open for only part of the year because of the severe climate. The second route followed the same path as the one known during the last war as the BURMA road.
This one was as dangerous as the first one and with the additional hazards of wild beasts and war-like tribes. The third route was the one most used by the end of the first century and recent researches have underlined its importance.
It was a sea route, starting from the Southern coast of China in the region of CANTON and was used as far as we know, exclusively by Indian traders.
From the Bay of Bengal, the merchants sailed up the Ganges to the highest possible point where the river could be navigated and then the merchandise had to be carried overland to the ports of the West Coast where it was collected by Persian, Arab and later, also by European traders.
We know that silk was part of the merchandise from the PERIPLUS of the ERYTHREAN Sea, which is a manual for merchant mariners known by PLINY.
It states that SERIC silk was shipped from the Indian ports together with Chinese furs, pepper, cinnamon, scents, metals, dyes and medical products.
Archaeological excavations enable us to differentiate between the products which came from Southern China by way of North East India and those which came from Northern China via the Central Asian routes.
Travel of any Kind was lengthy and dangerous. Safety and safe passage would depend on treaties and trade agreements.
Merchants from the West had to pass through Persian domains to reach the trading ports of the far east and consequently Persia was mostly able to dictate the terms.
The merchants would travel in caravans from PALMYRA and DAMASCUS eastwards, through BAGHDAD, HAMADAN, SAMARKAND and KHOTAN to LOPNOR – the great trading centre of the East where several roads met.
Many of these places have been investigated by archaeologists. One was Sir Mark Aurel Stein who from 1907 excavated at Lopier and sites nearby.
Among many other objects were found a roll of yellow silk about 40 cms wide and 6 cms in diameter, tied with string, numerous fragments of other silks in various colours, woman’s slipper in cream coloured wool woven with a geometric design and – most importantly – corms from the HAN dynasty.
Few caravans would go all the way to China. Most merchants travelled between 2 trading ports only so the goods changed hands several times between China and the West.
Only Christian missionaries, Budhists and special envoys would travel the whole distance into China.
When the 4 Empires collapsed there was a breakdown of relations between China and the Western territories.
The Huns overran the Northern parts of China breaking down the social order and the civilisation which went with it.
From 190 AD to the middle of the fifth century there was very little commerce with the West – or so we are told.
In the West the Romans were losing power while Persia’s strength grew with the SASSANIAN Dynasty beginning with the SHAHAN SHAH ARDASHIRI from 226 AD.
In 247 the Roman Emperor DIOCLETIAN took Mesopotamia and Armenia and for 40 years the Romans and the Persians were at peace. Constantinople became the head of the Roman Empire in 330 AD.
China was still ostensibly closed though there had been no significant falling off in the consumption of silk in the West.
By this time the Persian silk industry is known to be flourishing. The Persians too have a legend about the discovery of silk which involves Job, who is believed to have initiated their methods of sericulture. In 360 AD a Persian King SHAPUR II seized a number of Syrian dyers and weavers and carried them off to SUSA, where they are believed to have provided a powerful stimulus to the silk industry.
China opened her doors again in 440 AD. By then Constantinople had become the new focus for trade and industry in the West.
The emperors made all essential branches of both trade and industry into Imperial monopolies – coinage, metals and textiles including silk for which from 301 AD they decreed the price.
Subsequent fluctuations of silk prices closely reflected the political fortunes of Byzantium.
Restrictions caused private workshops to suffer, but consumption of silk continued to grow.
The demand was increased by the Christian Clergy now officially recognised, who were beginning to wear silk vestments and use silk hangings in their churches.
Silk is mentioned in Customs Laws, Peace Treaties, Corporation Statutes, SUMPTUARY Laws and even in sermons denouncing its use.
When, during the third century, Persia overthrew the KUSHAN Empire and took control of Afghanistan and Northern India, she became a sea-faring nation more powerful than Rome.
From the beginning of the fourth century Persia regulated the flow of silk into Byzantium and levied so many taxes that the price of raw silk was often too high for many of the workshops to pay and both dyers and weavers suffered. This resulted in difficulties on many levels which were further added to for the dyers, when they were forbidden to manufacture certain varieties of purple, namely BLATTA, OXYBLATTA and HYANCINTINA. We cannot now define these colours but know they were reserved for the use of the Imperial Court and manufactured in the Imperial Workshops.
Infringement of the rules could be punished by death.
The Imperial Workshops set up in the 5th Century were called GYNAECEUMS because they employed women. Their production included the long straight robes similar to Median robes of earlier times (which were eventually seen in Byzantian Mosaics).
In China new products arriving from the West began to influence Chinese design which in turn influenced the western artists and designers. One newly discovered fruit was the POMEGRANATE which in the West came to symbolise fertility. In Central Asia the pomegranate was the symbol of the water goddess ANAHITA. The exchange of goods paved the way for a cross inspiration of designs going both ways. Other imports included Persian and Syrian Brocades with gold and silver threads and sometimes pearls too to accentuate woven design and the lines of garments. Many early designs have continued to change and spark off new ideas till our own day.
An example is the cloth with feathers mentioned in the tribute of YU. This technique was used again in Byzantium to produce a very costly material called OPUS PLUMARIUM.
During the Middle ages similar fabrics were woven in the monasteries – at first with real feathers as before and then feather designs done in weaving or with embroidery such as the hangings on a bed of about 1700 with a design which was originally made for weaving. The necessary exactness of the drawing of the repeats is obscured by the different colours used by the embroiderer for the repeats of the design.
The supplier of raw silk to Byzantium had to come through Persia. Several attempts were made to by-pass her, but mostly in vain so it must have been very nice for the Emperor Justinian in 551 AD to welcome back the 2 monks who, if we are to believe the legend, had been commissioned by him to bring silk moth eggs from China.
The work of a 16th century painter Philippe Galle shows their return. They had the eggs in their hollow staffs and were also able to instruct people in the art of sericulture so that silk could now be spun by worms bred in the West.
However, there was a long way to go before Western sericulture could even begin to rival the Chinese and
supplies of raw silk were still needed and still had to pass through Persian domains.
In 562 a 50 year truce was agreed between Byzantium and Persia.
The truce came after yet another attempt – fairly successful at bypassing Persia which had been made by the Sogdians whose country SOGDIANA was placed right in the middle of Asia where many of the trade routes met or passed through.
The Turks had taken over Sogdiana in 550 AD but had left her people at liberty to pursue their vocation for trade.
One of the Sogdian ambassadors – a Prince called MANIAK – eventually concluded a treaty with the Byzantines allowing silk caravans to travel overland by using the CAUCASUS routes which had long been abandoned as too dangerous, but now was returned to use for as long as the treaty lasted.
In 636 4 years after Mohammed’s Death nomadic MOSLEMS known in the west as SARACENS began to dominate large areas of land.
They conquered Syria, then Alexandria and the whole of North Africa and eventually Spain in 712-713 and there they became known as MOORS.
They now controlled most of the Mediterranean, including the trade in Chinese raw silk, which had come through their Asian possessions and soon their silk production rivalled that of China.
In 751 AD they had captured some Chinese silk weavers and papermakers and learned their techniques, which spread through the Moslem world with great rapidity.
In Spain, by the 9th century, they had made Seville, Granada and ALMERIA famous for their woven and embroidered silks.
They also introduced paper making into Spain.
In the 11th century when the Moslems were driven out of Sicily, Roger the Norman brought Greek prisoners there to continue the silk industry started by the Moslems or believed to have been started by the Moslems.
By this time Venice had become the channel for the silk trade into Europe.
In 1203 the Venetians ruled over the principal centres of the Silk Trade and the knowledge of sericulture began to spread over Europe though Eastern imports continued alongside home manufacture.
At the close of the 13th century, Marco Polo tells us that at CAMBALO, the Royal City of China, no fewer than 1000 pack horses and carriages a day came loaded with raw silk to be manufactured or exported from there.
By 1306, the rearing of silkworm in Modena had become important enough to yield a revenue to the state of Italy. In Florence many thousands of people were known to be employed in the manufacture of silk. Silk weaving is still done there.
In Venice, silk was held in such high regard that the business of a silk factory was considered a noble employment to be practised by the higher classes.
Other famous silk centres were at BOLOGNA, GENOA and MILAN.
In France, silk weaving began at TOURS in 1480 but it was not until 1521 when FRANCIS I brought eggs from Italy that silkworms were reared in the RHONE valley at LYONS and in PROVENCE at AVIGNON.
Other countries in Europe introduced sericulture and silk manufacture with various degrees of success.
Edward III introduced the industry into England and we know there was a ribbon manufactury in London in 1363 using raw silk from Italy.
James I encouraged Italian silk weavers to set up broad silk weaving and Charles II offered a grant of naturalisation to weavers from abroad.
In 1685, following the revocation of the Edict of NANTES thousands of French HUGUENOT weavers came to England.
Many of these weavers settled at Spitalfields. Others went to Bristol, Canterbury, Norwich and Southampton and a great many to Ireland which were all areas with existing textile factories and some skilled labour.
There were said to have been 40,000 people employed in the silk industry by the turn of the century. The East India Company imported Persian, Indian and subsequently Chinese raw silk by way of the Cape of Good Hope and made London the chief raw silk market until the middle of the 19th century.
In 1701 a law was passed to prohibit the import of either yarns or woven silks from China, India or Persia. A silk throwing industry in Britain now developed in which mainly women were employed.
By 1713 there were stated to be 300,000 people employed in the British Silk Industry, who produced 20 times the amount produced 50 years earlier.
When protective import duties were removed in 1860, the silk industry had already begun to decline – perhaps because of competition with the Jaquard looms at Lyons, but more likely because of the inability of British Manufacturers to compete in the production of silk with continental looms employing labour at less than half the rate of wages for British labour.
In 1851 about 130,000 persons were employed in the silk industry. In 1881 about 64,000 and by 1907 the number was down to 29,000 persons.
From 1869 and the opening of the Suez Canal, London ceased to be the distributing centre for the world’s crop of silk.
Silk was now taken to Marseille through the Suez Canal and the Mediterranean and from Marseille to Lyons and Milan for distribution.
England no longer produce brocaded silks and embroidery silks in quantity.
In America too, silk weaving had been encouraged by James I and had initially met with some success, because of this Royal Patronage but already in 1622 the King had to send instructions to colonists that, “They should apply themselves diligently and promptly to the breeding of silkworms bestowing their labours rather in producing this rich commodity than to the growth of that pernicious and offensive weed, tobacco”.
His words were to little avail then or at any time since, because the cultivation of silk is only an economic proposition in those countries where plenty of cheap labour is constantly available.
Japan’s sericulture began on a big scale after 1885 and the escape of the Japanese silk worms from the dreadful PEBRINE disease which ruined the French and eventually most of the European silk industry from 1850.
The investigation of this disease led Louis Pasteur to discover the existence of germs and how to find means to control them through examination and strict hygiene.
The Japanese have devoted a great deal of study and intensive work to sericulture and their success has been due largely to their scientific approach to the many problems besetting a unique industry.
Since 1945 they have produced about 3 times as much silk as China. Next highest production figures come from the Soviet Union, India and Italy.
It is believed that Japan and China have always produced the largest amounts of raw silk with most other countries a long way behind.
Silk is perhaps the most beautiful of all the fibres we use for the creation of textile objects.
It is lustrous, soft and capable of taking dyestuffs well and so dyes into rich and vibrant colour.
Silk is a protein fibre, which is obtained from the cocoons of caterpillars of the silk moth of which there are several different kinds.
Cultivated or domesticated silk worms belong to the BOMBYXIDAE family of which BOMBYX MORI, is the most important for the production of silk.
The Bombyx Mori live exclusively on the leaves of the white mulberry tree. There are about sixty members of the BOMBYXIDAE family of which at least 6 varieties may be found in India.
So called wild silk is produced by the SATURNIDAE species. Among the better known members is the Chinese Bombyx Mandarinae Moore, which feeds on the leaves of the wild Mulberry tree and may be a primitive relation to the Bombyx Mori. Another Saturnidae lives on the leaves of oaks growing in the Northern provinces of China and yet another on the leaves of the AILANTHUS GLANDULOSA, the “tree of heaven” which grows in both China and India.
Strong magnification show a deal of differences between cultivated and wild silk. The cultivated fibres are smooth and vary in colour from nearly white or light cream to yellow while the wild silk fibres are grooved and dark in colour.
Wild silk is not as glossy as cultivated silk because the grooved texture reflects less light than does the
smooth surface of cultivated silk.
The quality of the products of the wild silk moths varies widely, but the Chinese kind is usually superior. The names TUSSAH or TUSSORE were first applied to Chinese wild silk but now seem to be applied indiscriminately.
Rearing cultivated silk worms
The work of rearing silk worms is timed to start when the Mulberry leaves come out in the spring.
First the eggs are warmed after having been kept in a cool place during the winter. In a modern establishment warming is done by spreading the eggs on trays in a warm hatching shed.
They hatch after a few days and are then only 1/8th of an inch long.
1 oz of eggs should yield 36,000 worms and they will eat 1 ton of Mulberry leaves and make about 140 lbs of cocoons to produce 12 lbs of raw silk.
When the worms begin to appear from the eggs, perforated paper is placed over the trays with Mulberry leaves on top so that the worms can climb through the holes and start eating in clean surroundings.
Apart from solid eating, the worm has four periods of sleep of about 1 day each during which it sheds its outgrown skin and grows a new one.
After the fourth moult it feeds for 10 days when it will eat 20 times its own weight in leaves. It will now be 35 days since its hatching and the worm is 10,000 times as heavy as when it was born and will be about 3″ long and weigh about 1/4 oz or 7 grams.
The silk it will spin is now a liquid contained in two glands.
When the worm has settled down between the straw or twigs provided it is ready to spin its cocoon.
From the glands the liquid silk flows along two channels to a single exit called a spinneret in the silk worm’s head. As it emerges the liquid hardens into fine filaments which are coated and stuck together by a gummy substance called SERICIN which comes from 2 further glands. So silk spun by the caterpillar or worm is a twin filament held together as a single strand with sericin cement.
As it exudes the thread, the silk worm moves its head backwards and forwards in a figure of 8 movement and thus surrounds itself with a strongly built cocoon made from a continuous silk strand up to a mile long. Spinning the cocoon takes 2 or 3 days while the silk worm shrinks and changes into a PUPA or CHRYSALIS. In the cultivated varieties the chrysalis is killed before it turns into a moth. The killing is done by various methods – stifling, baking or using liquids such as salty water or spirits. When the chrysalis is dead the silk from the cocoon is ready for unreeling but the cocoon can be safely kept until a convenient time.
In nature the moth escapes from the cocoon by secreting a fluid to dissolve away a section and make a hole from which to crawl out.
This method of escape usually means that it would be impossible to unreel the silk in a continuous length, which is why silk from most wild silk moth cocoons has to be combed and spun instead of being unwound. However, there are some wild silk worms that simply use sericin to fill the last gap by their heads and this is dissolved by the moths as they exit, leaving the cocoon intact.
The silk from such varieties of moth may often be unreeled.
Before unreeling the silk the cocoons have to be softened in hot water. A revolving brush may be used to find the almost invisible ends. The filaments from several cocoons are drawn through a guide, given a slight twist to hold them together and steadily unreeled while the cocoons are still floating in the hot water.
Each filament narrows at the ends, a fact which helps to produce an even thread, because the joins may to some extent be hidden if the reeler is sufficiently skilful.
Unreeled silk in continuous strands is known as RAW SILK, NETT SILK or GREGE and can be woven without further preparation and in China it generally was.
In other silk manufacturing countries 2 or 3 of these multifilament strands would be twisted or THROWN together to form a stronger heavier yarn called ORGANZINE, which is used for warp yarns: Untwisted silk called TRAM is used for weft yarns. Each kind would have a distinctive twist of the hanks according to the country of origin.
The sericin acts as a size and protects the fibres from mechanical injury, and so is usually left on the silk during the reeling, throwing and plain weaving processes. It is generally removed if the silk is to be dyed.
Sericin may be removed in a solution of soapy water at a temperature of about 95° Centigrade.
In some places chemicals were used, but this practice was damaging and eventually forbidden in Britain.
Chemical treatments were known as SCHAPPING.
The terms for silk which could not be reeled off but had to be spun include SCHAPPE, BOURETTE and NOIL which is the name for silk made from very short ends to produce a coarse material – originally this had industrial uses, but now it is often used for clothing too.
The processes are essentially the same as those first designed in China, where tradition includes silk among the fundamental elements of Chinese civilization.
Among the facts about silk, which needs consideration during any form for conservation are the following.
A strand of raw silk is about 75% silk, 23% sericin, 1.5% fat and 0.5% animal salts.
The silk filament is a protein called FIBROIN which is similar to the sericin protein though different in physical behaviour.
The sericin makes the silk quite stiff and harsh to the touch and hides the gloss and shim, but its removal causes a loss in weight of about 20%.
Examination under a microscope shows that the sericin is very unevenly distributed to coat and hold together the 2 silk filaments which are triangular in cross section.
Silk fabrics differ fundamentally from the other animal fibres, which are made from keratin.
The fibroin molecules of silk contain carbon, hydrogen, nitrogen and oxygen, but not sulphur.
The fibroin is composed of long chain molecules linked by chemical bridges called peptide chains formed by a compound of amino acids.
In silk filaments, the peptide chains are fully extended whereas in the fibres of wool and hair they are folded when the fibres are in a relaxed state.
Silk has, in this respect, a close resemblance to stretched wool fibres, this accounts for the low elasticity of silk compared with wool. When silk fibres are stretched in any way they tend to stay stretched or to recover only very slowly.
Silk may swell to nearly double its normal circumference in water. At 100% relative humidity it can take up nearly ⅓ of its own size in moisture and like wool it can do so without feeling wet to the touch.
If there are salts or other impurities in washing water or in a polluted damp atmosphere, silk tends to absorb these too. During the various kinds of finishing treatments including dyeing with basic dyes this ability is used. In the weighting process too. The characteristic called “scroop” which we often associate with silk is produced by an organic acid treatment of the silk during manufacture.
This treatment was particularly fashionable at the turn of the last Century. The effect is obtained, because the acid treatment causes a surface hardening of the filaments – taken a step further this treatment can also be used to produce crepe effects.
Treatment with very strong acids may decompose the fibroin altogether into its component amino-acids.
Silk fibroin is also attacked by oxidising agents.
We know from the fact that sericin is removed from silk in hot soapy water that silk is less easily damaged by alkalis than is wool, nevertheless only neutral washing aids should be used except possibly for Tussah or wild silk which is particularly resistant – hence it’s used for blouses and underwear.
Weak alkalis such as soapflakes, borax and ammonia may cause little appreciable damage but more concentrated alkali will destroy the lustre of the silk and cause less of strength.
Silk will withstand higher temperatures than wool without decomposing. It can stay unaffected by heat at 140° Centigrade for long periods of time but it decomposes at 175°C.
Burning silk gives off a smell similar to that of burning hair or horn.
Silk is attacked by atmospheric oxygen and may suffer a gradual loss of strength if not carefully stored in dark enclosed conditions.
Sunlight encourages the decomposition of silk.
Silk combines high strength and flexibility with good moisture absorption, softness and warmth. It can be woven and knitted into all manner of materials from the sheerest chiffon to the richest of heavy pile velvets.
It is without doubt a very desirable textile fibre, but it will always have to be treated with care and understanding of its particular nature.
Smooth surfaced filaments by their nature do not hold onto dirt, but human perspiration can degrade the fibroin so careful washing of silk garments in regular use is generally considered necessary.
In conservation, we are often confronted with dresses which have been attacked by perspiration and have to accept that the affected parts cannot usually be restored but – at best – only disguised.
In conservation it is rarely necessary to iron silk, there are various ways in which fabrics may be coaxed into the smoothness and gloss we associate with silk (and which promotes the gloss usually characteristic of most silk objects). Flat objects are easy, because they can be rolled. If ironing is necessary, then it should be remembered that mulberry silk should be damp or ironed with a damp cloth, while wild silk should be dry and that ironing should never be done on objects not absolutely clean.
The plasticity of silk is made use of in finishes where heat and pressure are applied to the fabric after weaving, dyeing or printing – such as pleating or making embossed designs with heated stamps. Such fabrics should neither be washed nor ironed.
Silk fabrics may also have been given special glazed effects by pressure as in watered silks.
Obviously damage may be caused by similar methods. For example carelessly applied ironing may cause wrinkles or an undesirable shine. The fabric may be stretched and fixed into a wrong shape. High temperatures may cause already degraded fibres to disintegrate further.
It is often easier to make silk objects resume their proper shape if they can be washed – provided their shape is not dependent on those kinds of finishing treatments which may be removed by water.
Weighting with metallic salts or acid treatments to produce “scroop” are treatments not in themselves removable, but nevertheless weighted silks are often drastically affected by washing or dry cleaning because of the accumulated damage done by the metallic salts over the years – especially when combined with the effects of pollution which tend to make silk very dry and brittle.
Heavily weighted silks are very sensitive to the effects of light and air and they can deteriorate very rapidly indeed under unfavourable conditions.
Their dryness and consequent brittleness makes it seem desirable that they should be washed because even desiccated silk when lubricated becomes lustrous again and stronger to handle.
But the main difficulty about washing arises from the fact that degraded silk objects are less strong when wet than when dry and have to be very well and thoughtfully supported during the whole process including the drying. And this means that it may be necessary to construct means to keep three dimensional objects in their particular shape throughout the processes of both washing and drying.
Degraded silk objects cannot be hung up to dry. They always have to be coaxed into their proper shape while they are still wet. However, this is generally speaking only a problem when the object is three dimensional.
Flat pieces are laid out flat and aligned according to warp and weft directions and then fixed into place with suitable aids, but garments will usually have to be specially arranged with individual parts placed in such a way that every part will dry right.
An important consideration in storage and display concerns the fact that silk is a poor conductor of electricity, which makes it acquire a static charge when handled. It “comes alive”.
This causes difficulties during manufacture, and in storage and display.
The static charge is increased in a dry atmosphere.
If objects made from silk are put in a glass fronted case in a dry atmosphere and this case is polished regularly with a dry cloth the object tends to be destroyed simply through the action of keeping the glass clean because the silk is drawn towards the glass by the static charge produced.
This destruction may be delayed by cleaning the glass with a damp cloth.
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