Adapted from 3rd edition, Encyclopaedia of Occupational Health and Safety.
Silk is a lustrous, tough, elastic fibre produced by the larvae of silkworms; the term also covers the thread or cloth made from this fibre. The silk industry originated in China, as early as 2640 BC according to tradition. Towards the 3rd century AD, knowledge of the silkworm and its product reached Japan through Korea; it probably spread to India a little later. From there silk production was slowly carried westward through Europe to the New World.
The production process involves a sequence of steps not necessarily carried out in a single enterprise or plant. They include:
- Sericulture. The production of cocoons for their raw silk filament is known as sericulture, a term which covers feeding, cocoon formation and so on. The first essential is a stock of mulberry trees adequate to feed the worms in their larval state. The trays on which the worms are reared have to be kept in a room with a constant temperature of 25 °C; this involves artificial heating in colder countries and seasons. The cocoons are spun after about 42 days of feeding.
- Spinning or filature. The distinctive process in silk spinning is called reeling, in which the filaments from the cocoon are formed into a continuous, uniform and regular strand. First, the natural gum (sericin) is softened in scalding water. Then, in a bath or basin of hot water, the ends of the filaments from several cocoons are caught together, drawn up, attached to a reeling wheel and wound to form raw silk.
- Throwing. In this process, the threads are twisted and doubled into more substantial yarns.
- Degumming. In this phase, the raw silk is boiled in a solution of soap and water at approximately 95 °C.
- Bleaching. The raw or boiled silk is then bleached in hydrogen peroxide or sodium peroxide.
- Weaving. The silk thread is next woven into fabric; this usually takes place in separate factories.
- Dyeing. Silk may be dyed while in the filament or thread form, or it may be dyed as a fabric.
Health and Safety Hazards
Symptoms of carbon monoxide toxicity consisting of headache, vertigo and sometimes nausea and vomiting, usually not severe, have been reported in Japan, where sericulture is a common home industry, as a result of the use of charcoal fires in poorly ventilated rearing rooms.
Mal des bassines, a dermatitis of the hands of female workers reeling raw silk, was quite common, particularly in Japan, where, in the 1920s, a morbidity rate of 30 to 50% among reeling workers was reported. Fourteen per cent of the affected workers lost an average of three working days each year. The skin lesions, localized mainly on fingers, wrists and forearms, were characterized by erythema covered with small vesicles which became chronic, pustular or eczematous and extremely painful. The cause of this condition was usually attributed to the decomposition products of the dead chrysalis and to a parasite in the cocoon.
More recently, however, Japanese observations have showed that it is probably related to the temperature of the reeling bath: until 1960 almost all reeling baths were kept at 65 °C, but, since the introduction of new installations with a bath temperature of 30 to 45 °C, there have been no reports of the typical skin lesions among reel workers.
The handling of raw silk may produce allergic skin reactions in some reel workers. Facial swelling and ocular inflammation have been observed where there was no direct local contact with the reeling bath. Similarly, dermatitis has been found among silk throwers.
Respiratory tract problems
In the former Soviet Union, an unusual outbreak of tonsillitis among silk spinners was traced to bacteria in the water of reeling basins and in the ambient air of the cocoon department. Disinfection and frequent replacement of reel bath water, combined with exhaust ventilation at the cocoon reels, brought about a swift improvement.
Extensive long-term epidemiological observations also carried out in the former USSR have shown that workers in the natural silk industry may develop respiratory allergy featuring bronchial asthma, asthmatiform bronchitis and/or allergic rhinitis. It appears that natural silk can cause sensitization during all stages of production.
A situation causing respiratory distress among spinning-frame workers when packaging or repackaging silk on a spinning or winding frame has also been reported. Depending upon the speed of the machinery, it is possible to aerosolize the proteinaceous substance surrounding the silk filament. This aerosol, when respirable in size, will cause a lung reaction very similar to that of the byssinotic reaction to cotton dust.
Noise exposure can reach harmful levels for workers at machines spinning and winding the silk threads, and at looms where fabric is woven. Adequate lubrication of the equipment and the interposition of sound baffles may reduce the noise level somewhat, but the continuing exposure throughout the working day can have a cumulative effect. If effective abatement is not obtained, resort will have to be made to personal protective devices. As with all workers exposed to noise, a hearing protection programme featuring periodic audiograms is desirable.
Safety and Health Measures
Control of temperature, humidity and ventilation are important at all stages of the silk industry. Home workers should not escape supervision. Adequate ventilation of rearing rooms should be ensured, and charcoal or kerosene stoves should be replaced by electric heaters or other warming devices.
Lowering the temperature of reeling baths may be effective in preventing dermatitis. The water should be replaced frequently, and exhaust ventilation is desirable. Direct skin contact with raw silk immersed in reeling baths should be avoided as far as possible.
The provision of good sanitary facilities and attention to personal hygiene are essential. Hand washing with a 3% acetic acid solution has been found effective in Japan.
The medical examination of new entrants and medical supervision thereafter are desirable.
The hazards from machinery in silk manufacture are similar to those in the textile industry in general. Accident prevention is best achieved by good housekeeping, adequate guarding of moving parts, continuing worker training and effective supervision. Power looms should be provided with guards to prevent accidents from flying shuttles. Very good lighting is required for the yarn preparation and weaving processes.