IELTS Academic Reading Practice Test 27: Passage 1

For thousands of years, silk has been one of the most highly prized fabrics in the world, valued for its softness, its strength and the way it catches the light. Chinese legend credits its discovery to an empress who noticed that a cocoon which had dropped into her cup of hot tea began to unwind into a long, fine thread. Whatever the truth of the story, archaeological finds show that silk was being produced in China at least 5,000 years ago, and for many centuries the methods used to make it were a closely guarded secret.

Almost all of the world's silk comes from a single insect, the domesticated silk moth Bombyx mori. Thousands of years of selective breeding have made this species entirely dependent on people. It no longer exists in the wild; the adult moths cannot fly, and they do not even feed, surviving for only a few days, during which they mate and the females lay their eggs. A single female may lay several hundred eggs. Producers can store the eggs in cool conditions and warm them when they want them to hatch, so that the young caterpillars, known as silkworms, emerge when fresh mulberry leaves are available.

Mulberry leaves are almost the only food silkworms will eat, and they eat a great deal of them. Over a period of about a month, a silkworm increases its weight roughly ten thousand times, shedding its skin four times as it grows. Throughout this period the worms are kept on trays in rearing houses, where workers bring them freshly picked leaves several times a day and night. The worms are highly sensitive to their surroundings: sudden changes of temperature, strong smells and dampness can all harm them, and disease can spread rapidly through a crowded rearing house. In the 1860s, an outbreak of a disease called pébrine came close to destroying the French silk industry until the scientist Louis Pasteur showed how it could be controlled by examining the moths under a microscope and breeding only from healthy females.

When it is fully grown, the silkworm stops eating and begins to spin its cocoon. Silk is made in two long glands that run along the length of its body, and it emerges from an opening below the mouth as a liquid that solidifies as it is drawn out. The fibre consists of two strands of a protein called fibroin, held together by a gummy substance known as sericin. Swinging its head in a repeated figure-of-eight pattern, the worm wraps itself in layer upon layer of this fibre for two or three days, producing a cocoon made from a single unbroken filament that may be several hundred metres long. Inside, it changes into a pupa, the stage between caterpillar and adult.

If the pupa were left to develop, the adult moth would eventually force its way out, softening one end of the cocoon with a liquid it produces and breaking the long filament into many short pieces. For this reason, apart from a small number kept for breeding, the cocoons are heated, usually with steam or hot air, which kills the pupae inside. The cocoons are then soaked in hot water to soften the sericin, and the loose end of each filament is found, often by brushing the surface of the cocoons. Because a single filament is too fine to use on its own, the filaments from several cocoons are drawn off together and twisted to form one thread, a process known as reeling. The result is raw silk, which still contains much of its sericin and feels relatively stiff.

To give silk its characteristic softness and shine, the sericin must be removed. This is done by boiling the raw silk in soapy water, a process called degumming, which can reduce its weight by as much as a quarter. The silk can then be dyed, and it takes colour exceptionally well. Little is wasted: damaged cocoons and those from which moths have emerged cannot be reeled, but their short fibres are combed and spun, much as wool or cotton is, to make a cheaper material known as spun silk.

Bombyx mori is not the only source of silk. In India, now the world's second-largest producer after China, several wild or semi-wild species are also used. These include the muga moth of Assam, whose silk has a natural golden colour, and the eri moth, whose caterpillars feed on the leaves of the castor-oil plant. Eri silk is normally spun rather than reeled, so it can be made without killing the pupae, and it is sometimes sold as 'peace silk' for that reason.

Today silk accounts for only a tiny fraction of the world's textile fibres, but it has found new uses beyond clothing. Surgeons have long used silk thread to close wounds, and researchers are now experimenting with purified fibroin, which the human body tolerates well, in products ranging from edible coatings that keep fruit fresh for longer to materials that help damaged tissue to repair itself. The silkworm, it seems, has not finished surprising us.