Animal and mineral fibres

1Why this matters

On a cold night in the highlands of Musanze or Nyabihu, a blanket made from animal fibre keeps a family warmer than one made from cotton. Animal fibres come not only from sheep but from goats, rabbits, camels and even caterpillars, and each has its own feel and price.

There is also one natural fibre that is dug out of the ground, and it is dangerous. Until recently many roofs in Rwanda were made with it. This lesson describes the animal fibres and the one natural mineral fibre, asbestos, and explains why asbestos is being removed from Rwandan buildings.

2Animal fibres: hair and wool

Hair fibres are cut or combed from the coats of animals. Wool, from sheep, is the best known. Others are called speciality hair fibres.

Do not confuse the Angora goat, which gives mohair, with the Angora rabbit, which gives angora.

3Animal fibres: silk

Sericulture is silk farming. The steps are:

1. A female silk moth lays 300 to 500 eggs.
2. The caterpillars that hatch, called silkworms, eat mulberry leaves and grow quickly, shedding their skin four times.
3. After the fourth moult each silkworm spins a cocoon around itself from one continuous filament of protein, held together by a natural gum called sericin.
4. The cocoons are placed in hot water. This softens the gum (degumming) and frees the end of the filament.
5. Filaments from several cocoons are unwound together and twisted into a thread; this is called reeling.

Silk was first produced in China thousands of years ago, and China and India still produce more than 60% of the world's silk. Wild silk (tussar) comes from silkworms that feed on other trees; it is less even but strong.

4Characteristics of animal fibres

Animal fibres are made of protein, so they share these characteristics:

• warm: hair fibres trap air, which holds in body heat;
• absorbent, yet they do not feel wet quickly;
• elastic and resilient: they stretch, spring back and resist creasing;
• damaged by alkalis such as bleach and strong soap, and weakened by strong sunlight and sweat;
• wool and hair are eaten by moth larvae and carpet beetles, especially when stored dirty;
• they burn slowly, smell of burning hair, and often go out by themselves.

5The mineral fibre: asbestos

Asbestos is a group of natural minerals that grow as long, thin, fibrous crystals. The fibres can be spun and woven, and they do not burn, so for most of the 1900s asbestos was used worldwide in fireproof cloth, brake linings, pipes, and cement roofing sheets and ceilings.

Asbestos is toxic and causes cancer. When asbestos sheets are broken, cut, drilled or worn, they release tiny fibres into the air. Breathed in, the fibres lodge in the lungs and can cause asbestosis (scarring of the lungs), lung cancer and mesothelioma, a cancer of the lining of the lungs. The illness usually appears about 20 years after the exposure, so people do not feel harmed at the time.

Many countries have banned asbestos. Rwanda banned asbestos roofing in 2009, and since 2011 the Rwanda Housing Authority has been removing old asbestos roofs and ceilings from buildings across the country; most had been removed by 2026.

If you find an old asbestos sheet: do not break, cut, drill, sand or sweep it, and keep children away. Report it to your local authorities. Only trained workers with protective equipment should remove it.

Worked example: two decisions

(a) A blanket for a cold highland home. Gatete's grandmother lives in Nyabihu, where nights are cold. The family compares a wool blanket, a cotton blanket and a mohair throw.

• Cotton is absorbent and cool: good in heat, but it traps little warm air.
• Wool and mohair are protein hair fibres that trap air, so they are warm, and they spring back instead of flattening.
• Decision: wool for the main blanket (warm, cheaper than mohair). They must store it clean and dry in a closed bag to protect it from moths.

(b) An old grey roof on a neighbour's kitchen. The sheets are cracked, and the neighbour wants to break them up to reuse the pieces as paving.

• The sheets may be asbestos cement, a natural mineral fibre that causes lung disease when its dust is breathed.
• Breaking them would release fibres near the family's home, and the harm may not show for about 20 years.
• Decision: leave the sheets alone, keep children away, and report them to the local authorities so that trained workers can remove them safely.

Practice

Solution. (a) The Angora goat. (b) The Angora rabbit. (c) The cashmere goat (its fine undercoat).

Solution. Eggs hatch; silkworms eat mulberry leaves; the cocoon is spun; degumming in hot water; reeling.

Solution. Wool is a protein fibre, and bleach is a strong alkali that damages protein. It would weaken and yellow the fibres.

Solution. Used: it does not burn, and it is a good insulator that could be made into strong, cheap roofing sheets. Banned: its fibres, when breathed in, cause asbestosis, lung cancer and mesothelioma; and the disease appears only years later, so people are exposed without knowing.