Asbestos, a name applied to several minerals from which flexible, heat-resistant fibers are obtained. Asbestos is an excellent insulator against heat. Varieties that are low in iron impurities are also good electrical insulators. Asbestos fibers have high tensile strength. They can be woven into cloth and tapes, spun into yarn, or felted. Asbestos is moisture-resistant, resists attack by acids and other chemicals, and is not easily worn down by friction.
Asbestos has been known since ancient times. Until the end of the 18th century, however, it was primarily an interesting oddity. At that time, an increased need for thermal (heat) insulation for devices such as the steam engine aroused commercial interest.
Asbestos has had many uses based on its heat-insulating properties and its resistance to burning. For example, suits made of asbestos cloth have been used by firefight- ers. Asbestos has been used to provide insulation for furnaces, jet engines, automobile brake linings, and pipes that carry hot fluids. In electrical devices, asbestos has been used to furnish both thermal and electrical insulation. For example, heating elements in some household appliances have been made with asbestos coverings for protection. Asbestos has also been used in roofing, to furnish protection against fire, weather, and wear; in floor tiles, to make them durable and resilient; and in concrete pipes to protect them against corrosion.
The use of asbestos is regulated in the United States and a few other countries because asbestos dust can cause serious health problems. Asbestos dust can be released from asbestos-containing materials that are exposed and friable---that is, easily crumbled. Inhalation of the dust can lead to asbestosis (a chronic lung disease). It can also lead to lung cancer, mesothelioma (a usually fatal form of cancer affecting the lining of the lungs and abdomen), or certain other kinds of cancer.
In the United States, federal regulations issued in the early 1970's set standards regarding the amount of asbestos dust to which asbestos workers may be exposed. Since that time additional regulations have banned products that can release asbestos dust into the air and have established procedures for finding and, if necessary, removing from schools building materials that could be sources of asbestos dust.
Asbestos fibers are produced primarily from three minerals: chrysotile, amosite, and crocidolite. Asbestos minerals of lesser importance include anthophyllite, tremolite, and actinolite. All of these minerals are silicates---that is, they all contain an atomic group consisting of one silicon atom and four oxygen atoms.
About 95 per cent of the fibers used to make asbestos products are chrysotile. Veins of chrysotile are lustrous green when mined, but the separated fibers are white. The fibers can be carded, spun, or woven, much the same as flax, wool, or silk.
Amosite, a form of amphibole, contains a high percentage of iron and varies in color from ash gray to brown. Because the fibers have a coarse texture, amosite is not made into cloth. Crocidolite, also a form of amphibole, is dull blue and is commonly called "blue asbestos." The fibers are stronger than chrysotile fibers, but less heat-resistant.
Asbestos minerals occur in cracks of rocks. These rocks, which are usually igneous, are older than the asbestos minerals. The process by which the fibrous structure of asbestos minerals was formed is not definitely known.
Asbestos is mined by both open-pit and underground methods. Asbestos ore contains fibers of various lengths mixed with much unusable solid rock.
The ore is crushed and the unwanted rock removed. Then the asbestos is crushed again in a special machine that separates the fibers without breaking them. The fibers are sorted by length. The longest fibers (34 inch [2 cm] and longer), which can be used for cloth, are the most valuable. The shortest fibers are used in combination with other materials, such as concrete and asphalt.
The world's leading producers of asbestos fibers are Russia and Canada; their primary output is chrysotile. The main producer of amosite and crocidolite is South Africa. Other important producers of asbestos fibers are China, Zimbabwe, Italy, the United States, and Brazil.