Asbestos is a fibrous mineral mined from rock deposits. There are approximately 30 types of minerals in the asbestos group. Of the six that have commercial importance, only oneotile, a hydrated silicate of magnesium that contains small amounts of iron and aluminum, oxides, is used in fiber processing.
Asbestos probably formed prehistorically when hot waters containing carbon dioxide and dissolved salts under high pressure acted upon rock deposits of iron, magnesia, and silica.
The ancient Greeks knew of asbestos as early as the first century A.D.; the name comes from the Greek word for inconsumable. But asbestos did not find commercial use until it was used for packing and insulation when the steam engine was invented. Currently the world's leading suppliers of asbestos are Canada and the former Soviet Union.
Useful for heat protection, the most notable characteristic of asbestos is that it will not burn. It can be spun and woven into textiles. However, asbestos is a known carcinogen. It is highly toxic when inhaled as dust particles. The American Conference of Governmental Industrial Hygienists has established a maximum exposure level to chrysotile fibers. A worker may be exposed, without adverse effects, to 2 fibers/cc more than 5 microns long.
The first step in processing asbestos fibers is to separate the longer fibers—from 3/4-3/8 in (1.91-0.94 cm)—from rock by pounding. The shorter fibers—up to 3/8 in (0.94 cm) long—are separated from the rock by crushing and screening. The fibers are next graded according to length. Different grades of fibers are blended, combed, cleaned, and aligned in webs. The web is separated into ribbons or rope-like strands. (Slivers are formed into rolls or laps for electrical insulation). The strands are spun and twisted into yarns of various sizes, depending upon end use. Finally the yarns are woven into fabrics or plaited into braids using mechanical braiders.
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