ASBESTOS, a generic name for a group of minerals possess ing crystalline fibrous structure and which can be spun or felted to make non-combustible fabrics for heat insulation and for other purposes where resistance to fire is essential. The name is derived from the ancient Greek name (acrf3EcfTos) for a fabulous stone which once set on fire could not be quenched, probably unslaked lime.
The non-combustible character and spinning qualities of asbestos fibre were undoubtedly known to the ancient world. Plutarch mentions "perpetual" lamp wicks used by the Vestal Virgins, and Pausanian records a lamp that had a wick which was not con sumed, being made of "Carpasian linen," or mineral fibre from Carpasius in Cyprus; shrouds of woven asbestos appear to have been used in ancient times for cremations. Pliny refers to it as linum vivum—"the funeral dress of kings." Varieties.—Three minerals possess varieties known as "as bestos." These are anthophyllite, amphibole and serpentine. The first two are silicates of lime and magnesia, with or without the presence of iron and related elements; the last named is a hydrous silicate of magnesia, with or without iron.
Anthophyllite is a magnesium-iron silicate which in some localities occurs in long thin plates and fibres. It is of mineral ogical importance only.
Amphiboles include five principal types characterized by fibrous structure : (1) tremolite, a calcium-magnesium silicate, occurs in long blade-like crystals or in long thin fibrous or columnar masses, grey to almost white in colour, and rather brittle ; (2) actinolite which is an iron-calcium-magnesium silicate that occurs in fibrous and radiating crystalline masses, bright green in colour, used for weighting paper, and as an inert adulterant. Both these and other varieties of amphibole which are free from alumina, pass by in sensible gradations into (3) as bestos, otherwise called "amphi bole-asbestos" or "hornblende asbestos," which consists of fine flexible fibres resembling flax, usually of a dull green colour and nacreous lustre; (4) mountain leather and mountain cork con sist of flexible sheets of interlaced fibres, difficult to separate ; moun tain wood is compactly fibrous, grey to brown in colour, and re sembles dry wood or petrified wood; (5) blue asbestos or cro cidolite, found in Griqualand, South Africa and in Australia, possesses a very silky lustre, is of a dull blue colour, and its fibres are elastic and occasionally sev eral inches in length. Chemically it is a soda-iron silicate ; it pos sesses higher tensile strength than chrysotile (see below) but is less resistant to high temperatures.
Serpentine is a hydrous silicate of magnesia, generally found in massive forms but also occur ring with a banded or slaty structure; closely related to serpentine is talc (q.v.) which is, however, soft and brittle. There are two fibrous minerals derived from serpentine : (1) Picrolite occurs in fibrous masses in fissures of the massive serpentine. The fibres are sometimes 1 yin. in length but they are not flexible nor easily separable; (2) Chrysotile occurs in narrow veins traversing the serpentine rocks as a mass of fine silky crystals. These veins are usually various shades of green but amber to brown shades also occur. The crystals are easily separated into fine, white, silky, flexible fibres. This mineral constitutes the great bulk of the commercial asbestos.
To be of commercial value asbestos must possess length and fineness of fibre combined with infusibility, toughness or relatively high tensile strength and flexibility. Asbestos fibre resembles fine polished wire, free from serrations of any kind. This adds to the difficulties of spinning. In cross section, fibres are either circular or polygonal. Under high magnification a fibre is seen to consist of numerous fine threads crowded together. The smallest fibre yet measured had a diameter of o•00075mm., according to Dr. H. T. Barnes, McGill The incombustible and insulating properties of asbestos make it invaluable to industry. The first modern experiments in weaving asbestos were made in Italy. The longer fibre can be carded and spun into yarns, either alone or with additions such as cotton or small sizes of brass or copper wire. Spun products are woven into fabric of varying weights, thickness and densities to meet numer ous industrial needs. Some of the principal uses are for drop curtains in theatres, for fireproof wall linings and for boiler pipe packing. Yarns spun with fine wires are used for fabric for brake linings ; woven asbestos belts convey cement clinker and other hot products ; when impregnated with rubber, asbestos is utilized for heat insulation and for gaskets; asbestos yarns, impregnated with graphite and suitable greases are used for steam and pump pack ings ; ropes, cords, twine and threads are made for various pur poses ; various products are used to insulate electric conductors; fibre shorter than spinning stock is used for making steam pipe, hot water pipe and boiler cover ings. Certain classes of short asbestos fibre are mixed with cement and moulded into various shapes for special purposes or pressed into millboard, wallboard, shingles or tiles. Asbestos sheets deaden sound and asbestos papers are used where damp-proof and fireproof coverings are required.
Asbestos products are manufactured in England, the United States, France, Germany, Austria, Russia, Italy, Spain, Belgium, Canada and Australia. The production of the raw material has risen from 200 tons (Italian) in 1868 to a total from all sources of over 250,00o tons a year. The United States is the largest consumer, and the asbestos factories there absorb about three fourths of the output of the raw material from the Canadian mines. In 1926, the exports of asbestos manufactures from the United Kingdom were 24,889 tons, valued at II,116,000, while the imports of raw asbestos, including fibre and waste, were 27,720 tons, valued at L709,708.
In tons of unmanufactured asbestos were imported into the United States, valued at $8,150,000; and the manufactures tons, valued at $2,177,000. The exports comprised a small amount of unmanufactured asbestos, and miscellaneous manufactures, with a total value of $1,918,000. (See Statistical Abstract of the United States, U.S. Department of Commerce.) BIBLIOGRAPHY.-F. Cirkle, Chrysotile-Asbestos (191o) ; A. L. Hall, Bibliography.-F. Cirkle, Chrysotile-Asbestos (191o) ; A. L. Hall, Asbestos in the Union of South Africa, Mem. 12, Geol. Surv. S. A. (1918) ; U.S. Asbestos and Mineral Corp., Asbestos from mine to finished product (1919) ; Imperial Mineral Resources Bureau 1921, Asbestos (1913-19) ; Asbestos, the mineral's automotive uses, Raw Material (1922) ; "Asbestos, Australian deposits of," Asbestos (Jan. 1925) ; Asbestos production of South Africa and the grades marketed, Eng. and Min. Jour. Press (May 1925) ; R. B. Ladoo, Asbestos, a ,chapter in Non-Metallic Minerals, bibliography (1925) ; L. Berlin raut, Russian asbestos mining reviving, Eng. and Min. Jour. Press (Jan. 1926) ; R. Rosenberg, "Die Verwendung von Mikro-Asbest fur die Herstellung plastischer Massen," Kunststoffe (Mar. 1927) ; also in Plastics (June 1927) ; Becker and Haag, Asbest (1927), bibliography ; "Asbestos textiles and their uses," India Rubber Journal (Dec. 1927) ; J. G. Ross, Chrysotile-Asbestos, monograph, with com plete bibliography, in preparation (available, 1929) . (A. W. G. W.)