Asbes'tos

Asbes'tos, a mineral very closely allied to tremolite, actinolite, and hornblende, and which, along with tremolite and actinolite, is often ranked among the varieties of hornblende. It consists chiefly of silica, magnesia, lime, and oxide of iron, and is of a fine fibrous character, the fibres sometimes combined together in a compact mass, sometimes easily separable, elastic and flexible. It is generally of a whitish or greenish colour, and has a silky or vitreous lustre. The variety called Rock-cork very much resembles cork, is soft and easily cut, and so light as to swim in water. Rock-leather and Rock-wood are varieties somewhat similar to rock-cork, but not so light. The finest fibrous variety with easily separable fibres is called Amianthus (from a Greek word signifying 'unpolluted,' as Asbestos is from a Greek word signifying 'indestructible'), because cloth made of it was cleansed by passing it through fire. This cloth was used by the ancients to enwrap dead bodies placed on the funeral pile, so as to preserve the ashes of the body unmixed. The finest amianthus is found in the Tarentaise in Savoy. It is abundant in Corsica, and is found also in Cornwall, at Portsoy in Scotland, and in several of the Shetland Islands. The main supply came, till of late, from the older crystalline rocks of the Alps and Pyrenees, from North America and Silesia, but is now obtained mostly from Italy and Canada, Tasmania, and New South Wales. Asbestos is now applied to many and very various purposes. It is largely employed in the form of cloth or canvas as a filtering medium for corrosive liquids, while as millboard or cardboard it has an extensive application in the form of washers for fire or acid proof joints. Steam stopcocks are now constructed in which the key or plug works in a chamber closely packed with fibrous asbestos; while for all kinds of engine joints, boiler manholes, and similar situations, the imperishable nature of asbestos renders it specially valuable. It has recently come into extensive use in the construction of gas fires. At first the coarser varieties only were used, being ground to powder, and then made into balls of a reddish-brown colour by means of fireclay. These being heated by some form of Bunsen-burner (q.v.), became red-hot, and radiated out a large proportion of heat. Later inventors, however, tend to use the filamentous white variety, in combination with a network of iron, for the purpose of producing a more cheery fire, the thin fibres being readily raised to a white heat. In the form of paint (made with asbestos powder, the medium being a secret) it has been much used to render wooden buildings more or less fully fireproof; but the tendency of the paint to peel off the wood has hitherto detracted from its value. Wood treated in this way does not burst into flame, but only smoulders away under the influence of heat. As a non-conductor of heat, asbestos has valuable properties; a sheet of it placed on a table enabling red-hot vessels to be put down without danger. It is used for so many and various purposes as packing for all classes of machinery, fire-escapes, firemen's clothing, furnacemen's gloves, fireproof putty, sheeting, boiler and steampipe covering, millboard for every purpose, paper, cloth for filtering acids and other similar uses; for covering rollers in print-works (where aniline dyes are employed, and it is necessary to resist heat and the action of the acids); for flooring-felt, roofing-felt, and wall-felt, more especially in timber-built houses; as a lubricant for every kind of engine; for portable fireproof safes, and for lamp-shades; for curtains in theatres; as a non-conductor in electrical work, as tubing for covering telegraph wires, and for electricians' gloves. Asbestos paper, printed with a pattern, has been used to cover wooden partitions. In its manufacture the lumps of crude asbestos are put through an ingenious crushing-machine, whose rollers have a parallel motion in addition to their rolling action over each other. This action effectually opens out the fibres, which are then boiled in large tanks. The shorter fibres having been ground down and reduced to a pulp, are converted into asbestos millboard by manipulation on gauze netting. The longer fibres on leaving the crushing-machine are woven into yarn and cloth in looms.