ASBESTOS (named from a Greek com pound word signifying inextinguishable, incom bustible), a fibrous fireproof mineral substance, one of the most remarkable found in nature. Several minerals are mined and sold as as bestos. The most important is chrysotile, the fibrous form of serpentine (II.Mg$S40.). Ac tinolite [Ca(MgFe)s(SiOs)41 yields a variety usually known as hornblende asbestos. An other variety comes from the mineral known as anthophyllite (MgFe)Si0.. The fibres formed by the chemical combination above given are perfectly smooth, and in this respect are different from all other known fibres. Par adoxically, it is at once fibrous and crystalline, elastic and brittle, heavy as a rock in its crude state, yet as light as thistledown when treated mechanically. Added to this, its fibres, soft, white and delicate, have, by their inherent qual ity of indestructibility, withstood the action of the elements since the world began. Asbestos is found widely distributed throughout the world, although the principal supply of crude asbestos suitable for the manufacture •of fire proof cloths and curtains comes from near Black Lake and Thetford, Canada, about 75 miles fromQuebec, where it has been mined since 1877. The Italian, Corsican and South European mineral has a fine, silk-like fibre, but is lacking in the essential characteristic of strength. The product obtained from South Carolina has a soft, woody, yellowish fibre, which quickly powders under pressure. The South African asbestos from Griqua Town is of a dark slate or black color, with exception ally long, strong fibres, but owing to its stiff and horny texture it cannot be manufactured into a fine fabric, hence the superiority of the Canadian asbestos and its large consumption in the United States.
The asbestos usually occurs in veins, most commonly in serpentine rock which has re sulted from the alteration of basic igneous rock, usually peridotites (q.v.). The chrysotile commonly occurs with the fibres across the vein, and is known as °cross fibre.° The hornblende asbestos more often occurs along slip fractures with the fibre parallel to the plane of slipping, and is called °slip fibre? The mining of asbestos differs radically from the mining of other minerals, since no shafts are sunk, but excavations are made in the open, somewhat after the manner of a stone quarry. Canadian asbestos, however, is found in narrow veins or seams about an inch and a quarter in thickness, and embedded in rock which is easily severed from it. The rock to which the mineral is attached shows on fresh fracture a serpentine mineral of a green shade containing finely divided particles of chromic and magnetic iron. The asbestos on cleavage presents a brilliant, dark-green surface by re flected light, but the fibres after they are de tached are perfectly white. The act of separat ing the mineral from its matrix of rock is termed ((hand cobbing,° and after this process the mineral is shipped to various factories in the United States.
The process of manufacture begins by plac ing the asbestos mineral in a chaser mill, a machine comprising a rotating edge-wheel re volving at the end of a radical arm in a trough, which crushes the mineral, dividing the fibres without destroying them. The result is a snowy
mass of mineral wool ready for winnowing, a method of removing the minute particles of rock still clinging to the fibres very much like the winnowing of grain; this is done by means of a blast of air, which separates and blows away the foreign matter, leaving the fibres in a re fined state and in proper condition for the third stage of manufacture. This is termed air fibre raising, and as the name implies, the fibres are raised by a current of air produced by a blower of large dimensions through a vertical pipe in clined at a small angle. The object of this pro cedure will be obvious, when it is stated that the air blown across the fibres causes those of coarser texture to be deposited in a compart ment near the bottom of the pipe. The medium fibres will be projected a little higher, and these will fall into a second compartment. The finer fibres will be blown to a higher point, and there collected, while the dust will be carried to the top and deposited. The fibres are in this way sorted into different lots according to their tex ture, and are ready to be made into articles for which they are best adapted. The fluffy stuff now goes to the carding room, just as though it were genuine wool sheared from a sheep or pure cotton fresh from the plant on which it grows, instead of a mineral substance that in its original state was mined like a lump of an thracite coal. A carding machine, similar to that employed in preparing wool, cotton or flax fibres before spinning, has been adopted by the manufacturers. The problem of mechanically combing these fibres was no small one, and the carding takes place in a machine having a large central rotating cylinder covered with card clothing, that is, strips of leather set with pro jecting wires termed teeth. Around the main cylinder there are a number of smaller cylin ders, also provided with card clothing, which engages the teeth of the central cylinder rotating in the reverse direction. This machine straight ens out the fibres and lays them parallel; after passing through the first breaker, they are fed into a second carding engine or breaker, which is set to a finer gauge than the preceding. A third and last carding process takes place in a machine called a finisher or condenser, when all the irregularities are eliminated, and the fibres are stripped from the final cylinder by means of a fly-comb and are converted into unspun threads, when they are delivered on a traveling apron or endless band, and are gath ered into rows by reciprocating scrapers; they are then condensed, and the process is continued in the coiling cans. In spinning the yarn, the rovings are delivered to the spindles on a car riage, which then recedes, when the fibres are twisted, and returns when the spun asbestos yarn is wound on the spindles. The spinning frames do not draw the yarn, and no strain is placed on it until after it is twisted. This brings the manufacture of the fireproof material to a point where it is to be woven into cloth, packing, or other forms; for asbestos is used for divers other purposes than those appertain ing to theatres.