Artificial abrasives belong to two principal groups: (1) The aluminous group, comprising alundum and aloxite; and (2) the silicon-car bide group, comprising carborundum and cris tolon. Alundum is crystalline aluminum oxide and is the most efficient of all abrasives for steel. The possibility of determining the de gree of toughness in the manufacture of this substance leads to a line of special alundum abrasives made purposely for grinding special hardened and toughened steels. Aloxite is of the same general composition as alundum, but with different qualities and adaptations. Its specific use is on malleable iron. Carbide of silicon, called in the United States (carborun dum," is the hardest of all abrasives but lacks the toughness of alundum. It is so hard that carborundum wheels have to be trued up with a diamond; no other known substance will cut them. Carborundum is the most effective abrasive for cast iron, chilled iron and brass. Crystolon is a very similar carbide, useful especially on copper, zinc, gold, tin and alu minum. Electrite is a still newer abrasive, with a composition between alundum and carborun dum. It is extremely tough, and amorphous in structure, breaking with a sharp fracture, which is durable under heavy work. The pro duction of artificial abrasives in the United States in 1915 amounted to 37,684,000 pounds, valued at $2,248,778. Abrasives are graded by the size of their fragments. After being crushed, the material is sifted through a series of screens, the number of the smallest screen through which the fragments will pass being given them. For mechanical use abrasives are commonly mixed with a bonding material and formed into wheels. These bonds are of four varieties,—vitrified, silicate, elastic and hard rubber. The first is made of fused clays, and
can be produced of varying degrees of hard ness. This bond does not completely envelop the grains of the abrasive, and thus affords a larger clearance than the other bonds. How ever, it is entirely without elasticity. The sili cate bond is of clay fluxed with sodium silicate at a low temperature. It is affected by damp ness and cannot be made into a thin wheel. The elastic bond is made of shellac and other resins. It has a high degree of elasticity and can be formed into very thin wheels. The rubber bond is of vulcanite, also very elastic. By adapting the bond and the abrasive to the work to be done, almost any desired result may be attained. Another thing to be consid ered is the speed at which the wheel is to be run. With the artificial abrasives a piece of work may be done in a fraction of the time required by the best emery. It is common practice for an alundum wheel to deliver 400, 000,000 cutting strokes per minute, and twice that speed is not unusual. On special work the speed is sometimes equal to 2,000,000,000 cuts per minute. Crushed steel is extensively used in sawing, grinding, rubbing and polish ing granite, marble and other stone. The finer grades of this abrasive, known as (steel emery" and °steel rouge," are used for working glass. See special articles under the names of the different abrasives for further particulars. Consult Haenig, A., 'Emery and the Emery Industry' (London 1912) ; Grits and Grinds (monthly, Worcester Mass.) ; United States Geological Survey, 'Mineral Resources of the United States' (chapter (Abrasives" annually) ; Ries, H., 'Economic Geology' (4th ed., New York 1916).