EMERY, an impure variety of the mineral corundum (q.v.), reddish brown, black, blue black or gray in color and next to the diamond the hardest mineral known but is not crystal lized. It consists of nearly pure alumina (65 to 75 per cent) and oxide of iron and a small amount of silica and water. Emery occurs in large boulder-like masses, closely resembles a fine-grained magnetite ore in texture and is often mistaken for it. In its native form its value as an abrasive has been known from the earliest times and many references are made to it in books by Greek authors. Then as now it was used in cutting and polishing jewels and intaglii in the sculpture of statuary from the harder rocks and in polishing marble. It was undoubtedly used by the Egyptians and there are many evidences of the use of it or as hard a substance in the manufacture of prehistoric stone implements.
As now used, in its pulverized form, it is one of the most useful substances known to the arts. The rock is broken in powerful crushers and stamping-mills and separated into .powders of varying degrees of fineness by screens or by elutriation. These powders, varying from par ticles one-tenth of an inch in diameter to the finest flour, are sprinkled with water or oil upon the lead wheel of the lapidary, or spread upon wood, paper or cloth to which a thin layer of glue has been previously applied; or as has been found to be its most effective application, mixed with various adhesive substances and molded into solid wheels. Emery-stones of various shapes and sizes are also made in the same manner.
Emery-wheels are now made up to 36 inches in diameter and from four to six inches in thick ness and in every variety of coarseness from rough shapers to fine polishers for brass and steel. The cementing material is usually a secret with the manufacturer and upon this and upon the quality of emery used depends the cost and the subsequent life and usefulness of the wheel. Properly mounted and turned at a
proper speed it is our most effective cutting tool, tearing its way rapidly into chilled castings that the best file will not cut, or taking the teeth instantly off the hardest file. Special points to be observed are uniformity of texture, that the wheel may wear away evenly under use; care fully fitted bearings, that there may be no vibra tion under the high speed at which it is run ; the wheel must not be fitted closely to either mandrel or flanges, lest expansion by heat burst the wheel; and the cementing material of the wheel must be able to resist the tendencies to centrifugal disruption and to melting under the heat generated by its friction with the object being cut. Its effective speed must have been determined and tested and the degree of pres sure with which the work is to be applied must likewise be ascertained. Emery wheels that have become misshapen through use are turned true by various special contrivances, all of which must have a cutting edge of rough diamond. Wheels are often shaped for special work in the same manner.
The present supply of emery is chiefly from the island of Naxos and from near Smyrna, Turkey. A small amount is mined near Chester, Mass., and Peekskill, N. Y., and it is found in insignificant quantities elsewhere in the United States. Corundum and precious sapphire have been found in Georgia and North Carolina. Consult Merrill,
Minerals) (New York 1910) and Pratt,