USES OF EMERY AND CORUNDUM
Corundum is the natural (mineral) crystalline oxide of aluminum. Emery is a very fine-grained and intimate intergrowth of corundum and other minerals, chiefly magnetite, some varieties containing also important amounts of hematite, spinel, and chlorite. Both emery and corundum are very hard, and break into rough, sharp grains; hence they are used as abrasives for grinding, dressing, and polishing metals—chiefly iron and steel—and glass, and, to a less extent, stone, wood, and other materials. Emery and corundum are used loose in the form of grains, powders, and flours, and also as grains made up into solid wheels, cylinders, blocks, and files of many shapes by means of a great variety of binders. The essential uses are in work on iron and steel and glass. The softer metals and other materials can be worked in many cases to better advantage with other abrasives, such as quartz, tripoli, garnet and pumice.
The essential operations for which emery and corundum are used can be performed with the artificial carbide and alumina abrasives. For some work, however, such substitutions appear not to be advisable, as the abrasive quality and efficiency of both the natural and artificial abrasives depend not only on the hardness of these materials, but also on a number of other factors: among these being the physical qualities of the materials worked; the sharpness of edges and angles of broken particles of the abrasive; the manner in which the abrasive breaks down under use; the manner of, and materials used in, binding the abrasive particles; and the speed and pressure with which they are applied to the work.
Of the various kinds of material abraded, each calls for different grades and kinds of abrasives, and for variation in the above factors in the use of these abrasives in order to insure most efficient use. Consequently, it is almost impossible to determine arbitrarily the uses for which each of the various abrasive materials is essential. This much, at least, seems certain—that for finishing and polishing glass, particularly optical glass and plate glass, there is as yet no general agreement that satisfactory substitutes are available for the better grades of Turkish and Greek emery, although experiments in manufacture and use of suitable artificial abrasives have been successful.