EMERY (Ger. Smirgel), an impure variety of corundum, much used as an abrasive or polishing substance, and occurring as a granular or massive, dark-coloured, dense substance, having much the appearance of an iron-ore. Its specific gravity varies with its composition from 3.7 to 4.3. Under the microscope, it is seen to be a mechanical aggregate of corundum, usually in grains • or minute crystals of a bluish colour, with magnetite, which also is granular and crystalline. Other iron oxides, like haematite and limonite, may be present as alteration-products of the magnetite. Some of the alumina and iron oxide may oc casionally be chemically combined, so as to form an iron spine], or hercynite. In addition to these minerals emery sometimes contains diaspore, gibbsite, margarite, chloritoid and sillimanite. Indeed emery must be regarded as a rock rather than a definite mineral species.
The hardness of emery is about 8, whereas that of pure corun dum is 9. Its "abrasive power," or "effective hardness," is by no means proportional to the amount of alumina which it contains, but seems rather to depend on its physical condition. Thus, taking the effective hardness of sapphire as Ioo, J. Lawrence Smith found that the corresponding hardness of the emery of Samos with 70.10% of alumina was 56, that of Naxos, with 68.53 of 46, and that of Gumach with 77.82 of 47.
Emery has been worked from a very remote period in the Isle of Naxos, one of the Cyclades, whence the stone was called naxium by Pliny and other Roman writers. The mineral occurs as loose blocks and as lenticular masses or irregular beds in granu lar limestone, associated with crystalline schists, and has been found to contain 52.4% of corundum, 32-1 of magnetite, 11.5 of tourmaline, 2 of muscovite and 2 of margarite.
Important deposits of corundum were discovered in Asia Minor by J. Lawrence Smith, when investigating Turkish mineral re sources about 1847. The chief sources of emery there are Gumach Dagh, a mountain about I2m. E. of Ephesus; Kula, near Ala shehr; and the mines in the hills between Thyra and Cosbonnar, south of Smyrna. The occurrence is similar to that in Naxos. The emery is found as detached blocks in a reddish soil, and as rounded masses embedded in a crystalline limestone associated with mica-schist, gneiss and granite. It is believed to have orig inated by regional or contact metamorphism of bauxite deposits. The proportion of corundum in this emery is said to vary from 37 to 57%. Emery is worked at several localities in the United States, especially near Chester, Mass., where it is associated with peridotites.
The hardness and toughness of emery render it difficult to work, but it may be extracted from the rock by blasting in holes bored with diamond drills. In the East fire-setting is employed. The emery after being broken up is carefully picked by hand, and then ground or stamped and separated into grades by wire sieves. The higher grades are prepared by washing and elutria tion, the finest being known as "flour of emery." A very fine emery dust is collected in the stamping room, where it is deposited of ter floating in the air, and is used by lapidaries and plate-glass manufacturers. Emery-wheels are made by consolidating the powdered mineral with an agglutinating medium like shellac or silicate of soda or vulcanized india-rubber, and are used not only by dentists and lapidaries but also, on a large scale, in mechanical workshops for grinding, shaping and polishing steel. Emery-sticks, emery-cloth and emery-paper are made by coating the several materials with powdered emery mixed with an adhesive medium.
(See CORUNDUM.)