The American Cyclopædia (1879)/Emery
|←Emerson, Ralph Waldo||The American Cyclopædia
|Edition of 1879. See also Emery (mineral) on Wikipedia, and the disclaimer.|
EMERY, an impure, granular variety of corundum, intimately mixed with hematite or with magnetic iron ore. It has the appearance of fine-grained iron ore, for which it was long mistaken. Often the crystals of corundum are separable by washing. Its extreme hardness, derived from the corundum, and the ease with which it is obtained in large quantities, have led to its extensive use in the arts, for grinding and polishing hard stones, metals, and glass. Some of the localities from which it is obtained in the Grecian archipelago, and in the vicinity of Smyrna and ancient Ephesus in Asia Minor, were probably known to the ancient Greeks and Romans, as the use of a substance of this nature seems to have been required by the lapidaries of Magnesia, Ephesus, Tralles, and Tyre. In later times the island of Naxos in the archipelago has furnished all the supplies of commerce, the mineral being shipped from the port of Smyrna, and known by the name of Smyrna emery. From 1835 to 1846 the trade in emery was a monopoly granted by the Greek government to an English merchant, who so regulated the supply as to raise the price from its former rate of $40 a ton to about $140. This monopoly was broken up and the whole trade changed in consequence of the discoveries of Dr. J. Lawrence Smith of the United States, who in the course of his explorations in the service of the Porte discovered in 1847 a number of localities of the mineral belonging both to the Turkish and Greek governments. By an arrangement with the former, operations were commenced in the same year at some of the localities and afterward extended to others, so that the price has since been reduced to $50 a ton. At the Juma Dagh, 12 m. E. of the ruins of Ephesus, Dr. Smith found the emery scattered about upon the summit in loose pieces of all sizes up to masses of several tons weight. The rock to which it belongs is a bluish metamorphic marble, reposing upon mica slate and gneiss. In this rock the mineral is found in nodules and in amorphous masses, some of which are several yards in length and breadth, and of the weight of 30 to 40 tons. The structure of this rock is compact and tolerably regular, but the surface presents a granular appearance. Unless traversed by fissures, the rock is broken with great difficulty, and attempts to drill it are made in vain from its wear upon the tools. As the transportation from the quarries is only on the backs of camels or horses, many of the heavier masses are necessarily left behind. Some of the blocks, however, yield to the hammer after being exposed for some hours to the action of fire. The color of the powder varies from dark gray to black; but its shade has no relation to its hardness, and is consequently no index of the value of the article. The relative degrees of hardness of different samples were determined by Dr. Smith by collecting the powder just coarse enough to pass through a sieve of 400 holes to the inch, and with weighed samples of this rubbing little test plates of glass till they ceased to be further reduced. The rubber was the smooth bottom surface of an agate mortar. The loss in weight experienced by the glass plates gave the relative values of the samples of emery. On this plan Dr. Smith prepared a table exhibiting the different degrees of hardness; and making use of sapphire of Ceylon as the standard of comparison, the hardness of which he called 100, and the effective wear of which upon glass was equal to about four fifths of its own weight, that of the best emery was about one half of its weight. This table, to which were appended the results of the analyses of many samples of the mineral made by Dr. Smith, was published in the elaborate articles which he furnished to the “American Journal of Science,” second series, vols. x. and xi. The hardness of the sapphire as rated upon the mineralogical scale is 9, next to the diamond, which is 10. That of emery is not necessarily indicated by the proportion of alumina, for a part of this may be in combination with the silica. It seems to vary with the water present, those samples containing the least water being the hardest. In 1855 the annual production of emery was 2,000 tons of Naxos stone and 1,600 tons of Turkish. The whole business was concentrated in the hands of Mr. Abbott, who held the contract with the Greek government, extending for ten years, and had purchased the Turkish firman, unlimited in time, for the annual payment of $55,000. An arrangement was entered into with the house of Messrs. John Taylor and sons of England to employ a capital of £120,000 in this business, and supply the emery either in the stone or powder to all parts of the world, with the guaranty of its being free from adulterations, such as had previously impaired its qualities and reduced its value. — The principal consumption of the article is in polishing plate glass, and the increase of this business causes a constantly increasing demand for emery. The discovery of new localities is a matter of great importance, the few that are known in other parts of the world furnishing no supply capable of competing with that brought from the head of the Mediterranean. It is said to be found near Petschau in Bohemia, near Yekaterinburg in the Ural, near Miyask in the Ilmen branch of that range, and in Frederick valley, Australia. An extensive vein of emery, exhibiting blue crystals of corundum, was discovered a few years ago at Chester, Mass., by Dr. C. T. Jackson of Boston. The mineral, which had for some time been mistaken for magnetic iron ore, occurs associated with margarite, diaspore, and chlorotoid, in talcose schist. The “American Journal of Science” for September, 1873, contains an article by Dr. J. Lawrence Smith giving an account of the discovery of beds of corundum containing emery in North Carolina, Georgia, and Montana. The beds in North Carolina are very extensive; the corundum is described as being very beautiful, and is found in masses weighing from 600 to 800 lbs., “having large cleavages, and being remarkably free from foreign ingredients,” and in such quantities as to admit of its use as a substitute for emery. It occurs in chrysolite or serpentine rocks, belonging to a regular system of dikes lying on the N. W. side of the Blue Ridge, at an average distance from the summit of about 10 m., and which have been traversed for 191 m. Dr. Smith remarks that through all the range the corundum forms a geognostic mark of the chrysolite, as it does of the calcareous rock containing corundum, described by him in Asia Minor, and belongs to the same geologic epoch. For a notice of the Montana mineral, see Emerald. — Emery is prepared for use by crushing the stone under stamps, and sorting the powder into different sizes by appropriate sieves. For the most delicate uses of opticians it is separated in a small way by a system of washing over called by chemists elutriation. After being ground, the powder is thrown into water, or water containing gum arabic, or it may be oil, and allowed to subside for a certain number of seconds or minutes. The process being systematically conducted, the powder is sorted into many sizes, and named according to the time the fluid is allowed to stand before the substance in suspension is collected, as emery of 10 seconds, of 30 seconds, 2 minutes, 30, 60, 80 minutes, &c. Emery is applied to paper, thin cloth, and slips of wood, by dusting the powder upon these articles after they have been coated with thin glue. They are then ready for sale or for use under the name of emery paper, cloth, or sticks. Mixed with paper pulp and fine glass and rolled into sheets, it forms the patent razor-strop paper; and by a variety of other methods it is prepared for convenient application to its numerous uses of grinding and polishing.