CORUNDUM includes very dissimilar minerals, all consisting almost entirely of highly crystalline alumina, namely, - Precious Corundum, or the Sapphire and Ruby, - Common Corundum - and Emery. - The last two are the common abrasives of the Asiatics and Europeans respectively; and all are separately described under their respective heads in this catalogue. 1. - Common Corundum, says Phillips, probably from its texture, has received the name of imperfect Corundum; and from its hardness, and from its occasional pearly lustre, Adamantine Spar: it occurs everywhere from China to Bengal, and is met with of various colours, but more often of a greyish or greenish tint. Corundum is much used in India for Corundum Wheels and Rubbers, the methods of constructing which are described in the following articles: - 2. - Corundum Wheel. - "This kind of lapidaries' wheel is called in the Tamul language Couroundum-sane. It is composed of corundum, more or less finely powdered, cemented together by lac-resin: the proportions, by volume, consisting of two-thirds of powdered corundum, and one-third of lac-resin. The corundum powder is put into an earthen vessel, and heated over a clear fire; and when of a sufficient heat, (which is ascertained by a small piece of the resin readily fusing,) the resin is added in portions, carefully stirring at the time, to form an intimate mixture. When made into a mass, it is put upon a smooth slab of stone, and kneaded by beating it with a pestle; it is then rolled upon a stick, reheated several times, continually kneading it until the mixture is perfectly uniform. It is afterwards separated from the stick, laid again upon a stone table which has been previously covered with very fine corundum powder, and flattened into the form of a wheel by an iron rolling pin. The wheel is then polished by a plate of iron and corundum powder; and finally, a hole is made through the middle of it by a heated rod of copper or iron.
"These wheels are made with a grain more or less fine, as the coarser perform the first rough work, and the finer cut the stones. They are mounted on a horizontal axis, and the workman, sitting on the ground, makes them revolve with a spring-bow, which he moves with his right hand, at the same time holding the stone with his left against the wheel, the latter being, from time to time, carefully moistened and sprinkled with corundum powder. The polish is given by wheels of lead and very fine corundum powder." 3. - Corundum Rubbers. - "The proportions generally used in making the corundum rubbers are, for the coarse, lac, 8, corundum, 1; for the medium, lac, 12 to 16, and corundum 1, by weight. The fine rubber is made by mixing the grindings of agates, carnelians, and the like, with lac; and as the lapidary's wheels, upon which they are ground are made of corundum and lac also, the grindings must contain a portion of those materials; their proportion, in composition, must vary according to the nature of the stone from which they are ground; but 6 of lac to 1 of grindings, may be considered a good proportion generally.
"The lac is first melted, and the corundum, after it has been reduced to a powder, mixed intimately with it. The composition is then moulded in the shape of a brick about 6 x 4 x1 1/2 inch, with a handle of wood about 6 inches at one end, having a rise of about 30 degrees for the convenience of working it."
Some dentists employ old files thinly coated with a cement of emery and shell lac, in finishing the enamel or mineral teeth. The incorporation of the materials is greatly assisted when the emery or corundum is heated to the melting point of the gum resin.