Abstract: The invention relates to polycrystalline, sintered abrasive granules based on .alpha.-Al.sub.2 O.sub.3 with a density of >98% of the theoretical density of corundum, a Vickers hardness of 21 GPa and greater and a crystallite size of 0.05 to 1.3 micron, as well as abrasives made from these abrasive granules and the use of these abrasive granules for the production of ceramic, refractory products.The polycrystalline, sintered abrasive granules based on .alpha.-Al.sub.2 O.sub.3 are produced by .alpha.-Al.sub.2 O.sub.3 crystal nuclei being added to a boehmite dispersion, the mixture of the boehmite dispersion and the crystal nuclei being acidified and gelled and the gel being dried, crushed, and sintered. In doing so, 0.5 to 35% by weight of .alpha.-Al.sub.2 O.sub.3 crystal nuclei, relative to the sintered abrasive granules, is added and the mixture of the boehmite dispersion and of the crystal nuclei is refluxed, while the pH of the mixture is kept in a range of 2.5 to 3.5 by adding acid.

Abstract: A superrefractory dry ramming material based on zirconium oxide for the lining of induction-type crucible furnaces comprising a zirconium oxide low in SiO.sub.2 and conventionally stabilized with CaO, MgO, Y.sub.2 O.sub.3, rare earths or their mixed oxides which has a particle size ranging from 0 to 8 mm, and from 5 to 40 weight percent finely divided monoclinic, fused zirconium oxide containing less than 0.5 weight percent SiO.sub.2.

Abstract: An apparatus adapted to be mounted on a surface grinding machine is provided for testing abrasive grains in single-grit scratch tests. To permit specimen preparation and scratching to be carried out on a single machine without changeover and without reclamping the specimen, and to maintain the infeed in the scratching operation constant by giving the specimen a contour corresponding to the cross-feed error, a belt grinder and a scratching device comprising a support disk, mounted by means of a main flange and a counterflange, and a scratching disk mounted on the main flange and carrying a single grain secured to its periphery along with a force-measuring system disposed thereunder, are designed as a structural unit adapted to be mounted on the spindle of the surface grinding machine, with a belt tensioning arm mounted to the headstock.

Abstract: In corundum abrasive grain which is ceramically coated with ground frit, a binder and fine grain highly abrasive material for the purpose of improving adhesion of the abrasive grain in its processing to resin-bonded abrasives and their use in wet grinding. The fine grain material forms from 1 to 5 weight percent of the untreated corundum grain and ranges in size from 10 to 50 microns, which corresponds to from 1 to 10 percent of the average diameter of the initial grain.

Abstract: A composition comprising .alpha.-alumina in a concentration of at least 99.95 weight percent and process for producing such .alpha.-alumina wherein alumina having an alkali metal content between 0.05 and 0.60 weight percent and Al.sub.2 O.sub.3 content of more than 95 weight percent is electrically fused with silica and additional alkali, if desired, in an electric arc furnace, the heated composition is cooled, comminuted and the so-formed glass phase is removed from an alumina containing phase by mechanical or chemical means.

Abstract: A process for the manufacture of granular abrasives having improved grain toughness from refractory, inorganic hard substances which comprises crushing a lump or block of said refractory inorganic substance, thereafter subjecting the resultant grains to a grind sorting process such that substantially only the porous grains, grains containing flaws or coarsely crystalline grains are comminuted and separating them in the ground condition from the balance of the material.

Abstract: An electrical heating element composition comprising crystals of fused magnesium oxide improved by the inclusion of 0.05 to 5% by weight, based on the weight of said fused magnesium oxide, of an additive composition which has excellent lubricating properties in respect of the magnesium oxide crystals and reacts at 800.degree. to 1050.degree. C with active conductivity centers at the surface of one or more adjacent magnesium oxide crystals; an improvement in the preparation of an electrical heating element wherein a granular fused magnesium oxide composition is subjected to mechanical shaping and calcination which comprises employing as the electrical heating element composition a composition comprising crystals of fused magnesium oxide and 0.05 to 5% by weight, based on the weight of said fused magnesium oxide, of an additive composition which has excellent lubricating properties in respect of the magnesium oxide crystals and reacts at 800.degree. to 1050.degree.