Abrasive granular materials used alone or compounded, bonded to paper, fabric or tools to wear away a surface by friction.
The earliest industrial grindstones were made of sandstone and were chiefly used for sharpening tools. Sandstone is a natural grinding material because it is composed of particles of quartz held together with a bond of silica. Grinding wheels were difficult to shape because of the irregular size of the quartz grain and the hardness of the stone. Because there was no way to control either grain size or hardness, they could not be
considered production tools.
Emery Wheels
Emery wheels, which replaced grindstones, were made of various components, one of which is corundum, a natural aluminum oxide (chemical symbol: A1203). Another abrasive ingredient of emery is iron oxide. There are other impurities which do not add to
the value of emery as an abrasive. Some deposits of emery contain as little as 37% corundum, while others contain as much as 70%. Emery wheels, therefore, while far better than grindstones in their speed and ability to cut metals, were still irregular in their composition and hardness, due to the varying amounts of impurities.
Corundum is almost as hard as the diamond. Grinding wheels made of imperfect diamonds were used on special grinding jobs, but were too expensive for general use. Edward G. Acheson, one of the early experimenters, tried to produce artificial diamonds for grinding wheels by combining clay and carbon at high temperatures. Clay and powdered coke were mixed and heated in a crude electric furnace. Examination of
the result revealed a few bright crystals of material, which were hard enough to scratch glass. It was not known what the material was, but it was thought to be
a mixture of carbon and corundum. Acheson, therefore, coined the word “Carborundum,” and it became the trade name for the new substance. Chemical analysis later showed that it was indeed a new substance, one that does not occur in nature. The substance was
Commercial procedures developed for manufacturing silicon carbide and aluminum oxide made it possible to produce, with suitable binders, grinding wheels
with uniform particle size and hardness.