Abrasive-coated belts are used in many industrial sanding machines. These may be made of extra strong paper, or else a fine, strong cloth such as linen or gaberdine is used.
Grit blasting with a machine is a versatile technique. It has the important feature that the workpiece is abraded more or less evenly all over the surface that faces the blast. It is used for cleaning metal objects thoroughly before they are electroplated – electroplating will not stick to dirt or corrosion. It is also used for incising patterns on plate glass. The area of the glass that is to be left smooth is protected with a tough paper stencil that is only partly eaten away by the blasting. This technique has replaced the older one of etching patterns on glass with hydrofluoric acid.
Abrasive materials may be either natural or synthetic. Traditional abrasives are all natural, and the synthetic ones are a fairly recent innovation.
The oldest abrasive of all is sand, which was used for polishing stone weapons as early as 25,000 BC. Other abrasive materials in use from early times include garnet (a hard, glasslike gemstone), emery, pumice, and silica (silicon dioxide) which occurs in various forms as quartz, flint and agate. In the Middle Ages, grinding wheels of quartz and flint fragments naturally bonded together in rock were used. Gemstones were lapped or polished by the use of emery or sandstone powder rubbed on with metal plates.
Sandpaper was discovered slightly later, and was followed by emery paper and cloth, which are finer grained and longer lasting, and corundum, discovered in 1825.
The most important step in the development of synthetic abrasives was made in 1891, when Edward Acheson first produced crystals of silicon carbide. This material was called Carborundum, and has been one of the most versatile synthetic abrasives. The crystals, which can be made in any required degree of fineness, can be bonded together in a solid block or used for coating metal discs or belts for use in machine tools such as sanders.
Other, more recent developments include aluminum oxide, a synthetic form of corundum, silicon carbide and synthetic diamonds. These have not ousted natural diamonds, however, which are still better for bonding on to the steel discs which are used for cutting stone and concrete. Synthetic diamonds are used mainly for cutting and shaping other very hard substances, such as tungsten carbide. They are produced from carbon at high temperature and pressure, as are natural ones.
Already, improvements in abrasive technology have displaced some intermediate stages in the shaping and finishing of materials. As new wonder materials are developed to meet the needs of modern industry, this trend will continue and new super-abrasives will be required. In meeting this need, modern technology might not be able to improve on all the properties to be found in diamond but it can certainly attain a close second best.