Abrasion is produced almost entirely by the simple physical process of a hard substance shearing or fracturing small chips off another surface.
Abrasive materials are used in three main ways. One is to use the abrasive directly on a substance: sharpening a knife on a grinding wheel, for example. Another is to coat another substance, such as a piece of paper, cloth, or rubber, on a metal disk with granules of abrasive material and use it as a tool; sandpaper is the most common application of this technique. The third method is sandblasting or grit blasting, where a powerful stream of air containing abrasive particles is directed at an object to abrade its surface; buildings are often cleaned in this way.
Apart from their use in sharpening-stones and grinding wheels, direct-action abrasives are also used in powder form. Most domestic cleaning agents (except soap and washing powder) contain abrasives, which are generally silica, pumice, or aluminum oxide ground to a very fine powder. The chemical action of the cleaning agent is helped by the abrasive, and the two substances clean faster than either would alone.
Toothpaste also contains a mild abrasive, which is generally finely powdered chalk. Old-fashioned tooth powders often contained powdered pumice or silica, which wore the enamel off the teeth in a short time, but manufacturers now claim that the cleaning action of their product is mostly chemical.
Most abrasives used in industry are applied indirectly by being stuck to a backing. This method is cheaper because less abrasive is used. The simplest type of coated abrasive material is sandpaper, which is made by simply gluing granules of abrasive material to a sheet of paper. Abrasive papers come in a great many types and have many uses. One unusual application is in the printing industry, where a sheet of paper to be printed on both sides is laid on a sheet of abrasive paper called tympan (abrasive side up) as the second side is printed. In this way, the printed side can rest on the abrasive points, which hold it steady and prevent the wet ink from smearing.
Abrasive-coated belts are used in many industrial sanding machines. They 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.
Mohs’ scale of hardness
The most commonly used method of measuring the relative hardness of an abrasive was devised in 1812 by a German mineralogist, Friedrich Mohs. Abrasives are ranked in hardness from 1 to 10 on the Mohs’ scale. Hardness is determined by attempting to scratch the abrasive material with one of 10 minerals to which Mohs gave hardness values of 1 to 10. If, for example, an abrasive is scratched by apatite (hardness 5) but not by fluorite (hardness 4), then the abrasive must have a Mohs’ value between 4 and 5. The softest mineral on this scale is talc, with Mohs’ hardness 1; the hardest is diamond with Mohs’ hardness 10.
