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.

The abrading effect is produced almost entirely by the simple physical process of the harder substance shearing or fracturing small chips off the work-piece to smooth it.

Abrasives are used in three main ways. One is to use the abrasive material directly on a substance: sharpening a knife on a grinding wheel is an example of this. Another is to coat another substance, such as a piece of paper, cloth or rubber on a metal disc, with granules of abrasive material, and use this as a tool; sandpaper is the commonest application of this technique. The third method is sandblasting or gritblasting, where a powerful stream of air containing abrasive particles is directed at an object to abrade its surface.

What is the definition of Abaft. - Towards the stern of a ship ; back ; behind ; as, abaft the mainmast.

What is the definition of Abaka. - A fiber from which manilla rope is made. Ropes and cables of this material float in sea water.

What is the definition of Abate. - To bring down or reduce from a higher to a lower state, number or degree ; to lessen ; to diminish ; to con. tract ; to moderate ; to cut short ; as, to abate a demand.