abrasives

What is an abrasive?

Abrasives are very hard, brittle substances that have been crushed to create particles with cutting edges, much as breaking a glass produces shards with sharp edges. Hardness and sharp edges give abrasives the ability to grind and polish. As with a knife edge, the hardness helps retain sharpness, but with sufficient use the cutting edges of any abrasive, even diamond, become rounded over. An abrasive must be at least as hard as the substance it is to abrade. Today most abrasives are synthetic.

Some commonly used abrasives are:

flint garnet emery
aluminum oxide silicon carbide  
  diamond  

How are abrasives sized?

The size of the particles is important. Abrasive particles cut away the higher areas of a surface as they travel over it, but in doing so they leave behind scratches whose depth depends on the size of the particles. To remove these scratches, a smaller particle size must be used, and then a still smaller size to remove the scratches produced by the new particles, and so on until the scratches are so small they don't matter. Many processes depend upon using progressively finer grits, from grinding a mirror for the Hubble Space Telescope to finishing furniture. Abrasives are, therefore, almost always sold by the size of the grit.

An error amateurs often make in using products like sandpaper is using too fine a grade too soon, in the mistaken belief that this will produce a smoother end result. Starting with a grit much finer than the original surface imperfections will simply require more time sanding. So can skipping grades, depending on how long it takes to change grades. In changing grades, all traces of the previous grit must be removed. Otherwise, the very effort exerted to grind scratches out will cause rogue particles to dig more large scratches.

The usual way of grading grains of abrasive is to pass them through a series of sieves with ever-smaller holes. The names of the abrasive grades are taken from this means of grading, although grades finer than about 240 are actually sorted by the speed with which they settle in a liquid or a current of air.

sandpaper (coated abrasives)

grinding wheels (bonded abrasives)

oilstones

abrasive pads

flint

Its only virtue is cheapness. It is used when the sandpaper will quickly become clogged, for example, in removing paint.

garnet

A step up from flint, garnet is also a natural mineral. The abrasive is made from the garnet species almandine, which is typically dark red–thus the orange color of the paper.

Many woodworkers prefer garnet paper for sanding raw wood because it seems "sharper" than other papers. Garnet particles are not as strong as those of aluminum oxide or silicon carbide, and so they break more easily, exposing fresh edges more frequently.

aluminum oxide

All-around sandpaper, long-lasting, and can be used on wood or metal.

silicon carbide

Sandpaper made with silicon carbide is black; wet-or-dry sandpaper is an example. Hard enough to sharpen tool steel, but mostly used for finish sanding.

alumina-zirconia

A newly available combination of aluminum oxide and the diamond substitute advertised late at night on shopping channels. Very hard, very tough, expensive; used to reduce thickness rather than to smooth surfaces.

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