The Technique of FURNITURE MAKING

Abrasives

Old-time cabinetmakers used sand-, brick - and stone-dust, shagreen (shark's skin) and other primitive materials for the levelling of wood surfaces, and it always appears incredible how they managed to achieve such wonderful finishes to their furniture. The manufacturing of modern abrasives is now a highly specialized field, and an enormous range of products for the surface finishing of every kind of material is available.

In all wood finishing the function of the abra­sive is to wear away the wood fibres by the cutting or rasping action of innumerable sharp crystalline teeth, firmly anchored to a supporting base. The rate or speed of the cut depends not so much on the amount of pressure applied, for excessive pressure tends to defeat its own object, but on the size of the teeth, their relative sharpness, and their ability to keep their cutting edges without disintegrating. A poor abrasive is, therefore, one whose grits or teeth are relatively soft, or whose crystalline structure does not provide the requisite sharp facets, or which rapidly disintegrates either under pressure or by the heat generated by that pressure, or whose cloth or paper base fails to hold the grits securely so that they loosen easily and grind upon each other. The swirl-marks occasionally met with on surfaces finished with a machine-operated oscillating sanding pad are usually caused by these loose grits travelling round in erratic orbits. It follows, therefore, that in an age in which labour costs account for much of the expense of furniture production, it is always false economy to use the cheapest types of abrasive, for the rate of cut is slow and the sharpness of the individual grits soon wears off, crushing rather than cutting the wood- fibres.