First, on the substances that are to be ground and polished.

The objects or substances the grinding and polishing of which are described, will be found to include nearly all those materials from the vegetable, animal, and mineral kingdoms, which are commonly used in the mechanical arts; those especially of which mention has been made in the first volume of this work in reference to their preparation and figuration by means of cutting tools, so far as regards the substances which admit of being subsequently polished. To these will be added the cutting, grinding and polishing of various hard and crystalline bodies on which cutting tools are ineffective.

Part of these materials such as the woods, ivory, and some of the metals and alloys, marble, glass, etc, receive dissimilar treatment from different classes of artizans, the principal variations of practice will be respectively noticed and contrasted under their respective heads: but it will be readily imagined that many unimportant variations are made, that are based rather on prejudice than necessity, and the insertion of which would tend perhaps to confuse rather than assist, and therefore the ordinary routines will be alone adverted to.

Secondly, on the materials or abrasive powders, by the successive employment of which different substances are polished.

The grinding and polishing materials used in the arts will be found principally to consist of carbon, alumina, and silex, in various degrees of crystallization and admixture, and usually combined with the oxide of iron and some other substances as may be seen by their comparative analyses given in the annexed table.

Table Of The Analyses Of Polishing Materials

Carbon in its purest and most crystalline form constitutes the diamond, the hardest substance in nature, and which in the pulverized state is variously employed, as, for example, in the polishing of diamonds for jewellery; in the configuration of the rubies and sapphires used for the pivot holes of watches and chronometers; diamond powder is also used by the lapidary in slitting all stones, of course including even those which admit of being polished by abrasive powders of inferior hardness to the diamond. Carbon, in another of its conditions, also constitutes charcoal, and which, probably from the minute particles of silex disseminated throughout its substance, is employed in polishing copper and others of the softer metals.

Alumina when highly crystallized is the basis of the ruby, sapphire, and other gems which are next in hardness to the diamond. This earth, with the addition of a little silex and iron, constitutes the principal part of corundum, emery, and rotten-stone; abrasive materials that are largely employed in grinding and polishing the harder metals and mineral substances. Alumina, when decomposed, is the basis of most of the clays and loams, some of which, under different names, are likewise used in abrasive processes. It is fortunate for the mechanical arts, that emery, which is nearly the hardest and most useful of all abrasive substances, is also found in sufficient abundance to serve for every required application.

Silex, in its crystalline form, and variously coloured, assumes the names of quartz, amethyst, rock crystal, flint, agate, and when in a disentegrated state, that of sand. Silex, with the addition of a little alumina and foreign matters, constitutes also the major part of the abrasive materials known as grit or grindstones, rubstones, hones and slaty stones, pumice stone, tripoli, and some others, all of which are softer than those mineral substances which are composed principally of alumina.

Of the siliceous abrasives, the gritty and slaty stones are very largely employed in the formation and sharpening of tools; pumice-stone, tripoli, and others for polishing metallic and other substances, softer than those which, from their superior hardness, require the employment of emery and rottenstone, abrasives that have alumina for their common base.

To those abrasive materials of which carbon, alumina, and silex form the base, may be added the oxide of iron used under the names of crocus and rouge, and the oxides of tin and lead, or putty powders, these are artificially prepared; and a few mineral substances of no great importance as abrasives are used without any preparation, such as lime and chalk.

Thirdly, on the tools or apparatus, by the agency of which abrasive substances are applied to the objects to be ground or polished.

Some of the abrasive substances are employed in the solid forms in which they are first obtained, as the grindstone, oilstone, hones, charcoal, Dutch rush, and fish skin; a few are pulverized and mixed with various cements, thus the effective grindstone and razor hone of the Hindoo are corundum mixed with melted gum lac, and moulded into form; wax and crocus similarly mixed are used in optical works amongst ourselves; and of late years emery has been reunited into factitious stones. But metal, wood, paper, leather, cloth, or bristles, are the more common implements or vehicles, by aid of which the several powders are applied in a variety of ways, after the powders have been carefully separated into grains of similar magnitude, the sizes of which must be proportioned to the perfection of the surface to be produced, and with a gradual transition from coarse to fine. This succession is adopted upon the same principle as that in filing a coarse file is first used, because it may be made to act rapidly; but as the form of the work becomes gradually developed, a second cut file, a smooth, and lastly, a superfine file is used, and which progressive mode of action is in no case more distinctly seen in works of polishing, than in the manufacture of a highly finished razor, which is described under the head Cutlery in the following Catalogue.

The grinding powder is of course always harder than the substance to be ground, whereas the implement or grinding tool is softer, and generally agrees in form with the analogous cutting or moulding tools used for producing work of corresponding shapes in other substances, as practised in different branches of the mechanical arts. Thus turned works are often polished with blunt factitious turning tools of wood supplied with the powders; flat works require artificial saws, files, and planes; a convex surface requires a concave grinding tool, and so on.