ABRASIVES. Abrasion is a scratching action and depends on the relative hardness of the two materials—the abrasive and the one abraded. The degree of hardness of a material, as deter mined by scratching, has long been used by mineralogists for distinguishing one mineral from another, and F. Mohs in 1820 devised a scale of hardness for minerals, which is still in use. Mohs's scale is : I. Talc. 5. Apatite. 8. Topaz.
2. Gypsum. 6. Felspar. 9. Corundum.
3. Calcite. 7. Quartz. lo. Diamond.
Diamond at No. I o is the hardest of all minerals and indeed of all known substances. Steel (a knife-blade or file) will scratch felspar but not quartz, and it is itself scratched by quartz ; it can therefore be set at 61 on the scale. Window-glass at 51 can be abraded and polished by felspar. The higher a material stands in the scale the more efficient is it as an abrasive, diamond being the most efficient of all.
Mohs's scale is arbitrary and merely comparative, and it gives no absolute measure of the degree of hardness of a material. At tempts have been made to arrive at this by various methods : such as by measuring under a microscope the depth of scratch produced by a diamond point under a certain load ; or by determining the amount of work done in grinding away a certain weight or volume of material. From such experiments it is found that the gap be tween Io and 9 on Mohs's scale is even greater than that between 9 and 1, emphasizing again the efficiency of diamond as an abra sive.
Brittleness or friability is another physical character to be con sidered in addition to hardness. Carborundum although exceeding corundum in hardness is much more brittle, and it soon crushes and rubs down to a fine flour. Diamond with rough usage breaks along the directions of perfect cleavage in the crystal. For this reason carbonado, a compact aggregate of minute diamond crys tals, rather than a single crystal, is used in rock-drills.
The form in which abrasives are used varies for different pur poses. The material may be crushed and sieved to different de grees of fineness. This may be used as a loose powder for grind ing ; or it may be glued on cloth or paper, or bonded with cements in the form of grinding wheels or hones. Other abrasives are used in block form cut to a suitable shape, as grindstones, millstones, scythe-stones, whetstones, pumice blocks, etc. Besides being used for grinding and polishing metal, stone, wood, leather, etc., abra sives are also employed for cutting, drilling, and boring.
In addition to natural minerals, certain artificial products are also used for abrasive purposes. For example, powdered glass as glass-paper, crushed steel for stone cutting, and the important carborundum. Corundum is also produced artificially on a large scale. Carbides of boron, tantalum, and tungsten are extremely hard, and these may in the future find technical applications.
In the following list the various abrasive agents are arranged according to their degree of hardness, i.e., in the order of their efficiency. Further information respecting each of these materials will be found in the separate articles (see DIAMOND, etc.).
Diamond (Hardness = 10). This is a crystallized form of car bon, and only the natural mineral is available. Cloudy, spotted, and imperfectly crystallized stones, known as bort, and small frag ments that cannot be cut as gems are crushed to powder for use as an abrasive. Diamond powder is the only material with which dia mond itself can be ground and polished.
Carborundum (H.=91). This artificial product is a crystal lized carbide of silicon (CSi), and since its discovery in 1891 it has more and more replaced mineral abrasives. It is produced in large quantities by heating coke and sand in electric furnaces at Niagara Falls. It is used in the form of powder either loose or on cloth, or is made up with porcelain, shellac, or other bonding ma terial in the form of grinding wheels, hones, etc. This material is sold under a variety of trade-names—crystolon, carbolon, samite, etc.
Corundum (H. = 9) is the crystallized oxide of aluminium a mineral which in its gem varieties is ruby and sapphire. Rough corundum is found as large crystals in certain kinds of igneous rock in Canada, North Carolina, Ural Mountains, Mada gascar, South Africa, the last being now the principal producer of material for abrasive purposes. Emery is an impure granular va riety of corundum mixed with magnetite and other minerals, and it is not quite so hard as the purer crystal or rock corundum. It is mined in Naxos, Asia Minor, and the State of New York. Much corundum is now produced artificially by fusing bauxite in elec tric furnaces at Niagara Falls, the product being sold under the names alundum, adamite, aloxite, lionite, etc.
Garnet (H.= 61-71) as mined in the eastern United States and in Spain for abrasives is mainly the almandine variety, a sili cate of iron and aluminium. It is largely used for making garnet paper (often sold as "emery-paper").
Quartz (H.=-7) is a crystallized form of silica (oxide of silicon, and is the commonest of all minerals, occurring in a be wildering variety of forms. It finds extensive applications as an abrasive. Millstones, grindstones, and pulpstones (for grinding wood in the manufacture of paper) are made of quartz-rock, quartzite, burrstone, grit, or sandstone. Whetstones are made of hornstone, lydian-stone, Arkansas-stone, and other compact va rieties of quartz. In the form of sand, quartz is used as a sand blast, as sand-paper, in scouring-soap, and for cutting and grinding marble. Pebbles and flints are used for grinding cement materials and clinker.
Steel (H. = 61). Crushed crucible steel, crushed cast iron, and steel shot are used in stone cutting.
Felspar (H. = 6), the potash-felspar, orthoclase or microcline is used for polishing plate-glass and in scouring-soaps.
Glass (H. = 51), powdered glass as glass-paper or "sand-paper." Pumice (H. = 51), a natural volcanic glass blown by included steam into a cellular mass. The best block pumice is from the Lipari Islands, and enormous quantities of pumice-dust are avail able in the United States.
Opal (H.= 5), a hydrated glassy form of silica, found in a pow dery form as large deposits of tripoli, diatomaceous or infusorial earth, and largely used for polishing and in scouring-soap.
Rotten-stone, chalk, rouge, putty-powder, and other softer grinding and polishing materials might also be mentioned. Abrasives, in the household, are substances of an erosive nature used for polishing or cleaning purposes. Those best suited for this use are fine sand, emery, crushed pumice and chert, which consists of siliceous skeletons of sponges and marine shells. These materials are often mixed with a solution of soap, so that there is a cleansing action as well. In the cleaning of aluminium, steel wool, in combination with an abrasive powder or soap, has a wide usage. Abrasives are in very general use to-day and have done much to lighten the housewife's work. They may be purchased in paste, powder or cake form and used for cleaning and polishing all kinds of metals, as well as scouring paint and enamel ware.
(L. J. S.)