28 



UNITED STATES MINERAL RESOURCES 



stones for abrasive purposes are lost in prehistory. 

 Early in the first millennium B.C., the hard natural 

 abrasives diamond, corundum, and garnet already 

 were known, at least in India and Greece. By A.D. 

 1500, the artificial bonding of loose natural abra- 

 sive grains was introduced, first as sheets and later 

 as blocks and wheels. The first bonding agent was 

 shellac, the next was rubber, and in the last half 

 of the 19th century, synthetic resins, metals, and 

 ceramics were introduced. When powered grinding 

 equipment became available about 1860, the use of 

 abrasives assumed a large role in metalworking 

 operations that had been accomplished previously 

 by saws, chippers, shapers, lathes, and milling ma- 

 chines, but the major surge in the use of abrasives 

 came about 1900 with the commercial introduction 

 of the first of the artificially produced hard abra- 

 sive materials, aluminum oxide and silicon carbide. 

 The use of steel rod and shot as pulverizers in tum- 

 bling mills and of steel grit as a sandblast medium 

 also was introduced about 1900. Artificial aluminum 

 oxide and silicon carbide, mostly used for dressing 

 metals and slabbing stone, have dominated the 

 market for hard abrasives for many years and cur- 

 rently account for about 90 percent of all the hard 

 abrasives used in this country. 



The continued use of certain natural abrasives 

 testifies to their desirable qualities as yet poorly 

 matched by the artificial abrasives and to their 

 lesser cost per unit of work. It is not to be expected, 

 however, that those natural abrasives which still 

 dominate a few specialized applications will con- 

 tinue to retain their fairly exclusive use. As Goes 

 (1971, p. 33) insisted. 



corundum and emery * * * [and] several other natural min- 

 erals are used as abrasives although their total value is only 

 a small fraction of the value of the synthetic abrasives used. 

 Furthermore, their usage is highly specific. In time, exist- 

 ing synthetic abrasives will probably replace all of these, but 

 the economic incentive is now lacking to work out the tech- 

 niques for using them * * *. To replace these natural agents 

 by new synthetic materials is probably quite possible, but 

 the size of the market has, so far, seemed inadequate to 

 justify the necessary research. Development of such syn- 

 thetics probably must wait for the exhaustion of the supply 

 of natural material or a substantial increase in its cost. At 

 the moment this situation does not seem imminent * * *. 



In general, the need for hard abrasives in indus- 

 try in the United States is overwhelmingly satis- 

 fied by synthetic, not natural, materials, and the 

 trend toward lower demand for natural abrasives 

 will continue. Modern technology is increasingly de- 

 voted to automatic machinery operated at high 

 speeds and high pressures to produce finishes of 

 high precision and is, therefore, correspondingly 



dependent upon the uniformity inherent in the arti- 

 ficial abrasives as well as upon their capability of 

 being tailored to specific needs — characteristics not 

 shared by the natural abarsives. Industry is not 

 dependent upon the naturally occurring materials. 

 All natural abrasives currently in use can be re- 

 placed by artificial abrasives after a short change- 

 over period or after minor research, should the 

 need arise. Nearly all the current consumption of 

 natural abrasives is dictated by tradition or by 

 cost — principally by cost — and not by the unique 

 qualities or the availability of the abrasives, so 

 whether or not the United States has vast resources 

 of the natural abrasives is largely of academic in- 

 terest. The artificial abrasives, of course, are de- 

 rived from natural materials, but resources of these 

 materials seem to be abundant. 



THE NATURAL ABRASIVES 



The number of naturally occurring abrasive sub- 

 stances commonly in use is large. These substances 

 represent more than 30 different mineral species and 

 of some species, many varieties. The principal hard 

 natural abrasives are diamond, corundum, emery, 

 and garnet. Staurolite, kyanite, sillimanite, epidote, 

 andalusite, topaz, bertrandite, and other hard min- 

 erals may have, or may have had, a minor and 

 occasional local use. Natural abrasives of moderate 

 hardness include the silica minerals, quartz (includ- 

 ing quartz sand and river silt), chert, flint, jasper, 

 chalcedony, tripoli, diatomite, and rottenstone; the 

 feldspars ; the volcanic glasses, pumice, perlite, and 

 obsidian; and rocks such as basalt, granite, and 

 quartzite, mostly used as grinding pebbles. A few 

 of the more common abrasives of inferior hardness 

 are apatite, dolomite, calcite (including chalk and 

 whiting), clays, and talc. 



Not many of the natural abrasive substances are 

 valuable enough to be transported long distances. 

 Most are abrasives of opportunity, generally termed 

 "regional" abrasives. They are consumed close to' 

 the point of production to the exclusion of other 

 abrasives far more efficient but far more costly. 

 Examples of some of these regional abrasives are 

 quartz sand — several million tons of which is con- 

 sumed annually, river silt, ground limestone, and 

 ground feldspar. The resources of these materials 

 relative to their local consumption are virtually 

 unlimited. Only a few of the naturally occurring 

 abrasives deserve individual description. These are 

 the abrasives that are reputed to have certain desira- 

 ble qualities and applications to specialized purposes, 

 that are costly or in short supply, or that are not 

 readily replaced by artificial abrasives. 



