PREPARING HARD SPECIMENS 437 



days to sediment. The water is then drawn off from the deposit. 

 This final deposit is mixed with a little distilled water and trans- 

 ferred to a stock bottle. For use a little of the stock suspension 

 is added to distilled water, introduced into an atomizer and 

 sprayed upon the cloth-covered lap. , 



It is best to polish the specimen in two directions. The cloth 

 of the revolving lap must never be allowed to become dry during 

 polishing, nor on the other hand should it be too wet. 



General Methods for Preparing Hard Specimens. - - Grind to a 

 plane surface upon the proper wheel, using a high speed and 

 holding the specimen so that it just barely touches the rotating 

 surface. If pressed too hard against the wheel there will be 

 deep scoring and too much heating. Observe great care to 

 prevent the specimen from turning in the fingers. A properly 

 rough-ground specimen should show all the striations parallel and 

 of approximately the same depth. Next bevel or round the 

 edges of the specimen around the ground surface, then apply 

 the specimen to a finer-grained wheel or to a lap fed with finer- 

 grained powder, grinding so that the striations are at right angles 

 to the first. Continue grinding until when examined with a 

 low magnification no vestiges of the first striations remain. If 

 now the striations are very shallow, polishing may be begun; 

 if not shallow, grind with a third finer abrasive; again grinding 

 at right angles to the direction last taken and continuing until 

 all trace of the preceding grinding has disappeared. Polishing 

 is carried out in like manner, using finer and finer powders 

 moistened to a pasty consistency with water or oil or other 

 suitable vehicle. When oil, vaseline or a similar vehicle has been 

 employed in the grinding, especially when dealing with materials 

 which have a tendency to adsorb the grease, as for example 

 certain rocks, earthenwares, terra-cottas. porcelains, cements 

 and concretes, etc., it will be found that polishing proceeds with 

 far greater speeds and with much better surfaces when the pol- 

 ishing powders are suspended in a solvent for greases and oils, 

 than when water is employed. The best of these arc alcohols. 

 ethers and light petroleum products or mixtures of them. 



With each change in fineness, polish at right angles to the 



