ETCHING 439 



Grinding Hard Friable Material Like Glass or Porcelain. 

 Employ lap heads of block tin fed with emery powder and water 

 or turpentine. Emery does not cut as fast as carborundum, 

 crys talon or similar abrasives, but also does not so deeply score 

 the specimen and therefore the time lost in grinding is usually 

 gained in polishing. 



For grinding, the lap head should rotate quite slowly, two to 

 five hundred revolutions per minute being suitable for ordinary 

 work. In polishing a somewhat higher speed may be employed 

 with advantage. Polish with fine rouge and complete the 

 finish with " putty powder." 



Grinding Soft very Friable Materials. Materials of this sort 

 have a tendency to chip or pit. This difficulty appears to be 

 largely eliminated by grinding wet and in a single direction. 

 The lap or wheel is kept continually wet with a stream of water 

 (or other liquid) and is never allowed to reach the condition 

 which may be designated as moist. Grinding in a single direc- 

 tion instead of turning at right angles as is the standard practice 

 with alloys will usually yield a surface free from the chipping 

 out of tiny particles. The final polish should be in the same 

 direction as the grinding. 1 



Etching. - - This step has for its object the development of 

 the crystalline structure of the specimen. It is based upon the 

 principle of submitting the polished specimen to the action of a 

 corrosive liquid of such a nature as to dissolve some components 

 more rapidly than others. 



The surface to be treated being a mirror surface, free from all 

 striations, it follows that the slighest attack by an etching liquid 

 will be easily seen by means of the microscope. 



Suppose, for example, we have an alloy consisting of a single 

 crystalline phase and an eu tec tic. Two systems of attack 

 would reveal the nature of its structure; a reagent could be 

 employed which would dissolve the eutectic leaving the crys- 

 talline phase unattacked, or another reagent could be selected 



1 For suggestions for the preparation of specimens of Coal and allied materials, 

 the reader is referred to: Thiessen: Structure in Paleozoic Bituminous Coals, 

 Bui. 117, U. S. Bureau of Mines, 1920, p. 10. 



