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, 
crystalon 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.^ 
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 eutectic. 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 
' 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. 
