PREPARING OPAQUE OBJECTS 
431 
is said to have been polished in relief. Since in almost all the 
materials commonly studied we deal with components differing 
in hardness, it is exceedingly difficult to obtain polished speci¬ 
mens which do not exhibit some relief polishing. Practice and 
a light touch are the only effective preventives. 
The wearing or cutting off of irregularities so as to obtain a 
flat surface is termed roughing. Roughing is most easily accom¬ 
plished by holding the specimens against rapidly revolving ab¬ 
rasive wheels. 
The most useful American abrasive wheels are emery, co¬ 
rundum, alundum, crystalon and carborundum. Emery and 
corundum are natural products, while alundum, crystalon and 
carborundum are products of the electric furnace; the first three 
mentioned consist of crystallized alumina, the last two consist 
of crystalline carbide of silicon. Of these, emery cuts or wears 
away specimens the least rapidly, crystalon and carborundum 
the most rapidly. 
All three steps, grinding, polishing and etching, require 
patience, practice and a certain inherent technical skill. Prac¬ 
tice, and practice alone, will enable the student to properly 
prepare specimens. The selection of the proper sequence of 
abrasives, the right pressure of the specimen against the grind¬ 
ing material, the rate of speed or motion in grinding and polish¬ 
ing all enter into the preparation of the specimen. No specific 
directions can, therefore, be given, but merely a general outline of 
the steps to be taken and the special precautions to be observed. 
So, too, in the etching much depends upon the individual. The 
proper concentration of reagent (which differs for different alloys 
of the same type), the way in which the specimen is immersed or 
submitted to the action of the reagent, the time of exposure, 
temperature of the room and reagent, thoroughness of removal 
of the etching liquid by washing, etc., each enters largely into 
the preparation of really satisfactory specimens and all con¬ 
tribute to the elucidation of the problem or to the confusion 
of the investigator. 
Grinding wheels are made from powdered abrasive mixed 
with a suitable binder, pressed into moulds and fired in an oven. 
