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. 



