PREPARING OPAQUE OBJECTS 385 



The character of the binder and the degree of incipient fusion 

 characterizes a wheel as hard or soft. The degree of hardness 

 or softness is technically spoken of as the grade or hardness of 

 the wheel. American manufacturers usually indicate the grades 

 of their wheels by letters of the alphabet, but the scale of hardness 

 as indicated by the letters is by no means uniform with different 

 manufacturers. Consequently, a letter indicating a grade can- 

 not be interpreted without reference to the scale of hardness of 

 the particular firm from whom the wheel was obtained. For 

 example, we find that a wheel marked U may be "hard" as 

 supplied by one firm, but if we purchase a U grade from another 

 firm we will obtain a "very soft" wheel. In selecting wheels 

 for grinding specimens, it is safe to be guided by the general 

 rule that a soft wheel will cut more rapidly and deeper than 

 a hard one, will clear itself more readily, but is more easily 

 worn away, and therefore more liable to be spoiled. The soft 

 wheels as a rule must be run at higher speeds. Hard wheels 

 on the other hand tend to glaze over, cause more heating of the 

 specimen and often yield aggravated cases of surface films or 

 surface flow of soft components, but they cut slower, hence do 

 not so deeply score or furrow the specimen through injudicious 

 pressure and may be employed to better advantage when only 

 low speeds are available. 



Besides the grade or hardness of grinding wheels as influencing 

 their suitability for certain work, the diameter and the uniform- 

 ity of the individual particles employed in building up a wheel 

 must be taken into account. The size of the component par- 

 ticles is called the grain or grit. Grain is obtained in manu- 

 facturing by screening the abrasive powder. The number of 

 linear meshes to the inch through which the powder will pass 

 is the grain number of the wheel. For example, in a wheel 

 marked 50, the component particles will pass through a sieve 

 having fifty meshes to the inch. 



The grain numbers employed by different manufacturers are 

 not comparable because the size of wire employed in the sieves 

 used for the grading is not always the same. Since it is the 

 number of linear meshes to the inch and not the diameter of 



