76 CORUNDUM TN THE UNITED STATES. [bull. 180. 
great deal of the smaller fragments could and should be utilized in the 
manufacture of watch jewels. 
Both corundum and emery are used in the manufacture of abrasive 
materials, and these are on the market in three forms, as wheels and 
blocks of various shapes and sizes, as emery paper, and as grains or 
powder. The last two need no further explanation, but a description 
of the first is of importance. 
The shapes of the corundum and emery wheels and bricks or stones 
are extremely varied, being adapted to all kinds of grinding. The 
principle of these wheels is the same as that of the rotary files, and 
as the points of a file become dull from using, so also do the grains or 
points of the emery and corundum in the wheel. In making a wheel 
it is necessary, therefore, to make it of such a temper or grade that 
when these grains become dull or rounded they will fall away or will 
be readity removed by a truing tool, leaving fresh, sharp grains 
exposed. The grade of a wheel depends upon the character of the 
work for which it is to be used, and the bond should be such that it 
will wear away a little faster than the corundum or emery, and thus 
always leave the sharp edges ready for cutting. The greatest economy 
is effected when the bond does not wear away until the grains of emery 
or corundum have become rounded or dulled, thus permitting the 
wheel to do its greatest amount of work. 
Before leaving the factoiy all wheels should be thoroughly tested to 
a higher strain than that to which the}^ are to be subjected in actual 
use; as the wheels have to be run at a A^ery high velocity in order to 
secure the greatest efficiency, there is at times but little reserve 
strength, and a sudden blow will often cause them to fly to pieces. 
To give an idea of the number of different wheels that the larger 
emery-wheel companies are prepared to make, I can not do better 
than to mention what I saw and what was told me at the Norton 
Emery Wheel Company, of Worcester, Mass. The wheels are manu- 
factured for the special work for which they are intended, and vary 
in shape, in bond, and in grain of corundum. The sizes of corundum 
that are used are Nos. 12, 14, 10, 20, 24, 30, 36, 40, 50, 54, 00, 70, 80, 
90, 100, 120, 150, 160, 180, 200, and 6 grades of flour corundum. The 
bond has 26 degrees of hardness, represented by the letters of the 
alphabet, although a bond is seldom used softer than E or harder 
than M. There are 408 different sizes of circular wheels, so that the 
different grades of wheels possible are almost unlimited. 
There are three types of Avheels known to the trade, the vitrified, 
chemical, and cement, the names being derived from the process by 
which they are manufactured. In the manufacture of all, the corun- 
dum or emery used is in grains of uniform size, but varies with the 
grade of wheel that is to be made. The vitrified wheel is the most 
important and most generally used, although for some work one of the 
