164 CORUNDUM^ ITS OCCURRENCE AND DISTRIBFTION. 
very essential to determine the abrasive qualities of the corundum. 
Neither a chemical analysis nor a superficial examination of a corun- 
dum ore Avill determine its cutting qualities, and this can be obtained 
only by making the corundum into a wheel and testing it. 
In estimating the value of an ore it is also necessary to determine 
to what degree of purity it can be cleaned or what percentage of the 
commercial product will be corundum, and also what will be tlie 
nature of the foreign minerals. A foreign mineral will always be 
softer than the corundum and will to a certain extent reduce its 
abrasive power. Beyond this the presence of a small amount of 
foreign mineral does not materially affect the value of the corundum 
for making a cement or chemical wheel, but is often the reason for 
discarding it in manufacturing a vitrified wheel on account of the 
low fusibility of the foreign substance. Garnet is perhaps the most 
objectionable mineral in a corundum ore, it being very difficult to 
separate it fi'om the corundum, because the specific gravity of the 
two is nearly the same. Corundum containing even a little of the 
garnet can not be used in the manufacture of the vitrified wheel. 
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 at 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 the grains become dull or rounded they will fall away or will 
be readily removed by a truing tool, and fresh, sharp grains will be 
exposed. The grade of a wheel depends upon the character of the 
work for which it is to be used, and the bonding material 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 aAvay until i 
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 factory all wheels should be thoroughly tested to > 
a higher strain than that to which they are to be subjected in actual 
use; as the wheels have to be run at a very 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 
