78 CORUNDUM IN THE UNITED STATES. [bull. 180. 
CEMENT WHEEL. 
Iii the cement wheel, shellac, rubber, linseed oil, and other sub- 
stances are used as the binding material. This makes a soft wheel 
that is well adapted for roll and surface grinding when made with 
shellac, and for saw gummers and thin wheels when made with oils. 
Although an examination of a corundum property may show the 
existence of considerable quantity of the mineral, no mining should 
be undertaken until satisfactory tests have been made upon the 
corundum to prove that it has those properties that will make it of 
value as an abrasive. 
The value of a corundum deposit as an ore for abrasive purposes 
depends upon that property of the mineral which enables it to retain 
a sharp edge, known as a cutting edge, when it is crushed to grains. 
All corimdums do not have this property, and while many exhibit 
this in the first stage of the crushing, the finer fragments and grains 
do not. This is more apparent when the corundum has been made 
into a wheel, for when first used the wheel may do good work, espe- 
cially if it is a coarse-grained wheel, but as it wears away, the grains 
of corundum become rounded, instead of breaking to a cutting edge. 
In estimating the value of a corundum deposit, it is therefore very 
essential to determine the abrasive qualities of the corundum. 
Neither a chemical analysis nor a superficial examination of a corun- 
dum ore will 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 the 
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 iiresence 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 from 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. 
OTHER USES. 
An attempt has been made to use corundum as a source of alumi- 
num, but on account of its refractoriness and the percentage of ferric 
oxide and silica that it often contains, and on account of the cost of the 
ore, this use has not been found feasible. 
