74 CORUNDUM IN THE UNITED STATES. [bull. 180. 
sluice boxes can be built, the more or less finely divided ore may be 
carried to it by means of these. In this way the ore is partially 
cleaned b} r the time it reaches the mill. This is the method employed 
at the Corundum Hill mine. 
Where the corundum occurs in a gneiss, quartz-schist, or syenite, 
there should be no difficulty in mining by means of open cuts. 
METHODS OF CLEANING CORUNDUM. 
The difference between the commercial product and the ore as it 
comes from the mine is that the latter has been freed as far as possi- 
ble of all impurities, so that the resulting product is or should be 
nearly pure corundum or emery. Most of the impurities are easily 
removed by conveying the crushed ore into boxes through which a 
st ream of water runs, which is so regulated that the corundum readily 
settles to the bottom of the trough and the lighter minerals are car- 
ried off. Before this, however, the crushed ore is sieved, and all 
that will not pass through a No. 12 screen is recrushed and passed 
between rollers until it is reduced to the desired size. This wash- 
ing process will remove only the impurities that are entirely separated 
from the particles of corundum, but there are usually some of the 
impurities attached to the particles or grains that have to be removed 
by another process. The product is passed through a machine known 
as the screw or scouring machine, in which there is a coarse worm 
similar to the screw conveyer. This grinds out almost all of the impu- 
rities, and these are separated by again subjecting them to the 
washing process. The final impurities are separated from the parti- 
cles of corundum by means of a machine called the "muller" or 
"chaser " (see PL XIV). The principle of this is to cause each grain of 
corundum to rub against another and thus wear away the adhering 
foreign substances. The machine consists of a shallow tub, in which 
are two heavy wooden rollers which move around its circumference. 
The freshly ground corundum on being thrown into these tubs is kept 
constantly stirred up and then pressed down b}^ the rollers being 
passed over it. In this way the scouring motion is kept up between 
the grains. The impurities are thus gradually warn away and are 
carried off by a stream of water that flows continually through the 
tub. The corundum in the tub is kept stirred up by men with hoes 
or by plow-shaped iron blades in front of the wooden rollers. It 
usually requires from four to eight hours, according to the nature of 
the impurities that are attached to the corundum grains, to obtain 
a clean product. 
There are two methods of drying this product, by either of which it 
is removed from the mullers and allowed to lie overnight on inclined 
floors. By one method this product is conveyed by elevator belts to 
the second floor of the mill and dropped vertically for a distance of 
20 to 30 feet down the stack of a furnace. At the bottom it strikes an 
