METHODS OF CLEANING. 161 
a stream of water, so regulated that the corundum readily settles to 
the bottom of the trough and the lighter minerals are carried off. 
This washing process Avill remove only the impurities that are entirely 
separated from the particles of corundum, but there are usually some 
inq^urities attached to the particles or grains that have to be removed 
by another process. The product is therefore 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 impurities, and these are separated by again subjecting them 
to the washing process. The final impurities* are separated from the 
])articles of corundum by means of a machine called the " muller " or 
'' chaser." (See PI. XVIII, B.) 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 by men with hoes or by plow- 
shaped iron blades in front of the w^ooden rollers, and is then pressed 
dow^n by the rollers being passed over it. In this way the scouring 
motion is kept up between the grains. The impurities are thus gradu- 
ally worn away and are carried off by a stream of water that flows 
continually through the tub. 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, after it has been 
removed from the mullers and allowed to lie over night 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 
inclined surface that is just above the flames of the furnace and slides 
dow^n this surface into an iron box. By the other method the wet 
product is thrown in at the upper end of an iron cylinder, open at 
both ends, wdiich revolves about a coil of steam pipes. One end of 
the cylinder is lower than the other, and the wet mass is alternately 
carried up by the revolving cylinder and dropped on the hot coil of 
pipes, and thus gradually w^orked toward the lower end, where it is 
caught in a hopper and conveyed by elevator belts to the sizing room. 
Here it is automatically screened to the various sizes. 
In PL XVIII, A and i?, are views of the exterior and the interior 
of the corundum mill at Cullasagee. In the foreground of B are 
the boxes in w^hich the ore is first washed, and just beyond these are 
the mullers. 
Only within the last few years has any attempt been made to 
improve the methods of concentrating and cleaning corundum. 
Bull. 269— 06 M 11 
