166 
CORUNDUM, ITS OCCURRENCE AND DISTRIBUTION. 
1 
The corundum grains are mixed thoroughly in a paste of prepared 
chiy and other binding materials, which is then poured into paper 
molds and set aside in a drying room until hard enough to be readily 
handled. When the molds are sufficiently dry they are subjected to 
a dressing or trimming process and shaped to approximate dimen- 
sions on a potter's wheel or a shaving machine, and are then further 
dried. The excess of mechanically included water having evapo- 
rated, they are then ready for the kilns. The kilns nrc^ cone shaped, 
and the inside measurements vary from 12 to 20 feet in height and 
from 10 to 18 feet in diameter. Allien the kiln is filled Avith the 
emery wheels the entrance is closed and sealed and the fires are 
started. The temperature is allowed to rise slowly until all the water 
Pig. 26. — Dental points, wheels, etc., made of corundum. 
of mechanical admixture and of crystallization in the foreign mate- 
rials is driven oft*, when the tempeiature is raised to about 3,000°, or 
to a white heat, this heating process requiring several days. Where 
the foreign materials mixed with the corundum contain water of 
composition that is driven off only at a very high temperature th(^ 
wheels are apt to be broken by this water coming off Avhen the tem- 
perature of the kiln is raised to the fusing point of the clay. The 
clay and other binding materials fuse and form a porcelain setting for 
each fragment of corundum, which makes a very strong bond. The 
kilns are allowed to cool very slowly, several days being required for 
this. The kiln is then opened and the wheels are brought to a lathe, 
called the " truing machine," where they are turned to the exact 
dimensions desired, the hole is bushed to the exact size, and the wheel 
is then trued and balanced ready for shipping. 
