PRATT. i USES. 77 
others is preferable, and for very large wheels the chemical is especially 
adapted. 
VITRIFIED WHEEL. 
In the manufacture of this wheel more care is necessary in the 
selection of the corundum, for in the vitrifaction of the bond the 
foreign minerals containing" water are very likely to cause the wheel 
to burst.. The corundum grains are mixed thoroughly in a paste of 
prepared elay and other fluxes, and the mixture 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 arc sub- 
jected to a dressing or trimming process and shaped to approximate 
dimensions on a potter's wheel or shaving machine, and are then 
further dried. 
The excess of mechanical water having evaporated, they are then 
ready for the kilns. The kilns are cone shaped and the inside meas- 
urements vary from 12 to 20 feel in height and 1<> to is feel in diam- 
eter. When 1 he kiln is filled the ent ranee is closed ami sealed and the 
fires are stalled. The temperature is allowed i<> rise slowly, until all 
the water of mechanical admixture and of crystallization in the for- 
eign materials Is driven off, when the temperature is raised to about 
3,000 , or to a white heat, this heating process requiring several days. 
Where the foreign minerals mixed with the corundum contain water 
of composition that is driven off only at a very high temperature, the 
wheels are apt to be broken by this water coming off when the tem- 
perature of the kiln is raised to the fusing point of the clay. The 
clay and other fluxes fuse and form a porcelain setting for each grain 
or fragment of corundum, which makes a very si pong 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 tin 1 exact dimensions 
desired, the hole is bushed to the exact size, and the wheel is then 
trued and balanced ready for shipping. 
The heat necessary for the fusion in making the vitrified wheel 
apparently has no effect upon the corundum beyond a partial decolor- 
ization and the expulsion of the slight percentage of water in the 
corundum. 
CHEMICAL WHEEL. 
In this process, which is also called the silicate process, silicate 
of soda is used as the binding material. The silicate is thoroughly 
mixed with the emery or corundum and some drying material and 
tamped into molds. It is then subjected to an " oven " heat for twenty- 
four hours, after which it is removed and finished according to the 
method described above for the vitrified wheels after they are removed 
from the kilns. Wheels have been made by this process that were 
over 2,000 pounds in weight. 
