|Monthly Tech-Tip |
The only difference between these two cone 6 glazes is the silica. Both are the G2926B recipe, both were thickly applied and fired in the same kiln. The left one employs the 90 micron (200 mesh) grade silica and the right one uses 45 micron (325) mesh. These test tiles are about 6 months old. There was no crazing out of the kiln. The porcelain recipe is 25% silica, 25% nepheline and the remainder kaolin and bentonite. It appears the finer particle size silica is dissolving in the melt much better, this narrows the difference between calculated and actual behavior, especially relating to coefficient of thermal expansion.
On the left is the oversize from 100 grams of 45 micron US Silica (325 mesh): 3 grams.
On the right is the oversize from 100 grams of their 95 micron grade (200 mesh): 26 grams!
Clearly, if you want minus 200 mesh material, the #45 325 mesh grade is the one to actually use.
Silica, sold as a white powder, is pure quartz mineral. Quartz is the most abundant mineral, it is pure SiO2 silicon dioxide.