|Monthly Tech-Tip |
Quartz particles have a high melting point, they must enter the glaze melt by being dissolved by it (usually the last particles to do so). Obviously, the silica should be as fine as possible to increase its surface area to be more readily dissolved. The more that dissolves the closer the physical properties of the fired glaze will be to the theoretical (e.g. degree of melting, thermal expansion, transparency, durability). This brand of silica, #90 classifies as 200 mesh even though 2.8% remains on the 200 mesh screen. Not surprisingly, their #45 grade retains 1.9% on the 325 mesh screen. However, the most significant aspect is how much of the #90 is on the 325 and 270 mesh screens: 26%. The #45 grade only retains 2.6% on them! This is a huge difference and shows the value of using the finer material. It would take a typical ball mill hours to make this difference.
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.
The melting temperature of ceramic glazes is a product of many complex factors. The manner of melting can be a slow softening or a sudden liquifying.
Material substitutions in ceramic glaze and body recipes must consider their chemistry, mineralogy and physical properties
200 mesh (a sieve with 200 wires/inch) is the particle size that most minerals used in the ceramic industry are processed to.
Silica, sold as a white powder, is pure quartz mineral. Quartz is the most abundant mineral, it is pure SiO2 silicon dioxide.