|Monthly Tech-Tip |
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.
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.
This is G1916Q3 low-temperature transparent glaze (it is the G1916Q recipe plus 10% 45-micron silica). Silica is a highly refractory material, this low-temperature glaze should not have been able to dissolve that much of an addition without cost to the gloss, melt fluidity or transparency. But none of these has happened and the reason is, in good part, because of the finer particle size of the silica powder: 45 microns (or 325 mesh). It is applied fairly thick and is runny enough to have pooled at the base. And it is still ultra glossy. A big benefit of the silica being present and having dissolved so well in the melt well is an almost certain increase in the durability and hardness of the glaze. Another benefit is a reduction in the thermal expansion, that is proven by the glaze shivering off the rim (on the far right). This body is Plainsman L215. So, while not suitable on this body, this glaze is ready to be used on others that craze the parent G1916Q recipe. And it has a thermal expansion adjustable recipe, a base frit and about 15% each of a low and high expansion frit. That means some high-expansion Ferro frit 3110 can be introduced at the expense of low expansion Ferro frit 3249.
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.
325 mesh (a sieve with 325 wires/inch) is the finest particle size that most ceramic minerals can be practically processed to (premium grades).
200 mesh (a sieve with 200 wires/inch) is the particle size that most minerals used in the ceramic industry are processed to.