Silica At Cone 10

[glazenerd]

Just read a very informative article about silica properties in cone 10 stoneware bodies.

 

 https://studiopotter.org/pdfs/sp28_1_sohngen.pdf

 

Peter Sohgen did some extensive testing on silica, in regards to mesh size, % of addition, and the outcome. The information tracks very precisely the effects silica has on shivering and dunting in a cone 10 stoneware body. I also like that he examined feldspar additions to counteract the problem: and the effects. Even if you do not formulate your own clays: it is still good information to help you diagnose crazing issues.

 

One particular aspect stuck out to me: the quartz inversion. This inversion happens regardless of clay body type. It was a discussion in several threads just a few months back: dunting issues during bisque firing. According to the dilameter testing provided by Ron Roy: it was confirmed that quartz inversion occurs every time silica is heated. I long suspected that because I have seen it on the large (heavy) format tiles that I fire. (Inversion temp 573C) That does not necessarily mean it will be a problem: most likely not in 97% of the firings. It does however mean that if you are firing large scuptural pieces, large (heavy) pots with a large foot ring, or large tiles with alot of shelf contact. The key would be: heavy, and a large percentage of shelf contact. Obviously in bisque firings, a slow ramp through inversion is required. However, during glaze firings, if the above parameters are present: you might want to consider a slow ramp through inversion the second time.

 

Nerd

 

Additional information for cone 10 stoneware firing: even more applicable if reduction firing. Ron Roy sent me an email with the above referenced article: but also made some additional comments specifically regarding reduction firing.

The more iron involved the more complicated the problem becomes.