Clay in a glaze recipes. Why is it there? What happens when there is too much?

Wednesday 25th February 2015

If I put this on social, I might get 100 comments on the cause. Most would say it's applied too thick. That is not correct. At the top, it is too thick, but not at the bottom (where it is still cracking). Crawling during firing is almost certain.

Before offering any opinion, one needs to insight-live.com/glossary/304">sanity check the recipe. This one, GA6-B, contains a lot of clay. Why do dipping glazes contain clay, usually kaolin, ball clay? To suspend the slurry, slow down drying, gel the slurry to make it thixotropic and harden the layer on drying. But their chemistry is also important, clays supply the all-important Al2O3 and SiO2 to the melt (which would otherwise have to come from feldspar). Certain clays excel at suspending. 15-20% EP kaolin, for example, is all that is needed. It’s not that plastic, but sticky and thixotropic (Grolleg and New Zealand kaolins are similar or even better). 10% of a gelling ball clay might be enough. However, 50% of a silty non-plastic clay might be needed. When the clay has too much influence, glazes shrink too much as they dry and then crack like this. Ones lacking clay (or employing one that is not suitable) have poor application properties (for dipping) and are powdery. This glaze has 80% pure raw Alberta Slip. That is a plastic clay, no wonder this is happening! 50% of it needs to be roasted.

Pages that reference this post in the Digitalfire Reference Library:

Alberta Slip, Alberta Slip 1900F Calcined, Alberta Slip 1000F Roasted, Calcined Kaolin, What makes EP Kaolin unique? Are others as good?, Shrinking glaze = peeling glaze, Calcination, Powdering, Cracking and Settling Glazes, Crawling

This post is one of thousands found in the Digitalfire Reference Database. Most are part of a timeline maintained by Tony Hansen. You can search that timeline on the home page of digitalfire.com.