The unexpected reason for this crazing can be seen in the chemistry

Wednesday 28th November 2012

This liner glaze is 10% calcium carbonate added to Ravenscrag slip. Ravenscrag Slip does not craze when used by itself as a glaze at cone 10R on this body, so why would adding a relatively low expansion flux like CaO make it craze? It does not craze when adding 10% talc. This is an excellent example of the value to looking at the chemistry (the three are shown side-by-side in my account at The added CaO pushes the very-low-expansion Al2O3 and SiO2 down by 30% (in the unity formula), so the much higher expansion of all the others drives the expansion of the whole way up. And talc? It contains SiO2, so the SiO2 is not driven down nearly as much. In addition, MgO has a much lower expansion than CaO does.

Pages that reference this post in the Digitalfire Reference Database:

Talc, Ravenscrag Slip, CaO is a strong flux but it can cause crazing, GR10-B Ravenscrag transparent glaze (with 10 talc), You cannot fix this crazing with a process or firing change, Digitalfire Insight-Live, Calculated Thermal Expansion, Glaze Chemistry, Crazing, Glaze Crazing

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