You cannot fix this crazing with a process or firing change

This is severe crazing (at cone 10R). It is happening because of the chemistry of the glaze, not the firing. The first option to check when fixing crazing is: Can the glaze accept an addition of SiO₂? This glaze is an excellent candidate for that because the melt is highly fluid, it will surely be able to dissolve extra SiO₂. But it could also accept Al₂O₃ because it is highly glossy (a little extra Al₂O₃ will not matte it and would also reduce expansion and increase fired hardness and durability). What to do then? I would start with a 10% addition of a mix of two parts silica to one part kaolin (this mix has a 10:1 SiO₂:Al₂O₃ ratio, about the same as most glossy glazes).

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

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The 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? This is an excellent example of the value of looking at the chemistry (the three are shown side-by-side in my account at Insight-live.com). 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 COE of the whole way up. And talc? It contains SiO₂ (so the SiO₂ is not driven down nearly as much) and its MgO has a much lower expansion than CaO does.

ChatGPT was completely wrong about the cause of glaze crazing in 2023!

A ChatGPT answer to a question about crazing

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ChatGPT was parroting common wrong suggestions about the cause and solution of the serious issue of crazing. Yet it trained on thousands of internet pages about the subject! Crazed functional ware is defective, and customers will return it. So fixing the problem is serious business, we need correct answers. Consider its suggestions: #1 is wrong. There is no such thing as an "incompatible mix" of ceramic materials. Crazing is an incompatibility in thermal expansions of glaze and body, almost always a result of excessive levels of high-expansion K₂O and Na₂O in the chemistry of the glaze. The solution is reducing them in favor of other fluxes (the amount per the degree of COE mismatch). #2 is wrong, firing changes don't fix the incompatibility of thermal expansions. #3 is wrong, refiring makes the crazing go away but not the stress of the mismatch, it will for sure return. #4 is completely wrong. Firing higher takes more quartz grains into solution in the melt and should reduce the COE (and mature the body more which often improves fit). And melt fluidity has nothing to do with crazing. Furthermore, if a glaze does not run off the ware, it is not overfired. Of course, this is the worst it will ever be, expect better in future.

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Glaze Crazing
Ask the right questions to analyse the real cause of glaze crazing. Do not just treat the symptoms, the real cause is thermal expansion mismatch with the body.

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