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Update: Glazed 8 thin cylinders heavily in the white matte/clear combination and did the freezer to boiling water tests on all of them aaaand.....nothing. No cracks,  no crazing, no dunting. The only thing I've changed is the firing schedule as @Babs suggested - 100˚c an hour to 600 so this might have slowed the heating through cristobalite inversion sufficiently. I also slightly increased the hold time at peak 1230 as @neilestrick suggested and the cones suggested firing perfectly to cone 7 (slightly over towards 8  in middle) which is just slightly hotter than it was before. So perhaps it was the firing schedule and temp that caused the problem rather than the glazes. I move forward cautiously optimistic....

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1 hour ago, birdypotter said:

Update: Glazed 8 thin cylinders heavily in the white matte/clear combination and did the freezer to boiling water tests on all of them aaaand.....nothing. No cracks,  no crazing, no dunting. The only thing I've changed is the firing schedule as @Babs suggested - 100˚c an hour to 600 so this might have slowed the heating through cristobalite inversion sufficiently. I also slightly increased the hold time at peak 1230 as @neilestrick suggested and the cones suggested firing perfectly to cone 7 (slightly over towards 8  in middle) which is just slightly hotter than it was before. So perhaps it was the firing schedule and temp that caused the problem rather than the glazes. I move forward cautiously optimistic....

Good to know. Just curious looking at the Scarva site, is this is a cone 10/11 clay for full vitrification?

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26 minutes ago, Bill Kielb said:

Good to know. Just curious looking at the Scarva site, is this is a cone 10/11 clay for full vitrification?

It's a little unclear - clays here usually state a really wide firing range and say 'they start to vitrify' at 1200 for example, but obviously we know that starting to vitrify and fully vitrifying will be miles apart temp wise. I think this clay will probably be at peak vitrification somewhere around the 9/10 mark yes, though I haven't tested up that far. At 1230 on my current firing schedule it takes on about 1.7% during the absorption test.  Scarva says the firing range is 1200-1290 so I assume it will be fully vitrified closer to 1290 for sure.

Edited by birdypotter

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