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Pottery cracking during a glaze fire

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I was wondering if anyone could help me, I have around 10 glaze fired tiles, they are all stoneware and I fired to 1200 with a 30 minute soak. 7 were glazed with textured brush on glazes and 3 were painted with a transparent glaze. For some reason when I took them all out of the kiln, all three transparent glazed ones and one beige tile appear to have several cracks in the back. The clay is a white stoneware clay, I placed them directly onto a batt washed kiln shelf and all three transparent tiles were on the top shelf. Not sure if this may be a coincidence. Please let me know if anyone has any suggestions as to how I can fix this issue? 

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Did you put any grog on the shelves so the tiles could shrink and slide on it with very little stress.    The  rectangle you have carved out of the back could be causing it,  some clay's are very picky about changing thickness.   You may need to change your clay to one that has more body to it and can handle the stress.   I made a mural with a clay body that looked like your photo,   I tried every fix I could think of I still got cracks from the edges.   Next mural I changed clay bodies,  I didn't have problems with stress cracks.  Denice

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9 hours ago, phyllosceramics said:

...and all three transparent tiles were on the top shelf...

I don't think it's a coincidence that the ones on the top shelf cracked. Uneven cross section (like your tiles with the additions) plus too quick a cooling can definitely lead to cooling dunt cracks like in your images. You want to create a slow even cooling environment for the tiles, the ones with a shelf below and above them will be cooling at a much more even rate than the ones on the very top shelf with nothing to hold the heat in as the kiln cools. Bottom shelf can create the same issue. Best to keep things that have a proclivity for dunting between shelves. These might not be cooling dunts but it's a good practice to follow for thick(ish) flat pieces.

Is the glaze on the top of the tile where the cracks are sharp where the crack is or smoothed over into the crack? If it's a sharp edged crack that too points to a cooling dunt rather than a crack that happened on the rise up in temperature. 

Welcome to the forum!

Edited by Min
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