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Ceramic Cracking During And After Cool


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Hi. I am a community studio manager and we fire cone 10 reduction. Recently we have had some issues with ceramic pieces cracking during the cooling process and continuing to crack after they are out of the kiln. This has happened a couple of times in different firings but the pieces that cracked have had the same glaze on them. It is a cone 8 glaze called Waxy Matte and the kiln gets to cone 10 consistently. (because os previous years of uneven firing the studio has cone 8,9 and 10 glazes). I am working to get all of the glazes to cone 10 but it is a community studio and every glaze is someone's favorite...

 

I am inclined to think that the glaze is too tight on the clay but am surprised that it would actually crack the ceramic as well. These are hairline cracks that go all the way through the piece, sometimes breaking them apart.

 

Any thought on where I should start troubleshooting? Can I provide you with any more information to help?

Thank you!

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It is definitely possible for the glaze to crack the pot, especially if it was only glazed on one side (inside or outside), or if the glaze is very thick on one side (in or out) compared to the other. Differences in COE between the clay and glaze can also cause cracking.

 

Here's another recent thread that has had a lot of discussion on this subject:

http://community.ceramicartsdaily.org/topic/7052-tea-pots-cracking-violently/

 

If the glaze is the only variable, then I would definitely start with that.

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You can get clay body COE from the clay manufacturer or distributor. Glaze COEs can be found by entering the glaze recipes in any of several glaze calculation programs available. A good technical reference book is Hamer and Hamer's The Potter's Dictionary of Materials and Techniques. A very good discussion of the glaze fit and durability is Hesselberth and Roy's Mastering Cone 6 Glazes (the concepts apply equally to Cone 10). I'm sure there are other good books out there, as well as these.

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COE figures are good to look at when working with gloss glazes but they are not so helpful when using for a proper matte or semi matte glaze (crystalline structure, not underfired gloss). There is a bit of a steep learning curve when first using glaze calc software, it might be more time efficient to make some test cylinders and do various glaze combinations on the inside only plus inside and outside then do the freezer / boiling water tests. Thinly thrown cylinders with the glaze applied heavily (worst case scenario for dunting), with the claybodies used in your facility.

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Hey,

     Sometimes a student brings in their own clay as well as glazes....  Do some more tests

if possible.  Glaze the inside of the vessels with a consistant glaze and the outside with

the questionable glaze.  If it still occurs, it won't matter since the crackle glaze is on the

outside.  I used some cone 06 raku glaze on the outside of some cone 10 reduction stoneware vessels

and it came out mottled just like I wanted.  The name of the raku glaze was Pippinburg...

It was like a mystery glaze on Raku since it wasn't always the same.

     Anyway, make sure the student doing the glazing is doing so on the community centers' clay

just to rule that out and do the inside outside glazing too.

If it still persists, send pictures... they help.

Good luck

Alabama

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