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Topic: Warping/Cracking AFTER second firing?  (Read 1012 times)
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uglyshyla
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« on: July 09, 2008 06:57:08 AM »


I have been having problems with things like large doll heads or bowls warping and cracking after the second firing.
I use the wet method of green wear cleaning,where you let you your raw cast piece dry after a tiny bit of clean up like removing large seams,then once it's totally dry you fire it to cone 018 to harden it so it won't dissolve in water.
Then you soak the pieces in warm water and THEN you do all your cleaning like sanding and smoothing.
You let it dry AGAIN all the way and THEN fire it to cone 6 to fully mature it.
My prob is that on things like say the larger antique reproduction doll heads they will crack at the top of the head during the second or mature firing,they are never cracked in the first firing.And I know it's not something like temperature fluctuation from me opening the kiln too soon or anything like that.
I'm not sure what is causing this.I don't know if it's the weight of the dolls head since it's fairly large,or the way I load them into the kiln,I put them on the shelf top of the head down, because the top of the heads are almost like the top of a bowl and are cut off where you can stand them on there.Or maybe I'm putting the parts that crack to close to the heating element?Or maybe it wasn't fully dried from the wet cleaning even though the clay is "set" after the first firing.
If anybody has any clue what is causing this or how to fix it I'd appreciate it.
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sugarbeth
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« Reply #1 on: July 09, 2008 01:20:05 PM »

If it's happening consistently, I'd probably try another clay. Or I'd do the first firing to a higher temp like 04. It might be that you're stressing the clay out when you're sanding? If you put pressure on the form at that point, it might not respond until it has a chance to move. I had that happen with tiles before - trying to unwarp them when they were mostly dry, and it would look like I got away with it until the final firing where they would split in half.  Shocked
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Beth
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uglyshyla
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« Reply #2 on: July 09, 2008 01:28:44 PM »

Thanks for the tip,maybe my slip is to thick or too loose.It may be the too thick issue because I poured a body with some more water downed slip from the same jug and that didn't crack even thought I gave that piece hell.LOL


If it's happening consistently, I'd probably try another clay. Or I'd do the first firing to a higher temp like 04. It might be that you're stressing the clay out when you're sanding? If you put pressure on the form at that point, it might not respond until it has a chance to move. I had that happen with tiles before - trying to unwarp them when they were mostly dry, and it would look like I got away with it until the final firing where they would split in half.  Shocked
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celedon
« Reply #3 on: July 10, 2008 09:08:52 AM »

Are you actually using a casting slip, or just recycled slip?  If you are using recycled slip, that may be your problem.  You might also be firing too fast, your clay may not be compressed enough, or if you are messing with the piece before your bisque fire, it's "remembering" and than warping (bowls). 

The compressing issue is sort of hard, since you are slip-casting, but you might try taking a softer (blue) rubber rib to the part that is cracking when it is leather hard, or even your thumb (just rub it gently but firmly, to compress the clay in that area). 
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paintindiva
« Reply #4 on: October 19, 2008 05:48:37 PM »

Have you tried to clean the greenware before it is completely dry?    Once my greenware is cleaned and dry, i fired to 04.  then paint and glaze and fired to 06.

I believe there is a difference between 06 and 6.  that might be your problem.  too high of a firing temperature. 
Also I have never done firing of 018.  Just cleaned greenware, let dry and fired to 04.   I have never had any problems. 
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KeLLeY
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