Tuesday, February 24, 2009
boring slip cracking technical jibber jabber.
Ok, here is what I am hypothesizing: I use a fairly thick layer of kaolin slip, so it is almost like a thin layer of porcelain on the surface of the piece. This "porcelain" is not as shock resistant as the stoneware and is pretty much dunting in the cooldown. It is fused to the piece yet cracking independently, weird right? I noticed this morning that a few pieces near the flue had slip cracks as well so this leads me the conclusion that it is definitely happening in the areas where there is the most chance for thermal variance in the cooling(firebox and chimney). So one obvious solution is to make sure that the kiln is sealed up like tupperware, I think I can handle that. I cool for 36 hours but this next time I'll mud every crack and wait until I can touch the pieces on the top with my bare hands. I thinned down my slip just a bit today, I like the surface thick slip gives vs. thin slip so I don't want to go skim milk thin. Joe(Cole?) had a good idea of increasing the silica, much like you would to reduce crazing. I think I'll save this solution if my previous doesn't work, I don't want to change too many variables at once(thank you sixth grade science). Thanks to Joy and Joe for their suggestions, much help. Does this sound ok or completely ridiculous? I wouldn't call clay and glaze chemistry one of my strengths.
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5 comments:
I think that's good to not change too many variables, thinning the slip could really help, also plugging with mud. Good luck and let me know what you find out.
sounds good to me. you were always better than me at that thinking part of pottery!
If the slip is cracking at the firebox and the flues, but not inbetween then it certainly sounds like it's related to somekind of thermal shock. It's certainly worth trying a slow cooling to help with the slip cracking. The only thing being that with slow cooling your ash glazes might really start to crystalize. I do a lot of decorating under ash glazes so I always avoid crystalization, I don't think I've seen many of your pots with decoration under the glaze, so maybe you would enjoy the effects of crystalization? Or maybe you could get the best of both worlds by quick cooling to 1800 or so then clamming everything up tight after that? I think you would avoid the majority of the crystalization that way, and the pots will still have some pyroplasticity to them, so you might still avoid the cracking. Or I might just be full of sh*t.
Joe
thanks joe. i already crash cool to somewhere around 2000. i've found that residual salt fumes do some weird things similar to soap scum on the surface of the pots if the kiln is sealed up when the pots are still at peak temp. i think the cracking is occuring somewhere between 400-800. very few potters are scientists so we all may very well be full of sh*t!
Just catching up here and really not too much to offer other than to say I got the exact same thing often in my salt kiln. I crash cool and I tried clamming up tight but probably never really did a good and proper job. I too think it is happening during cooling.
So there, I really did nothing but repeat all you all had said already. So I'm going.
Pots look great man!
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