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.