Vince Pitelka on thu 4 aug 05
>I recently had a lot of pots crack in strange places during an electric
> bisque, perhaps the most bizarre were a number of covered casseroles that
> cracked vertically from the rim downward, lids fired on them as usual.
> Nearly everything was done exactly as it had been hundreds of times before
> and i have never had casseroles crack like that. I did make a relatively
> minor change to the clay body i use by adding more kaolin but i find it
> hard to believe that was the cause.
Paul -
Those do not sound like moisture cracks. In my experience, vertical cracks
extending straight down from the rim on a bowl or casserole most often
result from deformation of the shape as it is drying. Is there any chance
that some of these casseroles were drying slightly out of round, and that
you "persuaded" them back into shape at the soft-leather-hard stage or later
in the drying process? An addition of kaolin could increase the liklihood
of that kind of crack. As you no doubt know, porcelain bodies are extremely
susceptible to this kind of crack, because the body is so fragile in the
bone-dry and bisque-fired stage, as compared to a stoneware body with a
broader distribution of clay particle sizes. I don't know how much kaolin
you added to your claybody, so I do not know if this could be a cause.
Adding a little kaolin doesn't increase the chance of such cracks at all,
because it just adds to the range of particle sizes, which means more
contact points between particles in the bone-dry and bisque-fired stages,
and thus the wares are stronger at those stages.
Uneven drying can cause a casserole or a steep-walled bowl to warp, and then
if you try to straighten it at the soft-leather-hard stage, it sets up weak
zones that can develop into cracks in the bisque-firing. This is even more
likely in the case of a lidded caserole, because the lid prevents the bottom
from returning to it's "clay memory" shape. If this does turn out to be the
problem, the solution might be to dry your casseroles very evenly in a damp
box, in order to avoid warping and problematic clay memory.
Good luck -
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
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