Kris Baum on wed 10 dec 97
I am having a weird cracking pot phenomenon (yes, I know I probably
am a crackpot , but that's another story...)
In the last two kiln loads, I have had cracks in my pots that do not
go all the way through the clay body, but are very shallow
(they appear to be about 1/32-1/16" in depth.) They are not in the
bottoms of pots (I have had those, both inside and out, but doing a
better job of compressing during throwing and trimming seems to have
fixed those). The first was on a slab piece (red sculpture body)
that was painted with underglazes and had a thin clear glaze coat on
top - the crack was about 1/2" long on the top surface, at least 3"
in from any edge. The second occurrence was on a glazed bowl (white
sculpture body) on the outside, 2 perfectly horizontal cracks - one
about 3/4" from the foot and the other slightly above and offset; and
a third crack on the other side at the same distance from the foot,
but at an uneven angle (not horizontal). Both the slab and bowl were
perfect after the bisque firing - no sign of cracks at all.
I've used the clays and the glaze in various color formulations
before without these problems. The firings were my standard cycle --
(electric, ^6, slow cycle, 30 minute hold at peak temp).
Although I'm not sure when the crack occurred in the firing, one of
the cracks on the bowl appears to be under the glaze and the glaze
itself seems to be merely indented over that one crack, not broken
through, so I deduce the cracking is during the heating cycle, not
the cooling cycle (is that right, or not necessarily?)
Any insights would be appreciated ... I seem to have enough trouble
with cracks in the predictable places, and these new ones have me
stumped!
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Kris Baum, Shubunkin Pottery
mailto:shubunki@erols.com
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Louis Katz on thu 11 dec 97
Kris,
These sound like delamination cracks. Most often these come from not
wedging pugged clay, although they could be blamed on poor pugging. The
cracks are just places where layers of clay have been pushed together but
have not really stuck.
Some clay bodies are immune to these problems, others seem to really need
the kneading. Some pugmills are bound to work better than others in this
regard. I believe clay pugged wetter has fewer delamination problems.
Louis
lkatz@falcon.tamucc.edu
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