David W McDonald on mon 23 aug 99
Hi Ron, 8/22/99
In response to your questions about my bloating results; The
pieces I make these days are 15" to 25" slab rolled, drape formed
platters which have a coil added to the bottom and a foot thrown onto the
slab with that coil. Wall thicknesses range from 1/4" to 3/8" plus. Of
course the pieces are thicker right where the foot is added.
Sometimes the bloats are right in the thicker area where the foot
is. More often though, the bloats are in the outer two thirds of these
pieces, between the center and the rim. In other words, not in the
thicker areas. The larger pieces are not more susceptable to bloating
than the smaller ones. This leads me to think that the clay just can't
handle the tempurature, because as we know, thinner walls extending
outward from the foot of a piece are going to absorb more of the heat in
the kiln quicker than the thicker areas of the piece down toward the kiln
shelf where it sits. I hope you're following me here, this is kind of
hard to describe.
Earlier on I was thinking that carbon entrapment might be
contributing to the bloating, but now I see that that is not necessarily
so. I have broken pieces open which evidenced no carbon trapping, totally
clear, but fully bloated. The clay is quite vitreous; the cross section
shines in the sun, and it has less than one percent water absorption.
The bloats range in size from about 3/4" to 3", and are
definately not just blebs. Broken open cross sections of the bloats
themselves show that the clay has just torn itself apart inside as it
separated in the middle. No indication of any contaminants large enough
to see.
This information, along with the posts that follow, should give
you some pretty good indications as to the possible causes of the
bloating. I'd like to know what you think. Thanks so much. David
McDonald
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