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two questions: warping & air bubbles in large platters (yep, again!)

updated tue 26 jul 05


Andie Plamondon on mon 25 jul 05

I searched the archives to find an answer to my problem, and, ironically, I
came up with the identical question, posted in 2000 - by me:

"I am finally throwing large platters and bowls, ranging from 19" - 24" in
diameter. However, I am losing about 3/4 of them in firings to warpage. I
did find that stilting (with those porcelain cone 10 three point stilts) in
the glaze firing seems to contribute, but I have had some warp in the bisque
and some that weren't stilted warped in the glaze. They are thrown from two
clays (not mixed together, obviously - some from one, some from the other):
Standard 108 and 181. I let them dry slowly and completely upside down
and then bisque to 04 with a soak at 5 (medium). I glaze fire them to cone
The bottoms and walls are the same thickness, and I do trim and burnish the
very bottoms.
The warpage is in the bottoms of them, so when they go in they sit flat and
when they come out they don't. Occasionally one will also warp across the
top, so it goes in round and comes out oval. I have already tried varying my
bisque schedule, and also have tried firing lower in the glaze, to a cone
5/6 (6 conesitter), but nothing has made any difference. Any ideas? I don't
want to keep wasting clay until I can experiment with some possible
solutions, and would absolutely appreciate any ideas. I love making and
glazing them, and it's so hard to keep losing them over and over. I'm
willing to try new clays, new firing schedules, anything. "

Now, back then the responses I got suggested a wider foot ring (this seems
like a possible fix, and I will be trying this next) as well as drying
tips - but the problem seems to be in the glaze firing. I just ran a glaze
of 26" - 28" platters, and every one of them is warped on the bottom - went
in sitting flat, now they're not. I could scream.

I suppose I could not stilt at all, but I do use glazes that run.

Any new ideas?

And now for a 2nd, new question:

In throwing platters, I have been having some air bubble problems on the
bottoms - I wedge, put the clay on the wheel, center, start throwing. What I
am finding is that after I pull out the center, the bottom is a full air
bubble - so big that it seems to spread across the bottom, and I don't see
it or feel it until it pops off the bat and the underside is basically
warped, sunken, and uneven. Is this just a compression problem in the
throwing process? Maybe too thin of a bottom?

Is it possible I have some tiny warpage in my bats that is causing both of
these problems?

I LOVE throwing large bowls and platters, but the clay waste is getting so
high - I'm actually starting to think I should just scale back and give up
on the big ones.

Thanks -

:) Andie
Handful of Earth Pottery