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

updated thu 28 jul 05


Stephen Powell on wed 27 jul 05


On larger platters I usually turn two concentric footrings, and or, a
pointed nib in the centre of the base. When firm enough I allow them to dry
on garden trellis placed on the bench. I then cover with loose polythene,
permitting the inside and outside to dry evenly together. I fire them on a
shelf sprinkled with a good layer of silica sand which allows shrinkage but
reduces drag.
A tip to me from a long established potter was simply "Try another clay if
the one you are using doesn't work the way you want it to"----- Original
Message -----
From: "Andie Plamondon"
Sent: Monday, July 25, 2005 6:38 PM
Subject: Two Questions: Warping & Air Bubbles In Large Platters (yep,

>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
> 7.
> 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
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