Ron Roy on mon 19 sep 05
Hi Wayne,
This is fairly common - sides shrink in and force the bottom up or down.
It's why you need to make plates a bit rounded and deeper in the centre -
else the bottom may rise - which results in a complete lose of the "inside
space" or "emptyness" of a plate.
So if the bottom is concave - and it moves down a bit more during the
drying - then you need to have a foot rim of sorts to stop the rocking.
Not sure that is all clear - but there is no douby - it does happen -
bottoms move during drying - the bigger the bottom the more they can move.
RR
>I recently got (thanks to a dear friend who was going that way) a
>couple boxes of Lizella Clay to play with. It's kind of a sandy
>clay from GA, fires to a dark terracotta at cone 6-ish. Mostly used
>to make jiggered flower pots, I'm told. I'm also told that
>sculptors love it, and I can see why...but I digress.
>
>Just to play, I threw a platter with it. Nothing dramatic, about 14
>inches, 2 inch rim, maybe 5-6 mm (3/16 - 1/4 inch) thick (Lizella
>doesn't like too thin, tends to slump while spinning.) From the bag,
>it's very plastic, very moist. Nice throwing clay, I'll admit.
>After throwing, I cut it from the (plywood) bat but left it on it,
>upright, and set it aside to firm up overnight. It was leather hard
>in the morning, so I placed a bat on the rim, flipped the whole
>thing, removed the bottom bat and left it for another day so that
>the center would dry out some.
>Flipped it back over (upright) the next day and let it air dry
>completely.
>
>To my amazement, after it had dried, I noticed that the platter
>center had RISEN about 1/8 inch. There is a definite rise to the
>center that was not there before.
>
>I'm trying to figure out why something like this happens. (It
>doesn't happen with porcelain, and it didn't happen with the bottoms
>of the bowls I threw from the Lizella either.) Could it be that as
>it dries from the edge, the clay shrinks, tightening the
>circumference and causing the center to raise (path of least
>resistance) since the edges most certainly dry out first? No, I did
>not cover it in any way, and we're talking South Florida humidity
>here (about 40-80%). No cracking in evidence, so I think I got the
>compression right . Is it simply a case of having dried too fast?
>Too little humidity? Too much stupidity ?
>
>
>Best,
>Wayne Seidl
>
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Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
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