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cupping: flanges, casserole lids and test tiles

updated thu 22 jan 98


Stuart Altmann on wed 21 jan 98

Three problems with a common cause:

(1) Platter and bowl flanges curl upwards as the pot dries.

(2) My casserole lids are almost always too large when dry, even though I
measure carefully when I throw them.

(3) When leather hard, my test tile verticals overhang their bases more
than when I throw them. I make test tiles by throwing a large torus to the
periphery of my largest bat, then shaping it into an L-shape in cross-
section, with the vertical part on the outer edge but leaning inward
slightly for stability. When the clay is leather hard, I cut vertically
through it every couple of inches, thus producing a series of test tiles.
But I have noticed that the vertical part leans inward more when leather
hard (before cutting) than when freshly thrown.

I believe that in each case, the primary cause is more rapid drying--and
thus shrinking--on the upper and peripheral parts of the pot. Like
tightening the drawstring on a stuff bag. Thus the rim is pulled inward.
By the time the center and the bottom undergo their major shrinkage, the rim
is too dry to move much, and in some case this results in splits in the

These effects are exaggerated in the winter, with the very low relative
humidity that results from heating our studios, and probably are a year
'round problem in arid regions.

Possible solutions: Use a heavily grogged clay. Throw on plaster bats to
speed up bottom drying. Wrap the periphery of the pot (flange or whatever)
with plastic to slow down drying of the periphery. Paint the flange with
wax resist, for the same reason. Cover the entire pot and let it dry very
slowly, over about a two-week period, turning it every couple of days.

An excellent article, by Stanley Lee, on throwing large platters is in
Ceramics Monthly for September 1979. My one addition is this. Cut circles
of various sizes from firm, closed-cell foam rubber, such as a camping mat,
then stack up the circles of the right size: the size of the inside of the
platter base, to support the platter when it is inverted for drying and
trimming. The rim should hang free when you do this. There's much more in
Lee's article. If you throw large platters, get a copy.

Stuart Altmann m