Lili Krakowski on wed 1 aug 12
We just heard of problems with large pots--all the same design--25 =3D
inches tall,and, I gather, that wide-- cracked at the rim.
Jeff writes (and I cut) :"After glaze firing...the rim, about 2.5" wide, =
=3D
was badly cracked. In
places... Second one I single fired ... cracked rim
looked like some alien thing was swelling up inside it and the outside =3D
wall
shed palm-sized flakes ... This one had less grog in
it but was more even, with about 3/4" thick walls and a rim about 1.5".
Disclaimer : I do not make pots that size --but the 3/4" thickness =3D
strikes me as possibly too thin for that particular clay body.
This sounds odd, as it is the rim that cracked. But pots move an =3D
immensity in the firing--they wiggle, they twist, they swell, they =3D
shrink...and this can rip a rim or lip apart.
And: Jeff does not reveal were these pots thrown or handbuilt, or slab =3D
built....Duh?
Trimming: As said by others inverting the pot on its head for trimming =3D
is a huge stress on the top-now-foot. Clay is not good at =3D
break-dancing. Chucks, foam pads on the wheel head, very slow trimming, =
=3D
all are needed for large pots. OR inverting the pot for trimming can be =
=3D
avoided entirely if one designs the pot to be finished right side =3D
up....no trimming except clean up.
Drying: As far as I am concerned and experience has shown me, clay =3D
cannot be dried too slowly. Speed drying requires a body that "can take =
=3D
it." Many bodies suffer. I think big pots need to dry slowly, =3D
preferably in a damp box or similar. It reduces the stress created by =3D
the inevitable unevenness of drying. The last stages of drying can be =3D
fast. The first stages not.
Over all design: I do not understand what 2.5" wide rim means. Is it a =
=3D
flange? How flared is it? Could it be it just hangs there like the rim =
=3D
of a soup plate? And is too horizontal to sustain itself in the fire? =3D
Then try for a rim that tapers from the collar where it rises from the =3D
pot, and thins as it goes. A rim that wide needs its own support.
As to the flakes...No idea...
Lili Krakowski
Be of good courage
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