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slab plates warping

updated tue 21 sep 04


Vince Pitelka on mon 20 sep 04

> wondering if there was a way to keeping warping to a minimum in slab built
> plates. I always roll them on the slab roller and place them over a
> thrown
> plate form or over a bucket with cloth over the opening to get a nice
> curve. They warp during a glaze firing, never the bisque.

Susan -
If you are using the clay straight from the slab roller, that is likely the
reason. The slab roller tends to cause a unidirectional grain structure
lengthwise along the slab, and the drying and firing shrinkage will be
greater across the grain rather than along the grain, just like with wood.
The solution is to cross roll the slabs. Some people like to roll the slab
partway on one pass, turn it 90 degrees, and then roll it to the final
thickness. That seems to work well, but I prefer to equalize the grain
structure with a rolling pin. I just smooth another piece of canvas down
over the slab, and roll across it a few times, 90 degrees to the original
direction of rolling. I then grasp both sheets of canvas on the far side
and flip the slab over, and then roll it again. After this is done you can
peel off the canvas and slump the slab, and this very well might solve your
warping problems.

Do these slab plates have feet? Are they well-supported in the kiln? I
always add a coil foot to my slab plates, and then I cut a few openings
through the coil, giving a visual impression like separate raised feet.
This gives good heat and atmosphere circulation around the plate, decreasing
problems with cracking. But be sure that your slab plates are very well
supported. If some part of the bottom can settle even slightly in the
firing, the whole plate will warp.
Good luck -
- Vince

Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111,