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warped compression/tension

updated tue 14 mar 06


Stephani Stephenson on mon 13 mar 06

A potter learns that clay can be compressed(compression) or stretched
(tension) in the throwing process.
This is simplifying things a bit, but it is valuable to consider
compression and tension in tile making, whether you press tile or roll
out tile or extrude tile.
At the Indiana NCECA conference Dr. Carty from Alfred showed examples
of how a differential in compression /tension from the center of a
thrown plate to the rim causes a variety of cracking and warping
For example if the rim is under more tension than the center of the
plate, rim cracks can develop.

with clay
too much tension leads to cracking
too much compression ,warping .
tension: cracking
compression: warping

if you are hand pressing a tile
and stretching one area more than others
you may find that the tension created by that area of stretch will
(taking into consideration otehr factors such as clay body, etc)

If you are compressing clay unevenly along one side of the tile,
you may find that those areas will warp.

One time I had a whole batch of 6X6 crack in the center. the cracks
developed in drying
my first thought was that water had somehow puddled or condensed in the
center. These tiles had been dried slowly ,
and I even thought the slowness was the problem, though I had never
observed any moisture in the center.
Then I remembered that the clay had been somewhat stiff and my pug not
quite big enough to fill the mold. so I had started pressing in the
center then literally stretched the clay to the perimeter as I pounded.
Usually I take care to press straight down and not stretch the clay
across the face of the mold.
This time I remember the sensation was that I hasd 'stretched' it a bit
though didn't think I was pushing the limit of the clay.
I do remember using more force than brains pressing those tiles that
day though!

I have also observed the compression problem , usually when
hand-pressing a long tile, say 18" long, and unwittingly using uneven
compression along the edge (trying to jam more clay into one spot,
pressing hared in one area...)
A side may develop a particular little warp, almost like a hiccup
in one spot,
when I focused more on exerting even pressure .. the problem went
This differential is from center to edge, as with the thrown plate, but
also along edges, and can occur anywhere in the tile.

so... look at your forming habits.. there may be something going on you
haven't noticed. the little cracks in the center sound like this might
be happening... and a change in relief can amplify the problem,
especially if the clay is under greater tension as it is streched over
the relief.

I can't really deduce what is happening with the tiles bottom of the
kiln cracking and the shelves...

I bisque fire tiles on edge. much better... more tiles, less
refractory shelving, fewer cracks and warps!

Stephani Stephenson