DeBorah Goletz on wed 18 jul 01
Craig - yes grog does help "open up" the clay body - reduces shrinkage and
warping. However, since your smaller border tiles fired without cracking,
your problem may be in the way you are firing the larger tile. If you don't
already, try sprinkling grog on the shelf under the tile for the bisque. The
grog allows heat/air to circulate under the tile as well as acting like ball
bearings to ease shrinkage. For the glaze firing, try allowing extra air
space (sometimes several inches) above large tiles. Large tiles heat
unevenly if the shelf above them is close - particularly in a glaze kiln
which heats faster than bisque. This results in cracks which start at the
edges and work towards the center of the tile (in a curve). If these
suggestions do not help, then try changing your clay body.
Best,
DeBorah Goletz
(snip) Now when I bisqued the large
> tiles all were fine but 2, six were without cracks. I glazed them all
> and almost every large tile cracked with 1/4 inch crackssometimes into
> three pieces. There was no crazing of the glaze. Do you think it is
> mainly a clay body problem. The clay has no grog. Should I add some hand
> crushed grog of the same clay? 2 lbs to 25 lbs (10%) or what? Do I need
> to fire slower, I already fired very slowly?
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