Stuart Altmann on fri 18 jul 97
Joseph Herbert suggested one source of rough pot bottoms: "gunk," picked up
from drying or kiln shelves or from runny glazes. For those who work with
stoneware, I believe a much more common source is differential shrinkage of
clay ingredients. In particular, grogs, which are added to almost all
stoneware clays, are made of crushed clay that has been previously fired, at
least to bisque temperature. This means that the grog particles have
already undergone (some of) their fired shrinkage before you use them. So,
when you fire your pots, the matrix of clay in which the grog is imbedded
shrinks more than the grog. And the tiny protrusions in the rough
surface are the color of your grog, not your clay, right? So, no matter
how finely you smooth the bottom of the feet as you trim, they come out of
the kiln rough. I also suspect that fire clay shrinks less than plastic
clays with which it is mixed and may account for some of the roughness.
Stuart Altmann
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