Stephani Stephenson on fri 5 dec 03
Karin
It almost sounds like you have a contaminated batch of clay.
the only other thing I can think of is scumming.
Some terra cotta bodies have alkali; calcium and other salts, that
are soluble in water and that are carried to the to surface of clay
with water as the clay dries. As the water evaporates from the clay
surface, it leaves these residues behind, often concentrated in one
area, or on one surface...a lot of times these salts are deposited on
the upper part of a form or sculpture, you guessed it, which is
coincidentally a face or an extremity such as a hand, etc.!
the scum shows up differently at different temps...commonly whitish and
'scum' like, it can also begin to flux and melt as temperature
increases, becoming a shiny glazed area as the clay is fired higher.
maybe the self glazing aspects also interfere with iron in claybody
oxidizing or reoxidizing on on heating or cooling???
One way to see if soluble salts are contributing to the gray, odd areas
of coloration, is to notice how a piece dries, and then notice if the
gray areas coincide with water evaporation patterns , usually upper
side of form. I have covered forms with plastic, forcing water to
evaporate out the sides of form, and found scumming to subsequently
occur on the sides. I use to have one clay body that was really bad with
regard to scumming. I tried firing pieces to a higher temp, but the
white scum simply turned to greenish gray glassy smudges.
For awhile I turned sculpture upside down to dry, so at least the
scumming wouldn't occur on the top, center, front of a piece. Finally I
just switched clay bodies. Commercial terra cotta bodies often have a
little barium added to counteract scumming.
Stephani Stephenson
steph@alchemiestudio.com
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