Richard Mahaffey on tue 13 feb 07
Vince,
I think you are right about the solubles causing the problem.
A studio partner made porcelain pots with an albany glaze that was
applied over tape stencils. After the glaze was dry enough the tape was
removed. The soluble iron that was in the clay migrated to the area
where the water evaporated the most - yep, the area where the tape
stencils were (no glaze to hide the color). The Porcelain bisque
turned a yellow color and in high fire the marks turned dark. The
addition of a small amout of barium carbonate stopped the soluble iron
problem. Not washing or wetting the pots should stop the problem.
I wonder if the pots were wrapped with plastic while still wet, while
leaving the opening uncovered so the water had to dry through the inside
of the piece would cause the solubles to be deposited on the inside of
the pot would work...... Then a coat of Duncan Ceramic Matt Sealer on
the throughly dry piece help retain the non-yellow look.
Rick Mahaffey
Tacoma Community College
Tacoma, WA
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