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hawthorne bond clay users (cone 10)

updated sat 30 jun 01


Tom Wirt/Betsy Price on fri 29 jun 01

We've been having problems with clay bodies based on use of Hawthorne
Bond least that's what we've isolated the suspect down to.

Glazes which are low in silica, such as the infamous Moonlight Rutile
blue, pinhole and crater-badly. We have 4 different glazes exhibiting
the problem and 3 that don't. For us, it seems to be worse in
reduction, but it still happens in very light reduction or even slight
oxidation. And it can be spotty in a given kiln load. 5% to 25% of
the load.

We've done all the usual to get rid of the pinholes...long, oxidized
bisque....soaks during and after the firing (increased amount of
craters and pinholes).

One interesting clue is that upon re-glazing and refiring, the
pinholing and cratering is generally it seems to be a high
temp reaction.

Current suspect is that the Hawthorne (35 or 50 mesh) contains some
iron sulfite which is gassing at high temp. We just tried a heavier
body reduction firing than normal to keep the sulfides from becoming
sulfates, but no help...actually the worst firing in several months.

The clay company says that no one else is having these problems,
although we know of at least 2 others who are. We're about to switch
to another fireclay and see what happens. My initial suspect was the
grog...but we switched from Christy Minerals grog (Hawthorne Bond
calcined) to Ione grain, with no change so the only common ingredient
left is the HB.

I sure like to hear if any other are having similar problems and what
clay you're using.

Tom Wirt Clay Coyote Pottery