Jeff Tsai on wed 27 mar 02
Wow, lots of questions after your original post.
I read it, but felt Vince's advice was better and more exact than anything I
could give but here's my two cents.
Try creating a loose saggar around your pieces in the pit. Large pot shard
that loosely cover the surface or even large metal boxes with lots of holes
punched in. You loose some space in the pit, but the saggar might help trap
the heat so that the piece heats more evenly. Put wood in with the saggar so
that you'll still get the effect of flames moving over the surface.
Mullite does shine a little, but you might have seen mica in the clay which
can also, as far as I've seen, be used to temper a clay.
I am confused by the fireclay substitute to grog. Test the fireclay, cut out
a ten CM slab and fire it up to your bisque temperature. If it shrinks much,
I don't see why it would be used as grog. Also, I don't see how it would help
in even heating.
I don't know what your pit fired work looks like. Do you like a smooth
burnish, if you do, well, your problems will only increase as the use of
heavy particles will make everything harder to smooth. But try some finer
grog. Volcanic ash is used by some native groups as temper. More talc could
help also. I've also known people who fine very fine sawdust and add it to
their clay. This burns out at bisque, but is supposed to help in later
firings with even heating.
keep your pieces as low in the firing as possible. If your pieces on the side
were lower in the pit because they are taller than they are wider, this might
explain some of the less prominent cracks. I generally see pieces crack in
pit after the wood has burned down significantly because too much of the
piece is exposed to open air and that part cools too quickly. My best
suggestion would be to keep the big pots as burried in coals as possible.
Maybe pile more wood in sections you know to have more large pots so as the
coals burn down, initially, there is more coverage.