Harvey Sadow on wed 29 jan 97
Betty Burroughs wrote:>
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Hi Betty,
It seems to me that the primary issue in choosing a clay body for raku
is the kind of work that you are firing. If you make thick walled
pieces, you will very probably wish to work with a very open, grogged
body. If you work flat, a kyanite body will significantly cut down
warpage and cracking, and will frankly aid in raku firing almost
anything (though it does affect the working properties of the clay). If
you want the clay to be super smooth and slick, or if you are using
transparent glazes, porcelains or talc bodies are effective, though they
tend to work better for small pieces. I have to admit that my
experience parallels Vince's with regard to talc bodies (low temp white
clays). No firing problems, only nice glaze results. White
earthenwares tend not to lend themselves to my forms, however, they are
too flacid. I read the description of the soon to be released Plainsman
Raku 97, and for the following reasons, it is a clay I want to try.
In the late sixties I began experimenting with the concept that tightly
compressed, thinly thrown vessels would acquire and release heat very
evenly and quickly, creating less stress on the object. I reasoned that
a smooth stoneware body might allow me to make hyperexpanded, thin
walled, largish vessel forms; fire the pieces very quickly, and cool
them very quickly. As I worked out my dry surface color fields (you
now call them copper mattes, crusted glazes and such), I became aware
that controlled cooling and reduction/oxidation sequences were critical
to color development and tendency. This privided more incentive to be
able to release heat quickly. I developed a body based on plastic
stoneware clay, ball, expansion lowering materials and a very small
amount of fine grog. Around 1980, after I abandoned copper mattes, I
was able to use the same body to do multiple firings, as I fired on
layers of surface, sandblasted into the layers and fired on more layers.
I went from using my own grog/sand-free stoneware clays to using ROD'S
BOD, a stoneware clay originally made by Westwood Ceramics, now made by
Laguna. While the recipe seems to have changed from a slightly grogged,
creamy colored clay to a somewhat grittier, greyer clay, it still works
fairly well. I do my bisquit firings at ^05. I am currently working on
a new body and will keep you posted.
I think that most raku work cracks less because of the clay body than
because it is set onto a hot kiln shelf and expands at the bottom before
the upper walls and lip get hot, or because it is brutalized during
removal from the kiln or because it is poorly made. Simple things like
putting a layer of soft brick onto the kiln shelf helps tremendously.
I designed tongs which allow me to lift work from the bottom, instead of
by the lip. This cut down lip cracking considerably. Kilns which heat
evenly and quickly minimize cracking, too. I fire most of my pieces in
3-15 minutes, having put them into a kiln already at 500-600 degrees
Centigrade. When I do workshops, I often make or suggest small changes
in the raku kilns used at the host institution, and suddenly people are
firing faster with less cracking. This is not to say that the clay
makes no difference. I recently tested a body for a manufacturer. I
loved the throwing properties of the clay, but every piece I fired split
in half, though most of the clays I have tried over the years have fired
just fine in raku, commercial bodies and homemixed, if the kiln was
right and I did not beat up the pots with the tongs.
Have fun and try them all, Harvey Sadow
Dannon Rhudy on thu 30 jan 97
I must agree that the clay body seems to have less to do with
cracking/not cracking than other issues. I've used nearly
everything from porcelain to very coarse stonewares, and not had
substantial cracking problems, if: the pieces are well made,
reasonably thin, fairly even. I have made some special tongs
for lifting large forms from the bottom; even well-made thin
work will crack at the lip when lifted there if the piece is
large - too much weight, clay at fragile state. I used to
lift the pieces out with gloves, but got tired of melted
eyelashes, just made some tongs that slide under, cradle. Made
a slightly different pair for very tall slender pieces. The
other critical issue, I again agree, is not setting the work
or a blazing hot kiln shelf. I always put a fresh cold brick
on the shelf to place work on; use hard brick or soft, doesn't
seem to matter just so bottom of pot does not contact shelf
immediately. Things seldom crack, even student work which may
be thick or clumsy seldom breaks. We only use one clay body
in class, so it is a general body for throwing, not much grog;
works fine for raku.
Dannon Rhudy
potter@koyote.com
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Margaret Arial on fri 31 jan 97
what are some receipes for home made clay bodies in use that have nationally
available ingredients?Has anyone receipes to share on bodies any temp range
and if the cost of materials is known that too would be of interest.
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