Harry Peery on sun 24 oct 04
H'lo everyone,
I have a question for those of you that go out and dig for your own clay.
Someone in our little pottery class has done this on their family farm in
Alabama. It's the most beautiful yellowy-ochre colour - much different than
the 901 stoneware I'm using presently. It seems to be very fine and smooth.
My question is this. How does one determine at what temp to fire the stuff
they dig out of the ground? Do you just run a few firings, experimenting? If
so, and you only have a limited amount of clay, how do you decide without
reducing the already limited supply on hand? Does the decision have to do
with how the clay handles, the elasticity, amount of 'grog' or sand? Or can
you tell by its colour what the composition is and how to handle it in the
kiln?
Thanks,
Sue
Earl Brunner on sun 24 oct 04
First, put a chunk in a test pot (to catch it if it melts) and see what happens at high temperature. If it survives then you can do a porosity test to see how vitrified it is. If it melts, you can do the following.
My college professor used to do a lot of experimenting with local clays. he would make up a bunch of cones of the clay he was testing and then make a big cone plaque that started with low cones and ended up at the highest temperature you are going to test to. He would then place the cones made from the test clay next to them so that they were side by side and angled so that they would fall acrossed each other. (kind of like a melted cone braid) You could then tell between which cones the stuff melted at by seeing which ones the yellow clay fell down between.
Harry Peery wrote:
H'lo everyone,
I have a question for those of you that go out and dig for your own clay.
Someone in our little pottery class has done this on their family farm in
Alabama. It's the most beautiful yellowy-ochre colour - much different than
the 901 stoneware I'm using presently. It seems to be very fine and smooth.
My question is this. How does one determine at what temp to fire the stuff
they dig out of the ground? Do you just run a few firings, experimenting? If
so, and you only have a limited amount of clay, how do you decide without
reducing the already limited supply on hand? Does the decision have to do
with how the clay handles, the elasticity, amount of 'grog' or sand? Or can
you tell by its colour what the composition is and how to handle it in the
kiln?
Thanks,
Sue
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Earl Brunner
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Bob Masta on mon 25 oct 04
I can't improve on Earl's excellent advice, but
I'd just like to point out that you should keep
an open mind about what you will use this clay for.
In my own case, my garden clay makes an OK
low-fire body, up to cone 2 or so. But it makes
a great "albany slip"-type glaze at cone 6. And
if there is a limited supply (as I infer from your
message), you'll get a lot more mileage out of
it as glaze than body.
Also, my clay had bits of lime
inclusions that weren't obvious until they
caused lime pops when used as body clay.
Sieving at 80 mesh solved that, but it's a *lot*
of work for body clay (needed to be dried and
crushed first). That makes it all the more precious,
and makes the glaze a better use for me.
Plus, the hand-dug aspect takes center stage
as a glaze, whereas as a low-fire body clay it would
only get covered over with some other glaze...
and wouldn't be vitrified and waterproof, either.
Just a thought.
Bob Masta
potsATdaqartaDOTcom
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