Lili Krakowski on fri 12 jan 07
" I just bought an electric kiln that will fire up to
^8. In my ceramics class we fired our bisque to ^07 and our glaze to ^ 9 or
^10. I was wondering if you can use the same clay bodies that are used for
^10, if you are firing glaze to ^8? Also, should I fire the bisque at a
different ^ or is ^07 OK? "
The question comes round almost as often as "Why do birds sing in the
Springtime" .
1. Clay bodies are formulated to fire to a range of temps/cones. The
reason being that
large kilns, fuel burning kilns esp. will have cooler and warmer spots. An
expert firer
of a large kiln will know just what spot to put which glaze in to get a
desired result.
So producers of clay bodies will say in the catalog: C.4-8, or c.6-10 and
like that.
There also will be shrinkage and absorption figures. If you have ever
cooked a pot roast
you know there is this relationship, AS you cook the potroast LONGER is
shrinks MORE
but becomes more tender. There is a similar business with clay. When you
fire higher.
shrinkage increases, while absorption decreases.
The manufactures have worked out the kinks. I would buy a clay body that
is
for my firing temperature. Schools often need to have clay bodies that go
from here to
there, but the individual potter does not. I would find a clay body that
is for c.8, or, to put it another way, where shrinkage is no more than 12%,
and absorption
no more than 1.5% (The last my personal opinion, criteria, etc.
So as I have no idea what your school used...I cannot tell you what it will
do
at c.8.
HOWEVER! The rule used to be that you never fire an electric kiln HIGHER
than
2 cones BELOW what it is listed as. This I think was to not push the
elements
beyond their endurance, and to allow some elbow room in case one overfired.
PLEASE CHECK THIS with one of the Kiln Experts....
Now there was another question, this one about specks. Reminding one of
Teresa
of Avila's great dictum that more tears are shed over unanswered prayers
than
over answered ones.
I will leave aside consideration of how dangerous granular manganese is in
the
firing. One always can use iron for specks. BUT: once specks have been
introduced into the studio, they will not
leave! Just be aware of that. They remind me of all kinds of animals, esp.
insects, that come into the country accidentally or because someone
brought them in as pets, or like that. And then, the import takes over,
destroying native
flora and fauna.....Unless you REALLY TRULY can clean up your studio
totally between kinds of clay....expect the day when you have made something
special, meaningful, an order, all that.....and there in the middle of
a perfect plate--two little dark, blurry spots.....
(I will now sit back and listen to the sarcasm of those who think
I am an alarmist.)
Lili Krakowski
Be of good courage
Ron Roy on sat 13 jan 07
Some manufactures have worked the kinks out - but most still sell clays
they say have a range of 5 cones - like from cone 6 to cone 10. If you are
looking for a clay that will not leak but have that range - well I would
like to see that.
In the end the best policy is to fire any clay you are going to use - to
the temperature you want to fire at - and test for shrinkage and
absorption. And do it for each batch of clay you buy. It's easy - it takes
little time - if you need to know how I'll send instructions.
Some clay makers test each batch - if they do just ask for their results -
if they are doing it they will be glad to send you the results. You still
should test your clay as you buy it - you will be surprised how much that
will help you understand problems - and who is testing their product.
Any clay company that tests their clay will do it according to firing time
by the way - it will be helpful to know that - in case your firing time is
different.
I think Lili is giving good advice about 1.5% absorbency - for stoneware
clays - closer to none for porcelains though. 12% shrinkage for a throwing
or hand building clay seems reasonable - lower than 12% shrinkage and
workability suffers - over 13% shrinkage and cracking and warping may
become a problem.
RR
Lili said,
>The manufactures have worked out the kinks. I would buy a clay body that
>is
>for my firing temperature. Schools often need to have clay bodies that go
>from here to
>there, but the individual potter does not. I would find a clay body that
>is for c.8, or, to put it another way, where shrinkage is no more than 12%,
>and absorption
>no more than 1.5% (The last my personal opinion, criteria, etc.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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