Sandra L. Sumpter on thu 11 jan 07
Hi fellow clay artists! I'm new to this site and have found it to be a
wealth of information. 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? Thanks for all the info.!!
Snail Scott on fri 12 jan 07
At 01:27 AM 1/11/2007 -0500, Sandra wrote:
>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?
A clay that is properly vitrified at ^10
may be a little porous at ^8, but yours
may work out OK; you'll have to find out.
You may have trouble finding a commercial
clay that vitrifies well at ^8. Test the
clay to be sure.
Most kilns rated for ^8 will actually hit
^10, but they will take a while and wear
out their elements much quicker.
A big reason for the popularity of ^5-6 is
that just that slight drop in temperature
will greatly increase element life by
shorten firings a lot. Each added cone
approaches the point of equilibrium where
the kiln can't pump heat in any faster than
it radiates out through the insulation.
(That's why temperatures can always rise
fast at first but then the rate slows down
as it gets hotter.) So, since you are
starting a whole new 'thing' - you'll
need new (or modified) glazes and maybe a
new clay - why not consider a change to
mid-range stoneware? ^5-6 clays are
readily available from most suppliers.
Another point: Was your class firing to
^10 in a gas kiln, or an electric? I ask
because most people who fire to ^10 do
so in gas kilns, and take advantage of
the reduction atmosphere that gas makes
possible. If you are accustomed to this,
you will find that your results will be
very different in an electric kiln with
its oxidizing atmosphere, whether at ^10,
or ^8, or ^6. If you were already firin g
in oxidation before, then this is a non-
issue, but of not, be prepared for the
change. Oxidation has real potential,
especially for producing bright colored
glazes, but clays will fire paler and
less brown. Since those bright colors
are even easier at ^6, that's another
reason many folks make the switch to
mid-range temperatures when they fire
in oxidation.
Whether you continue to fire high, or
to ^6 or lower, ^07 is a fine bisque
temperature. It really doesn't matter
very much, regardless of the firing
temperature of your clay. The only
time it might matter a lot is if you
are using low-fire commercial glazes,
which can burn out unless the clay is
bisqued to at least ^04.
-Snail
Ben Shelton on fri 12 jan 07
I like to bisque to cone 04. it burns out more organic matter from the clay.
Any change in time or top temp will affect your clay body as will a change
in firing atmosphere. If your class was firing in reduction and you fire in
oxidation in an electric kiln, you will notice differences. Check with your
clay supplier and see what they say about the clay bodies you are using. Ask
what the absorption rates are at both temps. If the absorption rate at cone
8 does not go up a lot compared to the cone 10 absorption it should be fine.
A word about max temps and electric kilns. If your kiln is rated at cone 8
max. that is like a car having a max speed of 120 MPH on the speedometer. It
will do that fast but it will get used up faster. Same with your kiln. The
more time it spends at the max temp the faster the elements will wear out.
In addition, if this kiln is at your home you may not have enought
electricity to power it to cone 8.
I would try to fire nearer cone 6. Less wear and tear and there are many
more clays and glazes specifically formulated for this temp.
Like me you can now make pots and bisque at home, then fire them at class.
This way your creativity is freed from the class schedule. You can make
things whenever you have some free time.
Enjoy your new kiln.
Ben
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