Jon Pacini on wed 13 oct 04
Greetings All,
Officially Laguna recommends ^04 for bisque firing all the clays we make.
The reasoning being, with a hot bisque you increase the strength of the ware
and you have hopefully been able to burn out all the nasties that cause such
things as glaze pin holing. Yet this temperature still allows the ware to be
porous enough to accept a good coating of glaze.
If you wish to bisque at temperatures as diverse as ^012-06, you can of
course do it quite successfully, lots of potters do, I do myself. The
biggest draw back is that if the ware is fired rapidly it can retain much of
the organics that cause pin holing during the glaze firing. This is a quite
common fault. The next most common fault I see with soft bisque is dunts.
The body just doesn t have the strength to over come the thermal stresses
during cooling and cracks can occur.
My co-hort Joe Koons has a saying tattooed on his chest that says, bad
bisque ware equals bad glaze ware . By firing hotter you lessen the chances
of various faults.
In my own bisque firings, which are in an electric kiln, I like to sneak up
on the terminal temperature. I increase the temperature only 25*f an hour
for the final 2 hours. This allows everything in the kiln to soak up the
heat, hot and cold spots are minimized. Radiant heat in electric kilns is by
definition very directional. A phenomenon I like to refer to as shadowing
occurs and the faster you fire an electric kiln the more pronounced it is.
Also the larger the diameter of the electric kiln, the more pronounced it
is. A pot in direct line of sight to an electric element gets hotter than
the one in the shadow of, or nesting in, another pot. It s just like the
difference between standing in the sun or in the shade.
Thermocouples and kiln sitters tend to be installed on the perimeter of a
kiln, which in electric kilns is the hottest place. This can mean that your
monitoring devices are not always reflecting what is actually occurring in
the kiln. You can test this in your own kiln by putting a set of cones on
the edge of the shelf next to the element near the thermocouple or kiln
sitter. Put another set of cones on the bottom of the kiln in the center of
the shelf surrounded by pots. See what the differences are. If there is
little variation in the fired cones, you have a very well engineered
electric kiln. If you get a big variation, you ve found the cause of some of
the faults you ve been getting.
Best regards
Jon Pacini
Clay Manager
Laguna Clay Co
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