Earl Brunner on wed 10 jul 02
This is somewhat different then what you have been talking about, but I do believe
there are differences. We fire 1027 Skutts at the art center and I think that
larger pieces often experience heat shock problems either from the heating or the
cooling in the electric's that I don't see in my gas kiln. These cracking defects,
usually don't show up until after the glaze firing. Large cracks, with glaze
melted into the cracks. We use a modified firing schedule from "Cone 6 Glazes"
for the glaze firing, we heat slower at the beginning of the firing. For the
bisque I hold it at 190 degrees for 3.5 hours before ramping up to 1800 for the
bisque.
Karen Sullivan wrote:
> Tamara...
>
> I don't think there is a huge difference between an
> electric bisque or a gas bisque, except maybe the
> size and responsiveness of the kiln.
> Larger kiln means more time to get there.
> Electric kilns are small, and maybe only 5 cubic feet of
> space to heat.
>
--
Earl Brunner
http://coyote.accessnv.com/bruec
mailto:bruec@anv.net
Karen Sullivan on wed 10 jul 02
Tamara...
I don't think there is a huge difference between an
electric bisque or a gas bisque, except maybe the
size and responsiveness of the kiln.
Larger kiln means more time to get there.
Electric kilns are small, and maybe only 5 cubic feet of
space to heat.
Candling is about removing the water from the clay.
I would candle in my little electric kiln and know
that I was ready to start climbing, when I touched the
clay.
I would reach in a touch the pieces.
Be careful if you do that....the distance
from the lid to a pot way only 6 inches and
so it was easy with no danger of touching
the elements.
If the pots were still cool, they still contained water.
The clay pots will start accumulating heat
when the water is gone, and
get so hot they will singe your finger, even with
a candling heat. So my test was always to touch the
pots to know whether I had candled long enough.
There was a difference between my thin ware and
my regular pots, the thin ware I candled for
two hours and was ready to begin firing,
the regular pots I would candle for about
8 hours. I made pieces that were two inches
thick and I candled them for a week.
Once you begin to climb...just take your time,
let the clay react to the changes, pick a
schedule and perhaps bump the kiln up every
1/2 hour. A bump is an increment that is consistent,
not too drastic, but indicates an increase of gas.
You can hear the difference.
The goal is red heat, at 1000 degrees you have
color, or the beginning of color, a red glow, and
you have traveled through the quartz inversion.
Quartz inversion is when the silica molecules are
reacting to the heat change and inverting in
shape...think of a pyramid that is expanding
and turning inside out...so that happens going up
in temp and cooling...so classically it is the
danger zone in temperature changes. So go slow,
or consistently through that zone.
At that point (color) you can turn up the gas to a full
setting and from there it's a matter of waiting for
the kiln to gain enough heat to be hot enough.
The color I look for is the same color as the glow
from the end of a burning cigarette...comparisons are
up to you. Grabbing some one who still smokes and
placing them next to the peep to compare is one way.
Using a cone. Using a metal rod to poke into the
peep and touch a pot, then pull away. If you are
at bisque temps the rod will leave a black dot that
stays for a minute then disappears. I use memory to
judge the color and just look.
An important consideration is that you want to burn
away the organic material and that doesn't happen until
about 1400, so it is important to make it to 1850 for
the firing or there will be problems with the pots later.
Also you don't want to choke the firing, so that the
organic material can escape the kiln while firing.
All other considerations I think are a matter of
developing a working relationship with the kiln.
All kilns are different, and there will be a pattern,
rhythm that will develop with time. So as with the
beginning of relationships watch, listen and learn.
bamboo karen
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