Snail Scott on thu 15 feb 01
At 09:00 AM 2/15/01 -0800, you wrote:
>Lili wrote:
>
>3. AND I want to fire to the appropriate c.6 a couple of sculptural pieces
>and then pit fire them. I tried sawdust-firing ^6 pieces that were
>glazed on the inside only. I got some nice effects, but the pots cracked.
>
>Mary
I had a friend who did this: She used a claybody
with lots of kyanite, to reduce the thermal shock.
The smoke didn't penetrate the way it would with a
more porous body, but she did get some effects.
-Snail
Mary Lynch on thu 15 feb 01
Lili wrote:
3. AND I want to fire to the appropriate c.6 a couple of sculptural pieces
and then pit fire them. I have been told that a proper c.6. clay fired to
proper c.6 will not respond to pit firing because the surface wouldn't
soften enough to absorb the effects. Has anyone tried a light spray of
borax, or bicarb or something like that?
I'm interested in this as well - I tried sawdust-firing ^6 pieces that were
glazed on the inside only. I got some nice effects, but the pots cracked. I
was told that clay fired to that temp can't take pit/sawdust firings because
there's no room left for expansion/contraction in the clay body - it's
vitrified. True?
I have used ^6 clays that were only bisqued very successfully in sawdust
firings, but w/o glazes, terrra sig only - I would be interested in finding
out a way around the physics of this.
Mary in TN (I know, I know, always taking the difficult path rather than
just doing what I am supposed to do; stubborn, too!)
Cindy Strnad on thu 15 feb 01
Hello, Mary.
I've thought and thought about how to make ^6 electric pots take on smoke in
a pit firing. But I'm afraid it's an oxymoron--you know, like "military
intelligence" and "Internal Revenue Service". The reason smoke-fired pots
take on smoke (carbon) is that they're still porous. If you bisque them, you
bisque to maybe ^010 or even ^020. This helps them to withstand the fire,
but of course it doesn't make them all that strong.
Vitrified pots, on the other hand, will not permanently take on smoke much
better than glass will. You can use glazes in pit firing, but if you do,
it's just going to serve the purpose of decoration, because these glazes
won't be suitable for food, and they're unlikely to hold water, either.
(Though they might--for a short time.) But I've seen pictures of this, and
it can be rather attractive.
However, as you're stubborn and accustomed to taking the difficult path,
maybe you'll be the one to lead the way in this. If you come up with
something, I'd be very pleased to have you share it with me.
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
earthenv@gwtc.net
http://www.earthenvesselssd.com
STVC on thu 15 feb 01
Mary,
I often subject fairly large vitreous or partially vitreous pieces to
significant thermal shock and multiple (up to 7) firings. One must accept a
higher failure rate when working this way. Thermal shock can be reduced by
slow, even heating and cooling--hard to do with larger pieces in pit
firings.
Other than firing factors, the clay body is critical. Unfortunately, I
haven't found a ready mixed body that works great for me. Soldate 30 comes
about as close as any, better than some so called sculpture bodies. I
posted my favorite sculpture body a while ago, and will email it to you if
you are considering a custom mix/pug, and find the archive search unwieldy.
On the technical side:
Thermal expansion must be minimized. Critical range is through 1063 degrees
Ferenheit--the infamous "quartz inversion" phase. The lower the shrink
percentage, the better. Substitution of a combination of pyrophyllite and
"Sierralite" (low thermal expansion talc) for a portion of feldspar helps.
I use wollastonite in my sculpture bodies. I do not use whiting at all.
Wollastonite makes a strong body resistant to thermal shock. Spodumene and
petalite with their lithium-alumina-silica combinations also reduce thermal
expansion. I've used them with fair results in test batches I mixed myself.
A mix of grog and sand sizes seems also to help.
I have successfully fired large *partially vitrified* tiles--notorious for
differential expansion/contraction--in combustibles, and also subjected them
to "raku" processes with an acceptable failure rate. (My acceptable failure
rate might be another's clay apocalypse.) Failed tiles make for good
nesting/shielding in pit/mound firings.
Hope this helps...
Steven Van Cleave
Vista, CA
stvc@home.com
-----Original Message-----
From: Ceramic Arts Discussion List [mailto:CLAYART@LSV.CERAMICS.ORG]On
Behalf Of Mary Lynch
Sent: Thursday, February 15, 2001 9:00 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Cone 6 refired in pit/sawdust
Lili wrote:
3. AND I want to fire to the appropriate c.6 a couple of sculptural pieces
and then pit fire them. I have been told that a proper c.6. clay fired to
proper c.6 will not respond to pit firing because the surface wouldn't
soften enough to absorb the effects. Has anyone tried a light spray of
borax, or bicarb or something like that?
I'm interested in this as well - I tried sawdust-firing ^6 pieces that were
glazed on the inside only. I got some nice effects, but the pots cracked. I
was told that clay fired to that temp can't take pit/sawdust firings because
there's no room left for expansion/contraction in the clay body - it's
vitrified. True?
I have used ^6 clays that were only bisqued very successfully in sawdust
firings, but w/o glazes, terrra sig only - I would be interested in finding
out a way around the physics of this.
Mary in TN (I know, I know, always taking the difficult path rather than
just doing what I am supposed to do; stubborn, too!)
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