tomsawyer on sat 2 jun 01
I always bisque fire in my Olympia electric kiln and save my large =
combination gas/electric for glaze firings. In the past couple of weeks, =
however, I developed a real backlog of green ware which I decided to =
fire in my larger gas/electric. I turned the electric on overnight and =
in the morning with the temperature reading 1650 C, I turned on the gas =
burners and completed firing in 20-30 minutes [full oxidation]. When I =
opened the kiln the coloration of the bisque pieces was different that =
usual. Usually, the body is a warm buff and indeed several of the pieces =
were the expected color but several were a combination of buff color and =
stark white. Where did the white come from? The only difference from my =
usual firing was the addition of gas for the last 20-30 minutes with the =
flue wide open. If the color change was due to the gas - i.e. reduction =
why wouldn't the pieces be darker? indeed in reduction the clay body =
does darken. I'm not concerned about the glaze firing as I expect the =
glaze and refiring to even out the color. I'm mainly just curious as to =
what is going on. Thanks for any help.
Tom Sawyer
tsawyer@cfl.rr.com
Craig Martell on sat 2 jun 01
Hello Tom:
It's the flue. Your normal bisque in the electric is a static condition
with regard to air flow. With the flue open in your electric/gas you had
more of an oxidizing atmosphere.
regards, Craig Martell
Snail Scott on sat 2 jun 01
Back when I was in charge of firings at the university,
we bisqued all the student work in the big gas kilns.
(We only had three shaky electrics, reserved for those
pieces which tolerated absolutely no reduction (low-fire
reds and the like.)
I never got close to an actual reducing atmosphere (no
probes to tell for sure, though), but every piece that
ever came out of those kilns was a slightly different
color from every other one, and it did seem tied to
the position within the kiln.
It's my belief that you don't get the 'reduction look'
(darkening of the iron content) 'til around ^1 or so
at the lowest, and the degree of the effect increases
with temperature; the clay just doesn't react that way
until it approaches stoneware/vitrification temps. Even
when a bisque/earthenware firing is run with a heavy
reduction (usually owing to 'pilot error', not intent)
it doesn't look 'reduced'. (You might end up with vicious
carbon-coring when the stuff is finally high-fired; that's
about the only way to tell, that I know of.) Even without
an actual 'reducing' atmosphere, though, the atmosphere
will vary, being more or less oxidizing. There's always a
bit of variation when combustion is involved. No harm to
the work; just looks kinda odd, that's all.
-Snail
At 10:35 AM 6/2/01 -0700, you wrote:
>I always bisque fire in my Olympia electric kiln and save my large
combination gas/electric for glaze firings. In the past couple of weeks,
however, I developed a real backlog of green ware which I decided to fire
in my larger gas/electric. I turned the electric on overnight and in the
morning with the temperature reading 1650 C, I turned on the gas burners
and completed firing in 20-30 minutes [full oxidation]. When I opened the
kiln the coloration of the bisque pieces was different that usual. Usually,
the body is a warm buff and indeed several of the pieces were the expected
color but several were a combination of buff color and stark white. Where
did the white come from? The only difference from my usual firing was the
addition of gas for the last 20-30 minutes with the flue wide open. If the
color change was due to the gas - i.e. reduction why wouldn't the pieces be
darker? indeed in reduction the clay body does darken. I'm not concerned
about the glaze firing as I expect the glaze and refiring to even out the
color. I'm mainly just curious as to what is going on. Thanks for any help.
>Tom Sawyer
>tsawyer@cfl.rr.com
>
Wade Blocker on sat 2 jun 01
Tom,
The reason you did not get the color change in the bisque firing is
because the clay was never subjected to a reducing atmosphere. Mia in hot
ABQ
tomsawyer on sat 2 jun 01
Snail,
You wrote
"I never got close to an actual reducing atmosphere (no
probes to tell for sure, though), but every piece that
ever came out of those kilns was a slightly different
color from every other one, and it did seem tied to
the position within the kiln."
and
"It's my belief that you don't get the 'reduction look'
(darkening of the iron content) 'til around ^1 or so
at the lowest, and the degree of the effect increases
with temperature"
If your statement is correct and I have no reason to imagine it is not, why
was I taught to start reduction in a gas firing at about 1650 F? Nils Lou
mentions in his book that 1650 is the temperature to begin reduction.
Somewhere someone told me that body reduction comes between 1650 F - 1850
and that reduction changes in glazes occurs a little over 2000 or around
Cone 1. What puzzles me is when gas is introduced in a bisque fire why
doesn't the body darken [since the gas would use more oxygen than in an
electric kiln] if body reduction occurs below the bisque fire temperature
[assuming 1650-1850 is correct]? One person responded off line saying that
the color differences were due to some small amount of reduction that occurs
when gas is introduced. Heck, the older I get the more confused I get.
Actually I'm getting worried with all this over 60 talk. I sometimes do
capacity examinations for the Court in guardianship cases; what is really
frightening when giving an exam to an alzheimers patient is to forget the
question you wanted to ask.
Tom Sawyer
tsawyer@cfl.rr.com
tomsawyer on sat 2 jun 01
Graig,
In reply to my question why my bisque pieces were whiter in a gas firing
than electric you wrote:
It's the flue. Your normal bisque in the electric is a static condition
with regard to air flow. With the flue open in your electric/gas you had
more of an oxidizing atmosphere.
Damn, now this is an answer I can buy into. Who said sometimes were wise and
sometimes otherwise?
Tom Sawyer
tsawyer@cfl.rr.com
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