John Tilton on thu 2 mar 00
My take on early reduction is that the glazes sinter at a very early
temperature and if you do not get some reduction before that time, you
will not get the full reduction that you are looking for. How can the
reduction get through melted glass? It is in this early period of the
firing that the most resuction gets done and this is THE most important
time. Heavy reduction at the end of the firing is basically a waste of
fuel.
That said, the first time that I cleared the kiln for an hour at the end
of the firing, I was sure that everything would be reoxidized. It
wasn't.
John
--
John Tilton
16211 NW 88th Terrace
Alachua, Fl. 32615
904-462-3762
Web site: http://www.tiltonpottery.com
mailto:tilton@atlantic.net
vince pitelka on fri 3 mar 00
> My take on early reduction is that the glazes sinter at a very early
> temperature and if you do not get some reduction before that time, you
> will not get the full reduction that you are looking for. How can the
> reduction get through melted glass? It is in this early period of the
> firing that the most resuction gets done and this is THE most important
> time. Heavy reduction at the end of the firing is basically a waste of
> fuel.
Just to clarify the terminology, bisqueware is sintered. When sintering
occurs, the fluxes begin to get tacky, cementing together the refractory
particles. So a sintered surface is very porous, and reduction can easily
penetrate it. As temperature increases, the fluxes and glass-formers begin
to form a true glassy-phase, which begins to fill in the spaces between
refractory particles, eventually resulting in vitrification, which in the
case of a claybody is just a sintered structure with a fully-developed
glassy phase. When you over-fire, the glassy-phase becomes increasingly
solvent, dissolving the sintered bonds between the refractory particles, and
the mass begins to slump and flow.
I would agree that heavy reduction at the end of a firing is basically a
waste of time, but it is not true that all the essential reduction occurs
early. In order to preserve what you have accomplished at body-reduction
temperatures, you must retain a partial reduction as the kilns climbs.
Best wishes -
- Vince
Vince Pitelka
Home - vpitelka@dekalb.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Craig Martell on mon 6 mar 00
Hi:
John Tilton and Vince Pitelka say that high temp reduction is pretty much a
waste of time. I guess the premise is that a soft porous structure is
permeated quicker and easier than a glassy phase which occurs later in the
firing.
If one does a fairly heavy amount of reduction at the earlier stages of the
firing, less reduction may be required later on for the desired
results. We should be familiar with the oxides in our glazes and the
amount of reduction required to alter them. Look up Barium in Hamer and
follow his reasoning about what it takes to reduce this stuff. Anyway,
reduction requires an excess of carbon monoxide, which is unstable and
seeks oxygen to become carbon dioxide. CO2 will then liberate itself from
the glaze and, viola, you have done the reduction work. But, this can also
be accomplished during the latter stages of a firing if the work is
incomplete. When a glaze is very soft and glassy and chemical bonds have
been broken, ion exchange happens very quickly and effectively. If
reduction ceases, there may also be a chance of reoxidation.
This is my understanding of the process and I guess that I'm disagreeing
with John and Vince a little. Also, I fire a porcelain body to cone 10 in
Red. and begin reduction at cone 06, which I don't find too hot for this
body, which is still somewhat refractory at that temp. The body is reduced
well and I get fairly consistent copper reds. Maybe the jury will be out
on some of this stuff until hell freezes?
regards, Craig Martell in Oregon
vince pitelka on tue 7 mar 00
> This is my understanding of the process and I guess that I'm disagreeing
> with John and Vince a little.
Craig -
I guess I don't see where there is any dissagreement, because I agree with
what you are saying about reduction. We begin reduction a little earlier
than you do, because we are firing mostly stoneware, and because there are
often lots of shinos in the kiln. But we maintain partial reduction for the
duration of the firing in order to prevent premature re-oxidation of the
reduced effects. Perhaps "a waste of time" was unnecessarily strong
language on my part, but I did refer specifically to "heavy reduction at the
end of the firing." I'm sure there are glazes and glaze effects which
respond well to a period of reduction at the end of the firing, but ours
seem to do best with a fairly slow, gentle climbing reduction, and a brief
oxidation "cleanup" at cone 10 before shutting down the kiln and closing the
damper.
Best wishes -
- Vince
Vince Pitelka
Home - vpitelka@dekalb.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
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