Firing Schedules
peter pinnell on tue 26 mar 96
Hi David,
From my experience, a slow cooling actually helps copper reds, making
them deeper and more transparent. The huge kilns that the Chinese used
to produce copper reds during the Ching Dynasty took WEEKS to cool, not
days.
I would guess that the reason you’re not getting red is that either you
are not beginning your reduction soon enough, or there is too thin a
glaze application. Either of these can result in the result you described.
Firing and application requirements vary depending on the formula of the
glaze, but generally I would say that reduction should begin by about
Orton cone 012. This is quite a bit earlier than is required by most
glazes, but that is because these high alkalie, low alumina formulas
begin to flux at a much earlier point.Once these glazes melt, they are
hard to reduce. The spaces in the glaze matrix are just too small for
the reduction molecules to penetrate, kind of like trying to push
basketballs through a screen door.
During the firing, a copper red separates into several distinct layers.
Between the body and the glaze a layer of anorthite and wollastonite
crystals grow that provide a white backround for the glaze. Above that
is a red layer that is made up of a liquid/liquid phase seperation:
globules of red glass floating in a transparent glaze. Above it all is a
thin, clear layer. If the glaze is applied too thinly, the first layer to
go is the red one. It is possible to produce even more layers by adding
a small amount of zinc, which adds tiny orange flects, and titania, which
pushes the glaze towards blue.
Are you using the same kind of burners that you used before? Are you
measuring reduction? It could just be that you are not getting as much
reduction as you think.
Good luck.
Pete Pinnell
P.S. I had the best luck with copper carbonate, in a ration of one to
three with tin oxide. ( .33 % copper carb to 1.0% tin oxide)
Linda Arbuckle on thu 20 nov 97
Pete Pinnell gave an excellent workshop at ClayFactory in Tampa several
years back, and fired a bunch of copper reds at U to Tampa. He
demonstrated that his method of early reduction (beginning at cone 010),
then neutral up to almost temperature followed by at least 20 minutes of
oxidizing produces beautiful copper reds. What I was taught to do, a
“glaze reduction” at cone 9 or 10, didn’t matter and just wasted fuel.At UF we have a small gas test kiln, so that I could try this at home
cautiously and see if I could get the same results as a “trained
professional” in copper reds. Worked fine. As Pete pointed out, thick
enough glaze application and a good glaze are also needed, but not a
second reduction.Linda Arbuckle
