Ben on thu 16 sep 04
Had a failed reduction firing last week. Kind of puzzled over it. Yeah =
I know Buy a probe. Probably will eventually. Anyway.
While unloading, i dropped a post and broke a handle on a mug. No big =
deal. the center of the handle was a medium charcoal color. Same as =
when it's well reduced. This is a commercial buff.
So today I cracked the pot to look closer. At the lip it's almost grey =
all the way through, the outside has a thin layer of very lightly =
reduced to oxidized layer approx 1mm thick and on the inside it's about =
2mm. The grey varies, it seems with the thickness of the pot, but is =
continuous throughout the pot from 1-4mm thick.=20
So what I'm wondering is: could this be evidence that I had a good body =
reduction early (typical shino gas firing schedule) and then did not =
maintain a sufficiently reducing atmosphere in the balance of the firing =
resulting in the reoxidation (if not thorough) of the body which =
subsequent top end reduction couldn't help? I had shino's and copper =
reds and everything was totally flat. The various details of my =
management of this firing (flame damper pressure etc.)make me think it's =
quite possible it was oxidized in the middle.=20
Thanks for any input,
Ben
David Beumee on fri 17 sep 04
Hi Ben,
An oxy probe has allowed me to know the amount of reduction throughout the firing, and has greatly "reduced" the number of surprises and eliminated firings that are entirely ruined because of incosistant reduction. I also use the oxy probe in bisque firings to make sure I have good oxidation throughout the eight hour cycle.
I begin reduction in a glaze firing early, cone 012, and keep the atmosphere in medium reduction until cone 9 falls. Then I pull out the damper and/or slow down the gas pressure to bring the atmosphere into straight oxidation until cone 10 is down.
I find this schedule is great for copper reds, celedons, shinos, everything. For someone who depends on sales of pots to pay the bills and keep food on the table, the expense of an oxy probe paid for itself the first firing, more than a hundred firings ago.
David Beumee
Earth Alchemy Pottery
Lafayette, CO
> Had a failed reduction firing last week. Kind of puzzled over it. Yeah I know
> Buy a probe. Probably will eventually. Anyway.
> While unloading, i dropped a post and broke a handle on a mug. No big deal. the
> center of the handle was a medium charcoal color. Same as when it's well
> reduced. This is a commercial buff.
> So today I cracked the pot to look closer. At the lip it's almost grey all the
> way through, the outside has a thin layer of very lightly reduced to oxidized
> layer approx 1mm thick and on the inside it's about 2mm. The grey varies, it
> seems with the thickness of the pot, but is continuous throughout the pot from
> 1-4mm thick.
>
> So what I'm wondering is: could this be evidence that I had a good body
> reduction early (typical shino gas firing schedule) and then did not maintain a
> sufficiently reducing atmosphere in the balance of the firing resulting in the
> reoxidation (if not thorough) of the body which subsequent top end reduction
> couldn't help? I had shino's and copper reds and everything was totally flat.
> The various details of my management of this firing (flame damper pressure
> etc.)make me think it's quite possible it was oxidized in the middle.
> Thanks for any input,
> Ben
>
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Ron Roy on sun 19 sep 04
Hi Ben,
That is black coring - you are right about how it happened. You slipped
back into oxidation before the glazes - or some of the glazes sealed the
surface so some of the clay got partially reoxidized
Generally you want to maintain at least some reduction till near the end of
the firing.
If there are certain parts of the kiln that suffer from this more - direct
flame there.
I would expect it to happen more at the bottom of a kiln - because the heat
rise is usually slower there.
RR
>So what I'm wondering is: could this be evidence that I had a good body
>reduction early (typical shino gas firing schedule) and then did not
>maintain a sufficiently reducing atmosphere in the balance of the firing
>resulting in the reoxidation (if not thorough) of the body which
>subsequent top end reduction couldn't help? I had shino's and copper reds
>and everything was totally flat. The various details of my management of
>this firing (flame damper pressure etc.)make me think it's quite possible
>it was oxidized in the middle.
>Thanks for any input,
>Ben
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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