Gary Navarre on wed 1 sep 04
---------------------------- Original Message ----------------------------
Subject: Re: Grey clay from soda/salt firing
From: "Paul Herman"
Date: Tue, August 31, 2004 11:30 am
To: CLAYART@LSV.CERAMICS.ORG
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Hello Nancy and Marianne,
Well, you've gotten a lot of different theories on this one, so here's mine.
"We get that blue-grey color in the woodkiln. It seems to be related to
the intensity of flame/reduction. More flame/reduction, more grey. I agree
with Tony Clennell, it's carbon trapping in the clay body. Seems it makes
the clay brittle, and easily broken."
This is exactly what I'd griped about a couple of months ago and probably
the last few decades. "Black coring" is common in institional settings. I
reference "Ceramic Science for the Potter", W.G.Lawrence 1972 p.122-23:
#3 Defects Due to Kiln Atmosphere. "Probably the most common defects
caused by kiln atmosphere are "black coring" and "bloating. They are
closely related. As has been briefly mentioned, it is impoetant that
excess oxygen be present in the kiln atmosphere in the 350-700 C. range.
This is necessary for oxidation and removal of any carbonaceous materials
present in the body. Although oxidation starts at 350 C. it does not
procede rapidly until 600 C. is reached. If oxidation is not complete by
the time glass formation begins, the gases generated will be traped in the
glass and bloating will result.
If oxidizing conditions are not good and if the body contains iron oxide,
another factor enters the picture and compounds the problem. The carbon
present may react with the insufficent oxygen to form CO. This gas will
reduce the Fe(2)O(3) to FeO, a powerful flux which reacts with its
siliceous surroundings to form glass below 1100 C. This results in the
formation of "black Core" in the interior of the body. Once this happens,
it becomes vary difficult to burn out the remainder of the organic
matter. It is confined in the black glass formed which greatly hinders
the diffusion of oxygen to the interior of the piece. Any oxidation which
does take place will generate gas which is confined and may result in
bloating.
These effects are most pronounced in dense, tight bodies having large
cross section. Excess oxygen and slow firing in the 350 -700C. range
eliminate this problem. If body reduction is desired, it should be done
after this temperature range is past." Most of the cups and bowls I've
fired in others' gas kilns have long since disappeared and the pieces I
fired long and slow in my wood kiln 15 years ago are still in use because
they didn't turn out brittle. Some even bounced when accidently dropped
because there was no carbon coring.
"It got to be too much, and when we backed off the stoking, there was less
blue. ...perhaps back off on the fuel a little bit....."
Good salty firings,
Paul Herman
And:
>From: Nancy Udell
>To: CLAYART@LSV.CERAMICS.ORG
>Subject: Grey clay from soda/salt firing
>Date: Sat, Aug 28, 2004, 12:35 PM
>
> Hello clay list,
> We are having some effects from soda/salt firing that we don't like,
and >were wondering if anyone has any thoughts as to the cause. We've
tried a >number of different things from time to time (and probably
because we've >often changed multiple variables, cannot consistently
get rid of the grey >effect described below).
That's my $4 worth. I'd rather error on the sturdy side than dazzling
effects. Stay in there, Nancy, you'll get the hang of it.
Gary Navarre
Navarre Pottery
Norway, Michigan, U.S.A.
Ron Roy on thu 2 sep 04
Hi Paul - or is it Gary?
Good for you for bring this up - it is not well understood and needs some
taliking about.
I have found the term Black Coring to mean different things to different people.
Hamer has an illustration - different shades of grey to black in the cross
section of a reduction fired clay - with the inside stripe being darker
then the outside clay. This can be due to two situations - the carbon not
burned out properly in the bisque firing and/or during initial reduction
during a glaze firing - during which the clay is properly reduced - all the
way through - and then patially reoxidized before the glaze seals over.
The problems are - 1. If the clay is reduced differently in the pot them
some will fire differently then the other - creating strains that will
affect the general stability of the ware - the ware should be the same
colour all the way through in other words.
2. If the difference in colour is due to trapped carbonous material -
because of improper bisque firing - then over fluxing of the body - due to
iron - redudced in the bisque firing - will result in gas production and
bloating.
So the two problems are related to firing faults and can be eliminated by
understanding the firing and atmospheric processes.
RR
> This is exactly what I'd griped about a couple of months ago and probably
>the last few decades. "Black coring" is common in institional settings. I
>reference "Ceramic Science for the Potter", W.G.Lawrence 1972 p.122-23:
>#3 Defects Due to Kiln Atmosphere. "Probably the most common defects
>caused by kiln atmosphere are "black coring" and "bloating. They are
>closely related. As has been briefly mentioned, it is impoetant that
>excess oxygen be present in the kiln atmosphere in the 350-700 C. range.
>This is necessary for oxidation and removal of any carbonaceous materials
>present in the body. Although oxidation starts at 350 C. it does not
>procede rapidly until 600 C. is reached. If oxidation is not complete by
>the time glass formation begins, the gases generated will be traped in the
>glass and bloating will result.
> If oxidizing conditions are not good and if the body contains iron oxide,
>another factor enters the picture and compounds the problem. The carbon
>present may react with the insufficent oxygen to form CO. This gas will
>reduce the Fe(2)O(3) to FeO, a powerful flux which reacts with its
>siliceous surroundings to form glass below 1100 C. This results in the
>formation of "black Core" in the interior of the body. Once this happens,
>it becomes vary difficult to burn out the remainder of the organic
>matter. It is confined in the black glass formed which greatly hinders
>the diffusion of oxygen to the interior of the piece. Any oxidation which
>does take place will generate gas which is confined and may result in
>bloating.
> These effects are most pronounced in dense, tight bodies having large
>cross section. Excess oxygen and slow firing in the 350 -700C. range
>eliminate this problem. If body reduction is desired, it should be done
>after this temperature range is past." Most of the cups and bowls I've
>fired in others' gas kilns have long since disappeared and the pieces I
>fired long and slow in my wood kiln 15 years ago are still in use because
>they didn't turn out brittle. Some even bounced when accidently dropped
>because there was no carbon coring.
>
>"It got to be too much, and when we backed off the stoking, there was less
>blue. ...perhaps back off on the fuel a little bit....."
>
>Good salty firings,
>
>Paul Herman
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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