Vince Pitelka on fri 3 oct 97
> Over reduction is called "BLACK CORE" and can severly reduce
>the strength of a clay body, I have done this and it makes the work very
>brittle.
It is important to point out that "black core" is not over-reduction, but
excessive early reduction. No matter how long you glaze-reduce in high
fire, you will not get black core. Black core, or carbon-coring, results
from excessive early reduction, before normal body-reduction temperature,
which converts the iron in the claybody into a powerful flux, causing early
vitrification, sealing the carbon into the clay.
Regarding the notion of excessive reduction, the term is valid, because some
glazes and claybodies benefit from a very light reduction, and can suffer
from too much reduction. But that is a personal thing, and is dependent on
very specific reduction practices.
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
"Rafael Molina-Rodriguez (Rafael Molina-Rodriguez)" on sat 4 oct 97
Vince :
An excellent source of information on "black core" is James McKinnell's
article "Black Core and Bloating" in the Studio Potter Volume 6 Number 2.
Some of the most salient points are as follows :
Michael Cardew succinctly states : " It is caused by small quantities of
matter trapped inside the body by vitrification and can be solids (carbon),
liquid (water), or gas (CO2)." Black core is a blackening or darkening of
the interior of the body, frequently due to entrapped, inadequately
oxidized carbonaceous matter.
Althogh bloating can be due to other gases, severe carbon black core
normally results in bloating.
Bauleke states that while mild black coring does no real harm, black
coring can reduce the strength in some industrial applications - and is to
be avoided.
Wilson points out, the use of clays having a lower temperature of
vitrification and containing carbon "must be fired slowly - or the oxidation
period will extend into the sealing upon vitrification period. Red burning
clays usually give more trouble than buff burning clays, because with
buff clays a greater temperature difference exists between the oxidation
period and the vitrification period. If the carbonaceous material is in the
form of bituminous or graphite matter the difficulty of removal will be
greater."
....for thrown and hand built stoneware the bisquit cycle should be
slowed by double or triple if one has bloating or black core problems, in
order to ensure proper carbon and sulphur burn-out.
Some clays have too short a vitrification range, and too much carbon to
prevent bloating. Better to substitute another clay - or if necessary,
open it up with sand or grog.
If heavy body reduction is begun too soon in a glaze kiln (according to
Cardew below cone 05), or carried out for too long a period, bloating can
result.
In my experience, I've only encountered "black core" on a couple of
occassions. The first time was in the early nineties in a body we mixed
as a class claybody. IMO, the culprit was the OM4 Ball clay I was using,
the temp (^ 012 . Too low for any iron bearing bodies. This temp works
well for white stonewares and porcelain and , IMO, aids in color
development of copper reds, purples, and celadon blues) at which I
reduced, and the amount of reduction.
If you notice OM4 is much darker than other ball clays. I substituted
another ball clay, began reducing at a higher temp (^ 05) and shortened
the time and amount of reduction. This alleviated the problem.
I must say in a school setting it is a difficult task to arrive at claybodies,
glazes, and firing cycle that work together in a common firing. Some
students want that toasty look of a body reduced stoneware, others
prefer whiter firing porcelainous stonewares, and still others work with
true porcelain (grolleg and no ball clay or bentonite). I eventually arrived
at a firing cycle that avoided black coring in the darker stoneware bodies
some students use and glazes that meet with the students demands of
color and texture (copper reds/purples, rutile blues, carbon-trapping
shinos, wood-ash, oribe green, seafoam, woo blue-brown, kaki, oil spot
temmoku, celadon, etc.,...).
Rafael
>>> Vince Pitelka 10/03/97 09:35am >>>
----------------------------Original message----------------------------
> Over reduction is called "BLACK CORE" and can severly reduce
>the strength of a clay body, I have done this and it makes the work very
>brittle.
It is important to point out that "black core" is not over-reduction, but
excessive early reduction. No matter how long you glaze-reduce in high
fire, you will not get black core. Black core, or carbon-coring, results
from excessive early reduction, before normal body-reduction
temperature,
which converts the iron in the claybody into a powerful flux, causing
early
vitrification, sealing the carbon into the clay.
Regarding the notion of excessive reduction, the term is valid, because
some
glazes and claybodies benefit from a very light reduction, and can suffer
from too much reduction. But that is a personal thing, and is dependent
on
very specific reduction practices.
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
paul m wilmoth on tue 7 oct 97
Dear Vince,
I use to think that a clay body could not be reduced past body
reduction. Wrong!! I use to body reduce at ^08 for 45 min. back the
reduction off to a slight "climbing" reduction until I reached ^10. This
usually took 6-8hrs to reach final temp afterbody reduction. (with this
procedure I never had black core and very good glaze results) During
grad school I was trying many experiments , one of these was a dull red
heat post-firing reduction to facilitate copper reds. The end result of
this experiment (after 4 to 5 tries) was no increase in the quality of
copper red , a noticable luster on the shinos and a new found black core
problem. When I quit doing the post firing reduction the black core
went away.
I was once told that a body was too tight to receive reduction
after ^01 - I don't beleive that any longer!!
regards -- Paul Wilmoth
Vince Pitelka on wed 8 oct 97
> I use to think that a clay body could not be reduced past body
>reduction. Wrong!! I use to body reduce at ^08 for 45 min. back the
>reduction off to a slight "climbing" reduction until I reached ^10. This
>usually took 6-8hrs to reach final temp afterbody reduction. (with this
>procedure I never had black core and very good glaze results) During
>grad school I was trying many experiments , one of these was a dull red
>heat post-firing reduction to facilitate copper reds. The end result of
>this experiment (after 4 to 5 tries) was no increase in the quality of
>copper red , a noticable luster on the shinos and a new found black core
>problem. When I quit doing the post firing reduction the black core
>went away.
> I was once told that a body was too tight to receive reduction
>after ^01 - I don't beleive that any longer!!
>regards -- Paul Wilmoth
This is very interesting. I have not encountered this, but it is a pretty
specific situation. I hope that John Neely is listening, and responds to
this, being the king of reduction cooling. Although I think he stops his
reduction cooling about that temperature. Who out there knows more about this??
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
JOHN NEELY on fri 10 oct 97
Vince Pitelka wrote:
> This is very interesting. I have not encountered this, but it is a
> pretty specific situation. I hope that John Neely is listening, and
> responds to this, being the king of reduction cooling. Although I
> think he stops his reduction cooling about that temperature. Who out
> there knows more about this??
OK, Vince I'll crawl out of lurk mode and take the bait. I'm always a little
hesitant 'cause I sound like one of those guys that only knows one lick on the
guitar and plays it over and over. Last week when Ron Roy said:
> Are there those of you who think that clay can be over reduced? If so
> - I would like to understand how it can happen - which oxide or
> oxides are over reduced in order for this to happen.
I think the question was more than a little rhetorical, and the implication
dead on the mark. In most of the clay bodies that potters deal with, the only
oxides that are of much significance in this oxidation/reduction business are
those of iron and carbon. Is the carbon elemental carbon, carbon monoxide or
carbon dioxide? Is the iron FeO (black) or Fe203 (red) ? (If it is just Fe,
metallic iron, then you do indeed have over reduction - go directly to jail
and do not pass go - you're out of the realm of ceramics and oxide chemistry
and in to the realm of metallurgy - this is, however, VERY unlikely.)
At least part of the problem stems from confusion between black iron oxide and
black elemental carbon. The great American Raku Scare of the 1960's seems to
have aggravated the situation by calling what was primarily a superficial
post-firing *carbon impregnation* by the moniker "reduction." Like entropy,
fuzzy thinking seems to be the law, and the belief developed that "reduction"
in a fuel burning kiln was somehow impregnating the clay with carbon. Tain't
so! Even in a misguided attempt to create a reducing atmosphere with a soot
(read "carbon") laden flame instead of one rich in carbon monoxide (a very
effective gaseous reducing agent) it is next to impossible to get carbon into
the clay.
If there is carbon already in the raw clay (not an unusual situation) it
behooves one to burn (oxidise) it out of the body before allowing the body to
vitrify. Failing to do this can result in bloating when the carbon (a
relatively ineffective solid reducing agent) finally steals oxygen from the
surrounding clay and forms a gas (probably CO, not CO2, FWIW), which takes up
much more space than the carbon did, and forms a bubble, or bloat. Since Fe203
gives up oxygen much more readily than alumina or silica, in general, iron
bearing clays are more prone to bloating than white clays. As I understand it,
especially in fast firing where there is insufficient time for the reduction
reaction to take place, the carbon can also remain in the clay as carbon -
hence black coring in white clays - with out the bloats. I suppose that the
presence of reduced iron (FeO) in the case of iron rich clays that have
suffered this carbon core fate does serve to further confuse the issue - two
sources of black colour.
As for this business:
>During
>grad school I was trying many experiments, one of these was a dull red
>heat post-firing reduction to facilitate copper reds. The end result of
>this experiment (after 4 to 5 tries) was no increase in the quality of
>copper red , a noticable luster on the shinos and a new found black core
>problem. When I quit doing the post firing reduction the black core
>went away.I can't acount for the black core
The copper red question is an entirely different bag of worms - short answer
is that reducing atmosphere during cooling won't affect the results one iota.
Melted glaze is not very gas permeable, not easily affected by atmosphere.
What WILL affect the colour, in very dramatic ways, is the cooling curve. And,
no, there is no ideal cooling curve for all copper reds. It all depends on the
make up of the base glaze. Other kinds of glazes, especially high flux mattes,
or glazes in which crystals precipitate out during cooling (classic example is
kaki or persimmon) are quite susceptible to superfical oxidation and
reduction. This is also the case with clay, and why "normal" reduction clay is
brown on the surface.
I can't account for the black core, either, at least not from this account,
but I'll bet a Texas barbecue dinner at NCECA that it wouldn't take long to
demonstrate that there is no cause and effect relationship between the post
firing reduction and the black core.
> > I was once told that a body was too tight to receive reduction
> >after ^01 - I don't beleive that any longer!!
This one, too gets an "It depends" answer. First I'd quibble with the
terminology - clay doesn't "receive" reduction where I come from - and if the
clay isn't vitreous it would certainly be subject to reduction at cone 01. The
*surface* of even vitreous clay will respond to atmosphere at temperatures as
low as 800 C or dull red heat. BTW, if more than the surface of your clay
reoxidises during an oxidised cooling after a "normal" reduction firing, then
your clay ain't vitreous.
with apologies for the verbosity,
John Neely
Utah State University
Louis Katz on sat 11 oct 97
First thanks John for adding to my uderstanding of bloating. The release of
CO fills in the gap.
But although glaze is relatively impermeable after it has melted, it is not
impermeable and reduction in the kiln after the glaze is melted does have
an effect. This effect can be seen by firing a Raku glaze such as Copper
Penny or most other glazes of that ilk firing it in clean oxidation until
it is melted and then reducing the kiln while holding the temperature or
allowing it to drop slightly. During this process if your peice is close
enough to the spy you can watch the reduced copper "floc" together in
little clumps (probably some sort of crystalization rather than
flocculation). If the pots are pulled and quenched the coloration stays.
Cooled normally in the kiln it disappears.
However, It used to be where I went to school that stoneware kilns would be
fired in oxidation between cone 1 and cone 7 or so. Once we started firing
in light reduction all the way from cone 08 we had much better reduction of
our glazes and more consistent results. We assumed and I still beleive that
this was because the reduction was eeasier to acheive before the glaze
melted.
It is still raining, heard reports of floating cars in town. Out here,
close to the water my street drains in both directions. My dehumidifier in
my studio is making lots of soft water for our sigs and glazes.
Louis
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