Paul Herman on sun 27 apr 08
Hello Gene,
It sounds like you have done a good reduction firing, but are using a
clay with a considerable amount of iron in it. The dark interior is
just the iron content of the clay body, reduced to black iron oxide. I
recommend you try a white stoneware and a porcelain body, to give
yourself some perspective. Sometimes, high iron clay bodies cause
problems of dunting or brittleness when fired in reduction.
After commencing reduction at around 1550F or so, I keep it in mild
reduction until the end, then oxidize the last few minutes. My kiln is
very much like yours, with the same burners, but with an arch on top.
And for what it's worth, after making pots for almost 40 years, I
still don't know what "black core" means. Anyone care to enlighten me?
Please, a definition. What the hell is "black core?" Also "fireclay"
if you don't mind.
Best wishes,
Paul Herman
Great Basin Pottery
Doyle, California US
www.greatbasinpottery.com/
On Apr 27, 2008, at 4:20 PM, Gene Arnold wrote:
> I posted this friday and I haven't recieved any clayart until today
> (sunday)
> and only 6 at that, so I'm re-posting to see if this one makes it
> through.
>
> I really don't know if I have black core of not, I don't think I do.
> My clay
> (standard 119) after it is fired is a dark charcoal gray through out
> not
> just in the center. Is this what reduced clay with iron looks like
> on the
> inside, or am I reducing to heavy. I'm firing to cone 10 and
> reduction is
> started at 1600 degrees and held for one hour. My reduction is done
> with the
> damper only. I have big flames at the top peep and a small flame at
> the
> bottom peep. The flames inside the kiln at this point seems to be
> lazy and
> licking around with not a lot of turbulence.
>
> I bisque fire in an electric kiln to cone 06.
>
> After reduction do you keep the kiln in a mild reduction or do you
> go back
> into oxidation until the end of the firing?
>
> My kiln is a 35ft. MFT with two b-4's ronsom venture burners. Fired on
> propane.
>
> All the glaze colors seem to be correct but that clay color through
> out sure
> is dark. The bottoms of the pots on the un-glazed portion is a nice
> toast
> brown. But just under that if it is chipped it is almost black.
>
> As I've said in the past this is my first gas kiln and there are not
> any
> others real close by me. So I'm kinda flying by the seat of my pants
> here.
>
>
> Gene
> mudduck@mudduckpottery.net
> www.mudduckpottery.net
Gene Arnold on sun 27 apr 08
I posted this friday and I haven't recieved any clayart until today (sunday)
and only 6 at that, so I'm re-posting to see if this one makes it through.
I really don't know if I have black core of not, I don't think I do. My clay
(standard 119) after it is fired is a dark charcoal gray through out not
just in the center. Is this what reduced clay with iron looks like on the
inside, or am I reducing to heavy. I'm firing to cone 10 and reduction is
started at 1600 degrees and held for one hour. My reduction is done with the
damper only. I have big flames at the top peep and a small flame at the
bottom peep. The flames inside the kiln at this point seems to be lazy and
licking around with not a lot of turbulence.
I bisque fire in an electric kiln to cone 06.
After reduction do you keep the kiln in a mild reduction or do you go back
into oxidation until the end of the firing?
My kiln is a 35ft. MFT with two b-4's ronsom venture burners. Fired on
propane.
All the glaze colors seem to be correct but that clay color through out sure
is dark. The bottoms of the pots on the un-glazed portion is a nice toast
brown. But just under that if it is chipped it is almost black.
As I've said in the past this is my first gas kiln and there are not any
others real close by me. So I'm kinda flying by the seat of my pants here.
Gene
mudduck@mudduckpottery.net
www.mudduckpottery.net
Hank Murrow on mon 28 apr 08
On Apr 27, 2008, at 8:40 PM, Paul Herman wrote:
>
> And for what it's worth, after making pots for almost 40 years, I
> still don't know what "black core" means. Anyone care to enlighten me?
> Please, a definition. What the hell is "black core?" Also "fireclay"
> if you don't mind.
Dear Paul;
A geologist told me that fireclay is a term for clays which have been
stripped of their alkali content by the giant ferns and other
vegetation which grew rapidly during the Carboniferous era. That is
why fireclay is mostly found abelow or above coal deposits. The coal
is what remains of the plant material, while the fireclay is what
remains of the soil the plants grew in.
Cheers, Hank
Bill Merrill on mon 28 apr 08
Fireclay is much more coarse than ball clays. The particle size is much
larger. It also refers to the refractory quality. It will fire much
hotter and not bloat, warp or melt than earthenware or ball clays.=20
Black core is what happens to stoneware bodies when an excessive amount
of metallic oxides are added to the clay and reduction fired. Normally
with a clay body that has some iron in it from a red clay or red iron
oxide, the reduction changes the red iron to black iron. When cooling
the outside of the clay if left raw the exterior turns orange to brown
in color because it re oxidizes on. Since the body is vitrified the
interior cannot reoxidize. The interior remains grey or with excessive
iron the interior goes black and many times has gas bubbles and gas
voids in the interior.
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Hank Murrow
Sent: Monday, April 28, 2008 6:42 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: black core and more???
On Apr 27, 2008, at 8:40 PM, Paul Herman wrote:
>
> And for what it's worth, after making pots for almost 40 years, I
> still don't know what "black core" means. Anyone care to enlighten me?
> Please, a definition. What the hell is "black core?" Also "fireclay"
> if you don't mind.
Dear Paul;
A geologist told me that fireclay is a term for clays which have been
stripped of their alkali content by the giant ferns and other
vegetation which grew rapidly during the Carboniferous era. That is
why fireclay is mostly found abelow or above coal deposits. The coal
is what remains of the plant material, while the fireclay is what
remains of the soil the plants grew in.
Cheers, Hank
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Edouard Bastarache Inc. on mon 28 apr 08
Hank,
"That is why fireclay is mostly found abelow or
above coal deposits.
(Hank)"
that is why I have already recommended to
an expert of EPA to sample fireclays for dioxins.
Gis revido,
(A la revoyure)
Edouard Bastarache
Spertesperantisto
Sorel-Tracy
Quebec
http://www.pshcanada.com/Toxicology.htm
http://www.flickr.com/photos/30058682@N00/
http://www.flickr.com/photos/potier/20321056/
http://perso.orange.fr/smart2000/livres.htm
http://myblogsmesblogs.blogspot.com/
Vince Pitelka on mon 28 apr 08
Hank Murrow wrote:
"A geologist told me that fireclay is a term for clays which have been
stripped of their alkali content by the giant ferns and other
vegetation which grew rapidly during the Carboniferous era. That is
why fireclay is mostly found below or above coal deposits. The coal
is what remains of the plant material, while the fireclay is what
remains of the soil the plants grew in."
Dear Hank -
Hmmmm. A geologist told me that fire clay starts out as common clay or
surface clay, but it ends up bedded in alluvial layering. Dependent on the
amount of iron and other fluxes leached out by acidic groundwater over
geologic time, the clay turns into stoneware clay and then of the leaching
goes on even longer it turns to fire clay.
Anyone out there with a geologist friend who has yet another theory?
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net; wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka
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