Kathy McDonald on mon 4 dec 06
oxidation in an e-kiln)
As I rereead my post I realized I had not stated what the
refire process was sorry mel!
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG]On Behalf Of
Kathy McDonald
Sent: Monday, December 04, 2006 8:35 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Fluxing with Iron oxide - Bloating
Ron,
I agree with you, it is "something else", but I am not
scientific enough
to analyze it in sophisticated terms.
I will use a recent example to provide some illustration.
I normally don't bother to refire much stuff for the sheer
fun of it,but with recent illness and
lots of time on my hands I did.
I had originally fired some mugs (Tucker's 1080) which is a
porcelain type
cone 9/10 clay, as you know I'm sure. They had been slow
bisqued to cone 05.
I had glazed them with a glaze called CM white ( very stable
glaze...used it for 25 years)
with some iron /rutile under glaze decoration. Fired them
to cone 10 in natural gas.
This combination has been working for me for years, and I
have several pieces that are glazed
in the same glaze combo that are still cooking, serving and
baking after 20+ years.
Here's the part that related to what I think you might be
talking about.
They looked rather boring so I thought I'll jazz them up a
bit, and see what happens if I
refire them to C6 with some colorful overglazes (Mayco) and
a clear coat over the outside of the
mug (also Mayco c6).
I have done this in the past
with mugs glazed only in the CM White and they were fine.
That's the "control" part of this experiment.
they were fine with the exception of areas that had the
iron/rutile
glaze decoration. It was a crusty bubbled mess . You could
scrape it off with a kitchen knife.
All other areas are fine. It is highly unlikely that the
clay caused it because
it was fired to vitrification, and was a nice semi gloss
surface in all areas except those with the iron underglaze.
I broke a few just to see...there was that characteristic
blackened area almost to the core of the clay right where
the iron decoration was.
I also broke a piece fired a few firings ago in the same
glaze combo but NOT refired
and it showed no evidence of any blackened areas in the
clay.
Not sure what's in the Mayco glaze.. ....( here is where I
apologize to
all those who have highly descriptive technical
language)...it looks like the refiring
process and the layers totally mess each other up.
I've had bad black coring with some high iron bodies and
that's NOT what this is.
I am very interested in what you have to say about this.
Kathy........ (who is actually enjoying mucking about
instead of pumping em out!)
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG]On Behalf Of
Ron Roy
Sent: Monday, December 04, 2006 12:07 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Fluxing with Iron oxide - Bloating
Hi Vince,
Let me rephrase what I am trying to say.
Bloating is the result of over fired clay - the clay breaks
down - gas is
produced and bubbles occur in the clay - small bubbles are
blebbing - big
bubbles are bloating.
The reduced iron in a badly fired bisque turns the iron
oxide into a flux -
and that is what overfires the clay and produces the
bloating - do you
agree with that?
When there is a layer of clay - inside the body - from a
bisque firing that
did not get all the carbon burned out - that produces
overfired clay just
were that reduced clay is - so it looks quite different from
a body that is
simply overfired. The gases from that overfired "black core"
produce a more
dramatic result.
The reduced iron in a badly fired bisque turns the iron
oxide into a flux -
and that is what overfires the clay and produces the
bloating - do you
agree with that?
There is some good information in Hamer under black coring
and bloating.
What I am trying to point out about black coring - which can
lead to
bloating if fired high enough - is what looks like black
coring but is the
result of something else.
The clay gets reduced in different layers - or reduced and
reoxidized -
looks like black coring sometimes but is not the same
thing - it did not
happen in the bisque but during the glaze firing. I am
trying to separate
the two problems.
If you google Black Coring you will see that many don't
agree on what is
happening.
Perhaps - if we discus it a little we might come to a better
understanding
of what is going on?
RR
>Ron -
>I wanted to ask you about the above statement. In
terracotta bodies, I
>agree, overfiring causes bloating. I have also seen it
happen in high-iron
>cone 6 bodies intended for oxidation that were fired in
cone 6 reduction.
>But I have seen bloating happen on stoneware bodies that
also showed carbon
>coring, where there was no other explanation, and over the
years I have seen
>plenty of examples of this. Throughout my career in clay,
and among others I
>know in the field, the term "bloating" refers to a
condition resulting from
>volatilization of materials internally when the claybody is
no longer
>porous, resulting in pillow-shaped separations along the
grain structure,
>producing large bumps on the surface of the piece. When
the piece is broken
>open, the separation along the grain structure is very
obvious. When the
>same piece shows carbon coring, it seems the reasonable
explanation,
>especially when the clay has not been fired beyond its
normal maturation
>point. What are your thoughts on this?
>- Vince
>
>Vince Pitelka
>Appalachian Center for Craft, Tennessee Technological
University
>Smithville TN 37166, 615/597-6801 x111
>vpitelka@dtccom.net, wpitelka@tntech.edu
>http://iweb.tntech.edu/wpitelka/
>http://www.tntech.edu/craftcenter/
>
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melpots@pclink.com.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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