Vince Pitelka on sun 3 dec 06
> Bloating is not due to carbon but is overfired clay breaking down and
> producing gases. Black coring is something else in my mind - a difference
> in colour inside a clay body due to different degrees of reduction and/or
> oxidation - which - if it results in bloating can be viewed as something
> different from bloating.
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/
Ron Roy on mon 4 dec 06
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/
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Kathy McDonald on mon 4 dec 06
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/
>
>___________________________________________________________
___________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your
subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
____________________________________________________________
__________________
Send postings to clayart@lsv.ceramics.org
You may look at the archives for the list or change your
subscription
settings from http://www.ceramics.org/clayart/
Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
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Jim Murphy on mon 4 dec 06
Hi Vince, Ron, et al,
My understanding of bloating may differ slightly.
Bloating is normally attributed to localized viscosity differences in
glass-phase development during firing.
For example, different glassy phases develop [at different rates] around
various-sized particles of materials [Silica, Kaolin, Fe2O3, etc.] - some
glassy phases are less viscous [less resistant to flow] than others -
leading to more fluid "movement" in some regions than others.
The effect of bloating is typified by localized surface irregularities
[observed after firing] as may occur when different molten glassy phases
"heave" unmolten particles from their glassy matrix.
Although "fluxing" & gas bubbles play their role in glass formation [and
"movement" of glassy phases] during firing, it's possible the raw claybody
itself may be the cause of bloating due to a poorly designed
particle-size-distribution and/or insufficient "mixing" to a more
homogeneous claybody state.
Perhaps "fluxing with Iron Oxide" is getting a bad rap when it comes to
bloating.
Best wishes,
Jim Murphy
Vince Pitelka on wed 6 dec 06
Ivor Lewis wrote:
"I find it difficult to reconcile information being relayed among potters
and ceramic artists to explain the behaviour of Iron oxides and the historic
metallurgical record. "
Dear Ivor -
I find it difficult to reconcile information being quoted from the supposed
historic metallurgical record that seems to have no reasonable connection to
the real-life behavior of certain materials in a ceramic system.
Best wishes -
- 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/
Ivor and Olive Lewis on wed 6 dec 06
Dear Friends,
Bloating seems to happen when there is premature vitrification and an =
impervious layer forms either on the surface or within stratified clay, =
as suggested by Jim an Mel. It must be initiated by the evolution of gas =
from residual decomposable minerals that collect during sedimentation. =
When gas is released, increasing temperature causes expansion at, from =
memory, a rate or 1/273 part of the original volume per degree rise in =
temperature. This may seem like a small increment but take Calcite =
decomposing and releasing carbon dioxide at 899 deg C, then being heated =
to cone 10 the increase in volume is significant. The quantities are not =
insignificant. One hundred grams of Limestone would yield twenty two =
point four litres of carbon dioxide, one gram about two hundred mills.
Other minerals that might be present and react to discharge a gas would =
be Fluorite, Gypsum and Limonite. Nor should we forget organic material =
such as coal, lignite and peat that would burn as temperature rises.
One thing that concerns me is the fact that, in the literature of =
pottery, Iron oxides are thought to be heat stable until they enter into =
some sort of fusion with other ingredients, seemingly described as =
"fluxing activity". However when I read alternative literature, =
including Encyclopedia Britannica I learn that all Oxides of Iron will =
be changed in the presence of burning charcoal, or Carbon monoxide, to =
metallic Iron once the temperature rises beyond 600 Deg Celsius. This =
material is called "Sponge Iron". It has never been molten but yet it =
can be forged, hammered into useful shapes. During the forging process, =
a glassy slag is expelled from the sponge as the metal is consolidated.
I find it difficult to reconcile information being relayed among potters =
and ceramic artists to explain the behaviour of Iron oxides and the =
historic metallurgical record.=20
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Dave Finkelnburg on wed 6 dec 06
Ivor,
Have you tried making an oil spot glaze?
I spent two years working in the steelworks at Pt.
Kembla...feeding blast furnaces. I have some idea of
how iron ore, limestone and coke combine to make
molten iron plus slag. The temperature at which that
occurs is not the same as the temperature at which
iron oxide appears to decompose and an oilspot glaze
"boils" like a mud pot. I do not know why this is,
but it's a fact.
Bloating is caused by expansion of gas in a body.
Bloating requires that the body yield to the pressure
of the expanding gas. To do that, the body must
become plastic. Plastic behavior only occurs when the
body begins to melt and form some glass phase.
Typically there are only two ways to get enough
glass phase for bloating to occur. One way is if the
body is overfired. The other is caused by clusters of
feldspar in the body.
For porcelains, which do not contain iron in any
quantity of consequence, feldspar clusters have been
documented to be a frequent cause of bloating.
Premixing feldspar and some clay, then blunging the
clay slip with high shear, then dewatering the slip in
a filter press or by evaporation, is the best way to
avoid feldspar clusters and bloating.
For stonewares, which are typically high in iron
while feldspar content is low to absent, the only
possible explanation for bloating is the body is
overfluxed. Could it be iron, in reduction, is the
excessive flux?
Good potting,
Dave Finkelnburg
--- Ivor and Olive Lewis
wrote:
> I find it difficult to reconcile information being
> relayed among potters and ceramic artists to explain
> the behaviour of Iron oxides and the historic
> metallurgical record.
__________________________________________________
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Timothy Joko-Veltman on wed 6 dec 06
On 12/6/06, Ivor and Olive Lewis wrote:
> However when I read alternative literature, including Encyclopedia Britannica > I learn that all Oxides of Iron will be changed in the presence of burning
> charcoal, or Carbon monoxide, to metallic Iron once the temperature rises
> beyond 600 Deg Celsius. This material is called "Sponge Iron". It has never
> been molten but yet it can be forged, hammered into useful shapes. During
> the forging process, a glassy slag is expelled from the sponge as the metal is
> consolidated.
Ivor,
I don't see the conflict between the metallurgical and ceramic "discourses".
Here's why. It sounds as though the experiment cited by the EB was
conducted as a metallurgical investigation, hence it is likely that
the experimenter was not interested in vitrification, flux/refractory
properties, etc, all things a ceramic investigation is interested in.
If that is the case (as seems HIGHLY likely), then there is no basis
for comparison between the behaviour of iron oxide as cited, and in a
ceramic glaze for the very simple reason that tests done in a ceramic
environment will be conducted in the presence of silica, alumina and
probably a flux or two (or more) - any one (or all) of which is likely
to change the way iron oxide decomposes, perhaps even preventing it
from decomposing at all. On the other hand, metallurgists are more
interested in complete decomposition (viz., producing high purity Fe
metal from FeO, Fe2O3, and Fe3O4) than they are in creating a glass,
so naturally, they will conduct experiments to that end - that is,
experiments in environments containing strongly reactive substances
that are likely to lead to decomposition, and the other elements that
may have an effect are absent or rigourously controlled.
Regards,
Tim
John Hesselberth on wed 6 dec 06
On Dec 5, 2006, at 11:52 PM, Ivor and Olive Lewis wrote:
> I find it difficult to reconcile information being relayed among
> potters and ceramic artists to explain the behaviour of Iron oxides
> and the historic metallurgical record.
Hi Ivor,
But wouldn't you expect that? After all glassy systems are not
metals. Why should the behavior be the same? You seem to expect
materials to behave the same in glassy systems as they do in their
pure form. I see no reason why they should. We should expect some
unique behavior and, perhaps, some behavior that is similar. But the
system within which the material exists matters.
Regards,
John
Ivor and Olive Lewis on fri 8 dec 06
Dear Tim,
I did not know I was addressing the fate of Iron Oxide in a glaze. My =
remarks were related to clay and in particular, clay that has a =
proportion of Iron compounds. For a discussion on the nature of glazes I =
regards Kingery et al as giving the best picture, which they do without =
invoking eutectic behaviour. Unless objects to be glazed were made from =
highly ferruginous clay with a decent proportion of retained organics =
and reduction was started prematurely, what has been said so far seems =
to be of no consequence.
The ancient way of making Iron produced a very pure substance. =
Impurities depended on the nature of the ore. One of the bye-products of =
making Wrought Iron was a glass.=20
Regards,
Ivor=20
Ivor and Olive Lewis on fri 8 dec 06
Dear John Hesselberth,=20
What you say sounds most reasonable.
But the stuff used in the smelting of iron also contains many of the =
ingredients we would regards as "Ceramic" by nature.
The forging of Sponge Iron helps to consolidate the metallic mass and =
expel what remains of the "Gangue", the non metallic part of the ore. =
Remember we are not talking about pure refined oxides that potters use. =
Bog Iron, one readily available ore of Iron is usually contaminated with =
clay and silt. If the records I am reading are correct, since I have not =
seen this, the material is a glass. Note also that when making a =
"Smithy" weld with wrought Iron, "Silver" silica sand is used as a =
"Flux" to remove those oxides that form on Wrought iron when it is =
heated to 11 or 12 hundred degrees C. This suggests that there is an =
Iron Silicate with a melting point in that temperature region. Perhaps a =
liquid silicate wets the metal, preventing more oxide forming that would =
prevent solid state welding and acting as a solvent as well. Evidence of =
the existence of Iron Silicate can be found in the references already =
given.
Once, long ago I was employed in the metallurgical research department =
of a large steel works. One section, which employed about four people, =
was devoted to Ceramic Research. Believe me, there is a strong link =
between metallurgy and ceramics.
Best regards,
Ivor
Lee Love on sat 9 dec 06
I have found that bloating can be caused by certain types of
glazes on a high iron body. My cone 5 bizen is okay to cone 9
unglazed. If it is glazed, it will bloat at the higher temp.
--
Lee in Mashiko, Japan
http://potters.blogspot.com/
"Let the beauty we love be what we do." - Rumi
"When we all do better. We ALL do better." -Paul Wellstone
Ivor and Olive Lewis on sat 9 dec 06
Dear Dave Finkelnburg ,
For some reason or other I am getting out of sync with my postings. This =
seems to be due to the late delivery of Clayart Digest.
Right, Down to Brass Tacks.=20
I did not raise the issue of Bloating but I accept your summary of its =
causes as being authoritative.=20
As I understand Oil Spot Glazes, and as I recall reading the explanation =
given by John Britt in his CM article, this only happens when the kiln =
is not allowed to go into a reducing phase. Oil Spot is caused by the =
thermal decomposition of Red Iron Oxide. This give rise to a lower oxide =
and the discharge of Oxygen which accumulates as bubbles that break =
through the glaze surface.
Temperatures in a modern blast furnace range for atmospheric where the =
Charge is dropped into the reaction chamber, to well above the melting =
point of Iron at the hearth. The atmosphere is reducing through out the =
reaction chamber. The Chemistry is very complex but Greenwood and =
Earnshaw give a good summary and in their illustration give a =
temperature at the hearth of around 2000 deg C. In the same illustration =
they show 'FeO' being reduced to Fe(s) between 700 and 1200 deg C.
If, in the reducing conditions of a blast furnace, Red Iron Oxide can =
undergo solid state reactions I can see nothing to prevent that =
happening if the clay contains carbonaceous materials which are =
prevented from burning because of Oxygen deficiency in a kiln chamber.
Frank Hamer has a lot to say about Iron oxides but I think the following =
is interesting and I quote "Black Iron Oxide....FeO. The simplest =
compound of iron and oxygen. It is stable at room temperature but =
becomes excited at red heat. This excitement is a loosening of the =
molecular bonds thereby allowing black iron oxide to combine with =
silicates..." (p164, first edition). This sounds like a Solid State =
Reaction to me. Red heat ! Which ceramic compounds in clay or glaze melt =
at red heat? Perhaps Iron Silicate is the compound with the ability to =
flux clay. 2FeO.SiO2 melts at 1205 deg C as given on Plate 6 of the ACS =
Wall Charts
Thanks for your contributions to the marathon thread.
Best regards and enjoy your weekend,
Ivor
Greenwood and Earnshaw also give an insight into the historic processing =
of Iron ore. Iron ores from Iron Barron and the MIddleback Ranges have a =
high Silica content and are known as Jaspilite. At the Blast Furnace in =
Whyalla they are fluxed with Dolomite.
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