Vince Pitelka on sun 20 nov 05
> Great analysis and explanation of the problem. So, let me see if I've got
> it right. When we're talking about flameware, we're actually discussing
> high-fired claybodies designed to be used over a gas or electric heat
> source, as opposed to earthenware that is used over the same.
Philip -
I don't know if it is any sort of official designation. When I hear of
contemporary discussions of flameware, it seems that people are usually
tlaking about midrange or high-fired ware. As I said, that's the only ware
where there is a real danger of steam explosion.
- 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/
Vince Pitelka on sun 20 nov 05
Claudia McPhee wrote:
"Hi All, For two years all we had to cook on was charcoal-the only pots were
red earthenware. No one had anything else where we were living. We boiled
things in the open upright shaped pots and fried in the open shaped ones.
None exploded nor cracked. They were glazed on the inside and about an inch
or two on the outside. I also used these pots when we had wood to cook on.
They gave good service to us then and I suppose for thousands of years
before to the people who lived there. Beware? Beware of complex (and
expensive) solutions to simple questions."
Claudia -
You have hit the nail on the head. The issue here is vitrified stoneware
versus porous earthenware. We all know that you can heat up bisque-fired
pots very quickly, with no problems, whereas high-fired pots in a re-fire
heated that quickly will crack. For 10,000 years people have been cooking
on porous earthenware pots over open fires, because porous earthenware has
very high thermal-shock resistance. Round-bottom pots work best, because
the round surface is more able to accommodate uneven expansion and
contraction.
When we talk about flameware, we are often referring to high-fired ware
formulated to allow use on a gas or electric cooktop. That is where the
possible danger can arise, and I believe that the real danger is from steam
explosions rather than from thermal cracking. Have you ever seen what can
happen when someone refires a utilitarian pot that has been in constant use?
Occasionally, as in a teapot or coffee mug, moisture has impacted in the
slightly-porous body over time. It cannot find easy avenues for escape, and
when heated in the kiln, the pot can explode in spectacular fashion - enough
to ruin all other pots in its vicinity and even damage the kiln.
I saw this happen in a potentially catastrophic way. When I arrived at the
Appalachain Center for Craft in 1994, there was a huge stoneware pitcher
sitting atop the firebox on our Bourry-box wood kiln. I do not know who
made it or how long it had been there, but someone had left it as a tribute
to the kiln, and we all assumed it to be a permanent installation. However,
the roof over that kiln was minimal, and in heavy rains that pitcher would
collect water. During one firing around 1998, We had just finished firing
off the first chamber, and had moved around to the intermediate firebox in
order to finish firing off the second chamber. It was cold weather, and a
half dozen of us were sitting around the secondary firebox halfway down the
front of the kiln, while the primary firebox was off the end of the kiln,
and that pitcher was around the corner from us, out of line-of-sight.
Suddenly, that pitcher exploded with so much force that it practically
knocked us off our chairs. It sounded like a very loud gunshot, and
initially we were all baffled as to what the source was, until we started
finding fragments of the pitcher. I am convinced that if we had still been
firing the first chamber, people would have been blinded or worse. I am so
glad to have witnessed the power of a steam explosion from impacted moisture
in vitrified clay, in a situation where no one was injured.
So the point here is that the same thing could happen with high-fired
flameware. If there is any porosity in the claybody (and there always is in
high-fired studio stoneware), and if there is any exposed clay or a crazed
glaze, moisture can slowly impact in the claybody, and then, when no one
expects it, steam pressure could built up in the claybody to the point where
the piece explodes. This could not happen in an earthenware body, because
there are easy paths for the steam pressure to escape.
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/
2ley on sun 20 nov 05
From: "Vince Pitelka"
Great analysis and explanation of the problem. So, let me see if I've got
it right. When we're talking about flameware, we're actually discussing
high-fired claybodies designed to be used over a gas or electric heat
source, as opposed to earthenware that is used over the same.
Philip Tuley
Vince Pitelka on sun 20 nov 05
David Beumee wrote:
"It's untrue that all high-fired studio stoneware clay bodies are porous.
Either stoneware or porcelain clay bodies fired at cone 10 can be formulated
at zero percent absorption, and I have formulated and used dozens of such
bodies for decades. Such bodies can be made to be put directly into
preheated ovens with no possibility of steam explosion or cristobalite
dunting. High-fired flameware is a different matter entirely; risky
business!"
David -
I did not know that. It was my assumption that almost all functional bodies
retain a very slight porosity, and that there are other disadvantages that
appear if one tries to approach zero absorption. Can you talk about that a
little more? I'd love to know what's going on there.
I think it's probably a safe assumption that almost all commercial
claybodies intended for midrange or high-fire still have some porosity. I
wonder if it is possible to make a flameware body with zero percent
absorption. But then again, I wonder why anyone would bother, considering
the risk.
And regarding the claybodies that can be put directly in a hot oven, I don't
think that there is any chance of steam explosion in such a situation, from
any claybody, but cristobalite dunting is a different matter, and a real
risk with so many claybodies with low thermal shock resistance. Maybe I've
just been lucky. In production in the 70s and early 80s, I used two
Westwood (now Laguna) claybodies called Rod's Bod and Danish White (with
sand), and made many hundreds of casseroles. I did recommend that they be
placed in a cool oven and brought up to temperature with the oven, but I
know that not everyone followed those directions. I never had a casserole
returned with cracks.
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/
David Beumee on mon 21 nov 05
Vince wrote:
> So the point here is that the same thing could happen with high-fired
> flameware. If there is any porosity in the claybody (and there always is in
> high-fired studio stoneware), and if there is any exposed clay or a crazed
> glaze, moisture can slowly impact in the claybody, and then, when no one
> expects it, steam pressure could built up in the claybody to the point where
> the piece explodes. This could not happen in an earthenware body, because
> there are easy paths for the steam pressure to escape.
It's untrue that all high-fired studio stoneware clay bodies are porous. Either stoneware or porcelain clay bodies fired at cone 10 can be formulated at zero percent absorption, and I have formulated and used dozens of such bodies for decades. Such bodies can be made to be put directly into preheated ovens with no possibility of steam explosion or cristobalite dunting. High-fired flameware is a different matter entirely; risky business!
David Beumee
Lafayette, CO
-------------- Original message ----------------------
From: Vince Pitelka
> Claudia McPhee wrote:
> "Hi All, For two years all we had to cook on was charcoal-the only pots were
> red earthenware. No one had anything else where we were living. We boiled
> things in the open upright shaped pots and fried in the open shaped ones.
> None exploded nor cracked. They were glazed on the inside and about an inch
> or two on the outside. I also used these pots when we had wood to cook on.
> They gave good service to us then and I suppose for thousands of years
> before to the people who lived there. Beware? Beware of complex (and
> expensive) solutions to simple questions."
>
> Claudia -
> You have hit the nail on the head. The issue here is vitrified stoneware
> versus porous earthenware. We all know that you can heat up bisque-fired
> pots very quickly, with no problems, whereas high-fired pots in a re-fire
> heated that quickly will crack. For 10,000 years people have been cooking
> on porous earthenware pots over open fires, because porous earthenware has
> very high thermal-shock resistance. Round-bottom pots work best, because
> the round surface is more able to accommodate uneven expansion and
> contraction.
>
> When we talk about flameware, we are often referring to high-fired ware
> formulated to allow use on a gas or electric cooktop. That is where the
> possible danger can arise, and I believe that the real danger is from steam
> explosions rather than from thermal cracking. Have you ever seen what can
> happen when someone refires a utilitarian pot that has been in constant use?
> Occasionally, as in a teapot or coffee mug, moisture has impacted in the
> slightly-porous body over time. It cannot find easy avenues for escape, and
> when heated in the kiln, the pot can explode in spectacular fashion - enough
> to ruin all other pots in its vicinity and even damage the kiln.
>
> I saw this happen in a potentially catastrophic way. When I arrived at the
> Appalachain Center for Craft in 1994, there was a huge stoneware pitcher
> sitting atop the firebox on our Bourry-box wood kiln. I do not know who
> made it or how long it had been there, but someone had left it as a tribute
> to the kiln, and we all assumed it to be a permanent installation. However,
> the roof over that kiln was minimal, and in heavy rains that pitcher would
> collect water. During one firing around 1998, We had just finished firing
> off the first chamber, and had moved around to the intermediate firebox in
> order to finish firing off the second chamber. It was cold weather, and a
> half dozen of us were sitting around the secondary firebox halfway down the
> front of the kiln, while the primary firebox was off the end of the kiln,
> and that pitcher was around the corner from us, out of line-of-sight.
> Suddenly, that pitcher exploded with so much force that it practically
> knocked us off our chairs. It sounded like a very loud gunshot, and
> initially we were all baffled as to what the source was, until we started
> finding fragments of the pitcher. I am convinced that if we had still been
> firing the first chamber, people would have been blinded or worse. I am so
> glad to have witnessed the power of a steam explosion from impacted moisture
> in vitrified clay, in a situation where no one was injured.
>
> So the point here is that the same thing could happen with high-fired
> flameware. If there is any porosity in the claybody (and there always is in
> high-fired studio stoneware), and if there is any exposed clay or a crazed
> glaze, moisture can slowly impact in the claybody, and then, when no one
> expects it, steam pressure could built up in the claybody to the point where
> the piece explodes. This could not happen in an earthenware body, because
> there are easy paths for the steam pressure to escape.
> 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/
>
> ______________________________________________________________________________
> 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 on mon 21 nov 05
Hi David, Hi Vince,
I always formulate stoneware bodies with a little absorption - between 1
and 3% - always have - some of those clays have been used for over 30 years
now and I have never heard of any exploding or getting hot in a microwave.
Remember - an absorbency of 2% means the water can only get a little way
into the clay because the pores are discontinuous. Add paper fiber and the
channels left by burnt out fibers are continuous - in that case the
absorption goes form 2% up to 6 or 8%.
The significant part of vitreous flameware is it's low expansion -
cristobalite in ovenware is to be avoided because it gives the clay a high
expansion and that is what leads to cracking due to uneven heating - given
the clay glaze fit is engineered properly.
RR
>Vince wrote:
>
>> So the point here is that the same thing could happen with high-fired
>> flameware. If there is any porosity in the claybody (and there always is in
>> high-fired studio stoneware), and if there is any exposed clay or a crazed
>> glaze, moisture can slowly impact in the claybody, and then, when no one
>> expects it, steam pressure could built up in the claybody to the point where
>> the piece explodes. This could not happen in an earthenware body, because
>> there are easy paths for the steam pressure to escape.
>
>
>It's untrue that all high-fired studio stoneware clay bodies are porous.
>Either stoneware or porcelain clay bodies fired at cone 10 can be
>formulated at zero percent absorption, and I have formulated and used
>dozens of such bodies for decades. Such bodies can be made to be put
>directly into preheated ovens with no possibility of steam explosion or
>cristobalite dunting. High-fired flameware is a different matter entirely;
>risky business!
>
>David Beumee
>Lafayette, CO
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
David Beumee on tue 22 nov 05
Vince wrote:
It was my assumption that almost all functional bodies
> retain a very slight porosity, and that there are other disadvantages that
> appear if one tries to approach zero absorption. Can you talk about that a
> little more? I'd love to know what's going on there.
You are absolutely correct. My mistake! I formulate stoneware and porcelain bodies at 0.05% - 0.2% absorption, so a very slight porosity does exist in a properly formulated body. Zero percent absorption can be the cause of overfired bloating problems. 0% absorption for a clay body is not a healthy figure because there's no way to know if the body is being overfired or not; you're at zero % absorption; no way to tell how far into zero you are. My mistake Vince.
David Beumee
-------------- Original message ----------------------
From: Vince Pitelka
> David Beumee wrote:
> "It's untrue that all high-fired studio stoneware clay bodies are porous.
> Either stoneware or porcelain clay bodies fired at cone 10 can be formulated
> at zero percent absorption, and I have formulated and used dozens of such
> bodies for decades. Such bodies can be made to be put directly into
> preheated ovens with no possibility of steam explosion or cristobalite
> dunting. High-fired flameware is a different matter entirely; risky
> business!"
>
> David -
> I did not know that. It was my assumption that almost all functional bodies
> retain a very slight porosity, and that there are other disadvantages that
> appear if one tries to approach zero absorption. Can you talk about that a
> little more? I'd love to know what's going on there.
>
> I think it's probably a safe assumption that almost all commercial
> claybodies intended for midrange or high-fire still have some porosity. I
> wonder if it is possible to make a flameware body with zero percent
> absorption. But then again, I wonder why anyone would bother, considering
> the risk.
>
> And regarding the claybodies that can be put directly in a hot oven, I don't
> think that there is any chance of steam explosion in such a situation, from
> any claybody, but cristobalite dunting is a different matter, and a real
> risk with so many claybodies with low thermal shock resistance. Maybe I've
> just been lucky. In production in the 70s and early 80s, I used two
> Westwood (now Laguna) claybodies called Rod's Bod and Danish White (with
> sand), and made many hundreds of casseroles. I did recommend that they be
> placed in a cool oven and brought up to temperature with the oven, but I
> know that not everyone followed those directions. I never had a casserole
> returned with cracks.
> 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/
>
> ______________________________________________________________________________
> 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.
skiasonaranthropos@FSMAIL.NET on tue 22 nov 05
Hi David,
I hope you don=92t mind to a few comments against your last post, and that
they do not seem overly picky:
You note =930% absorption for a clay body is not a healthy figure=94 some
bodies are routinely produced with no open porosity, ie zero water
absorption
=93 ... there's no way to know if the body is being overfired or not=94 ...
determining water absorption is a very important technique to gauge the
firing of bodies but others are available, such as determining the bulk
density
It is also worth noting that different water absorption procedures exist,
and that these can give different results. Consequently one may indicate
0% absorption and hence suggest vitrification whilst another finds some
open porosity remaining
Regards,
Antony
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