Richard Aerni on fri 15 feb 08
Someone, Paul Borian, I believe, asked why talc would make a difference in
whether a body dunted or not. I've received some private inquiries about it
as well. Sorry for the delay in responding, but it's been pretty busy of
late.
Please understand that I am no expert at this. I was hoping someone who
really knew what they were talking about would answer this one, but I
haven't seen anything, so I'm going to take a stab at it and hope that all
the experts will correct my mistakes!
My understanding is that there is silica glass, free silica, and
cristobalite in the clay matrix above 2200 degrees Fahrenheit. Silica glass
is pretty non-reactive, but too much of it and the body is overvitrified and
weak. Free silica is what goes through the quartz inversion phase,
resulting in about 1% shrinkage/expansion around 1053 degrees Fahrenheit (my
figure may be off a bit...I don't have any texts here near the computer),
while cristobalite will goes through a similar phase, but more extreme
(around 3% (??)) in the 400-500 F range. This excess of cristobalite is
what causes much clay dunting, I believe.
In order to make sure your body is strong and functional, it's important to
balance the three types of silica in the fired body. In order to prevent
too much cristobalite from forming, you should make sure there are adequate
fluxes in your clay mixture. These fluxes are most often feldspars, and
most commonly potash feldspar. Sometimes in stoneware clay bodies, most
particularly in whiteware bodies (why, I'm not sure, but if you check clay
formulae, you will see that it's mostly in whiteware), talc is added for, as
I recall reading in Tichane, "tightening the body up." I assume this means
tightening as in fluxing. One of the particularly nice qualities of Nytal
is it's approximately 9% calcium content, which added considerably to it's
fluxing power. Above 2200 F, the kaolinite crystals start to decompose,
forming mullite (good thing for fired strength) and freeing up silica, which
will form cristobalite if the temp is held in or above that range for any
length of time. Having a good glassy melt will take enough of this
cristobalite into solution so that it will not constitute a structural
threat when cooling or in repeated heating and cooling. My take on the
problems of dunting caused by the absence of talc in one clay body in my
past is that the body was not fluxed properly at high temps, and so not
enough of the cristobalite was taken into the glassy melt, and therefore was
free to do it's damage upon cooling.
So, that's my brief take on this issue. I welcome comments from those who
know more than I.
Best,
Richard Aerni
Rochester, NY
Bill Merrill on sat 16 feb 08
Richard,
All clays have cristobloite in them. What we as potters have to do is
be aware of the clays that we put in our clay bodies. I'm speaking
about cone 10 reduction bodies. Clay bodies have clays, fluxes and
fillers in them. So if one want a a stoneware body one may start with
plastic clay (OM$), a high fire clay like Cedar Heights Gold Art plus a
plastic red fireclay like neuman Red, a refractory clay like A.P. Mexico
Missouri Fire clay, a flux (Custer spar) I use talc for a flux because
it helps develop an orangeish color and is a good flux. I use a grog to
give the body more tooth and cut the shrinkage of the body. After a
body is developed to your needs, and the body does what you want it to
do on the wheel, hand building, the one fires their clay, follows the
science of firing and cooling and if these these things fall into place
one will probably not have problems with your clay body.
=20
Peter Sohgen was a student of Val Cushings at Alfed. I just spoke to
Val about all the fuss people are making about cristobolite. We came to
the conclusion that cristobolite problems are rare with today's potters
because=20
most potters buy their clay from large pottery supply house that have
spent time developing a range of bodies or the potter has developed and
worked out any issues with their clay body. I still haven't heard a
potter on Clayart say they were having problems with dunting, shivering
or the compression and breaking of pots do to excessively tight glaze
fit etc. Lets not cry wolf about cristobolite. =20
Val was just saying that the supply room at Alfred doesn't allow Talc.
They have substituted something for talc and it doesn't work well. I
quess we will all have to wear Hazardous materials outfits to be
potters. When is it all going to come down to common sense on how we
handle ceramic materials. What's the tile industry going to use to make
tiles if talc isn't allowed. Sculptors who carve soapstone (talc) won't
be allowed to use it. Is part of the talc scare caused because talc has
a little asbestos in it? Certainly it must be handled with common sense,
Most potters don't use talc in their clay body. So What?=20
I know this, I'm not trying to convert potters to use talc in their
clay bodies. I just have to say "keep making pots, the best ones are
still in you". I know I'll just keep making mine. Some of these issues
are just too draining to keep talking about.
May your pots not dunt or shiver....
Bill=20
=20
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Richard
Aerni
Sent: Friday, February 15, 2008 5:07 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Talc, cristobalite, clay bodies
Someone, Paul Borian, I believe, asked why talc would make a difference
in
whether a body dunted or not. I've received some private inquiries
about it
as well. Sorry for the delay in responding, but it's been pretty busy
of
late.
Please understand that I am no expert at this. I was hoping someone who
really knew what they were talking about would answer this one, but I
haven't seen anything, so I'm going to take a stab at it and hope that
all
the experts will correct my mistakes!
My understanding is that there is silica glass, free silica, and
cristobalite in the clay matrix above 2200 degrees Fahrenheit. Silica
glass
is pretty non-reactive, but too much of it and the body is overvitrified
and
weak. Free silica is what goes through the quartz inversion phase,
resulting in about 1% shrinkage/expansion around 1053 degrees Fahrenheit
(my
figure may be off a bit...I don't have any texts here near the
computer),
while cristobalite will goes through a similar phase, but more extreme
(around 3% (??)) in the 400-500 F range. This excess of cristobalite is
what causes much clay dunting, I believe.
In order to make sure your body is strong and functional, it's important
to
balance the three types of silica in the fired body. In order to
prevent
too much cristobalite from forming, you should make sure there are
adequate
fluxes in your clay mixture. These fluxes are most often feldspars, and
most commonly potash feldspar. Sometimes in stoneware clay bodies, most
particularly in whiteware bodies (why, I'm not sure, but if you check
clay
formulae, you will see that it's mostly in whiteware), talc is added
for, as
I recall reading in Tichane, "tightening the body up." I assume this
means
tightening as in fluxing. One of the particularly nice qualities of
Nytal
is it's approximately 9% calcium content, which added considerably to
it's
fluxing power. Above 2200 F, the kaolinite crystals start to decompose,
forming mullite (good thing for fired strength) and freeing up silica,
which
will form cristobalite if the temp is held in or above that range for
any
length of time. Having a good glassy melt will take enough of this
cristobalite into solution so that it will not constitute a structural
threat when cooling or in repeated heating and cooling. My take on the
problems of dunting caused by the absence of talc in one clay body in my
past is that the body was not fluxed properly at high temps, and so not
enough of the cristobalite was taken into the glassy melt, and therefore
was
free to do it's damage upon cooling.
So, that's my brief take on this issue. I welcome comments from those
who
know more than I.
Best,
Richard Aerni
Rochester, NY
________________________________________________________________________
______
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You may look at the archives for the list, post messages, change your
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Richard Aerni on sat 16 feb 08
Bill,
Thanks for the reply. I'm not sure what you were replying to, however. I'm
having a bit of a problem with a talc change in my white stoneware body,
which I mentioned a few weeks ago. I also mentioned I'd had several
complete kiln loads of dunted pots 15 or so years ago when the clay company
neglected to add the talc to my custom body.
Someone asked why an absence of talc would cause dunting, hence my reply
below.
I guess I would say that cristobalite is only a problem when it's a problem!
Best,
Richard Aerni
Rochester, NY
On Sat, 16 Feb 2008 10:29:44 -0800, Bill Merrill wrote:
>Richard,
>
>All clays have cristobloite in them. What we as potters have to do is
>be aware of the clays that we put in our clay bodies. I'm speaking
>about cone 10 reduction bodies. Clay bodies have clays, fluxes and
>fillers in them. So if one want a a stoneware body one may start with
>plastic clay (OM$), a high fire clay like Cedar Heights Gold Art plus a
>plastic red fireclay like neuman Red, a refractory clay like A.P. Mexico
>Missouri Fire clay, a flux (Custer spar) I use talc for a flux because
>it helps develop an orangeish color and is a good flux. I use a grog to
>give the body more tooth and cut the shrinkage of the body. After a
>body is developed to your needs, and the body does what you want it to
>do on the wheel, hand building, the one fires their clay, follows the
>science of firing and cooling and if these these things fall into place
>one will probably not have problems with your clay body.
>
>Peter Sohgen was a student of Val Cushings at Alfed. I just spoke to
>Val about all the fuss people are making about cristobolite. We came to
>the conclusion that cristobolite problems are rare with today's potters
>because
>most potters buy their clay from large pottery supply house that have
>spent time developing a range of bodies or the potter has developed and
>worked out any issues with their clay body. I still haven't heard a
>potter on Clayart say they were having problems with dunting, shivering
>or the compression and breaking of pots do to excessively tight glaze
>fit etc. Lets not cry wolf about cristobolite.
>
>Val was just saying that the supply room at Alfred doesn't allow Talc.
>They have substituted something for talc and it doesn't work well. I
>quess we will all have to wear Hazardous materials outfits to be
>potters. When is it all going to come down to common sense on how we
>handle ceramic materials. What's the tile industry going to use to make
>tiles if talc isn't allowed. Sculptors who carve soapstone (talc) won't
>be allowed to use it. Is part of the talc scare caused because talc has
>a little asbestos in it? Certainly it must be handled with common sense,
>Most potters don't use talc in their clay body. So What?
>
> I know this, I'm not trying to convert potters to use talc in their
>clay bodies. I just have to say "keep making pots, the best ones are
>still in you". I know I'll just keep making mine. Some of these issues
>are just too draining to keep talking about.
>
>May your pots not dunt or shiver....
>
>Bill
>
>
>-----Original Message-----
>From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Richard
>Aerni
>Sent: Friday, February 15, 2008 5:07 PM
>To: CLAYART@LSV.CERAMICS.ORG
>Subject: Talc, cristobalite, clay bodies
>
>Someone, Paul Borian, I believe, asked why talc would make a difference
>in
>whether a body dunted or not. I've received some private inquiries
>about it
>as well. Sorry for the delay in responding, but it's been pretty busy
>of
>late.
>
>Please understand that I am no expert at this. I was hoping someone who
>really knew what they were talking about would answer this one, but I
>haven't seen anything, so I'm going to take a stab at it and hope that
>all
>the experts will correct my mistakes!
>
>My understanding is that there is silica glass, free silica, and
>cristobalite in the clay matrix above 2200 degrees Fahrenheit. Silica
>glass
>is pretty non-reactive, but too much of it and the body is overvitrified
>and
>weak. Free silica is what goes through the quartz inversion phase,
>resulting in about 1% shrinkage/expansion around 1053 degrees Fahrenheit
>(my
>figure may be off a bit...I don't have any texts here near the
>computer),
>while cristobalite will goes through a similar phase, but more extreme
>(around 3% (??)) in the 400-500 F range. This excess of cristobalite is
>what causes much clay dunting, I believe.
>
>In order to make sure your body is strong and functional, it's important
>to
>balance the three types of silica in the fired body. In order to
>prevent
>too much cristobalite from forming, you should make sure there are
>adequate
>fluxes in your clay mixture. These fluxes are most often feldspars, and
>most commonly potash feldspar. Sometimes in stoneware clay bodies, most
>particularly in whiteware bodies (why, I'm not sure, but if you check
>clay
>formulae, you will see that it's mostly in whiteware), talc is added
>for, as
>I recall reading in Tichane, "tightening the body up." I assume this
>means
>tightening as in fluxing. One of the particularly nice qualities of
>Nytal
>is it's approximately 9% calcium content, which added considerably to
>it's
>fluxing power. Above 2200 F, the kaolinite crystals start to decompose,
>forming mullite (good thing for fired strength) and freeing up silica,
>which
>will form cristobalite if the temp is held in or above that range for
>any
>length of time. Having a good glassy melt will take enough of this
>cristobalite into solution so that it will not constitute a structural
>threat when cooling or in repeated heating and cooling. My take on the
>problems of dunting caused by the absence of talc in one clay body in my
>past is that the body was not fluxed properly at high temps, and so not
>enough of the cristobalite was taken into the glassy melt, and therefore
>was
>free to do it's damage upon cooling.
>
>So, that's my brief take on this issue. I welcome comments from those
>who
>know more than I.
>
>Best,
>Richard Aerni
>Rochester, NY
>
>________________________________________________________________________
>______
>Clayart members may send postings to: clayart@lsv.ceramics.org
>
>You may look at the archives for the list, post messages, change your
>subscription settings or unsubscribe/leave the list here:
>http://www.acers.org/cic/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at
>melpots2@visi.com
>
>______________________________________________________________________________
>Clayart members may send postings to: clayart@lsv.ceramics.org
>
>You may look at the archives for the list, post messages, change your
>subscription settings or unsubscribe/leave the list here:
http://www.acers.org/cic/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots2@visi.com
Ivor and Olive Lewis on sun 17 feb 08
Addition of Talc or Magnesite to a clay body promotes the creation of =
Cordierite (2MgO.2Al2O3.5SiO2). This substance has a low CoE and does =
not undergo phase changes. It is hard and does not fracture easily. It =
has refractory qualities.
Talc, when heated dehydrates and alters to Enstatite (MgO.2SiO2) at 900 =
deg C. Enstatite is refractory.
Ivor Lewis.
Redhill,
South Australia.
Ron Roy on mon 18 feb 08
Hi Richard,
Talc is hardly ever used as a body flux in high fired clays - it is used in
white eathenwares for two reasons. It forms enstatite which helps raise the
expansion of low fired clay which helps to controll crazing. It also helps
control moisture expansion in non virious bodies - which also helps stop
delayed crazing.
One of the cone 6 clays I monitor is fluxed with talc - and I also made one
for a tile company - because they needed a high expansion body and it seems
to work OK at cone 6 - just a trace of cristobalite when I measured it.
I think you have a pretty good grasp but there are some gaps - I would not
place much store in Tichane - there are more modern texts with more
reliable info - you should read Hamer - the section on silica and get a
copy of Peter Sohngen's article - he really did nail this well.
RR
>My understanding is that there is silica glass, free silica, and
>cristobalite in the clay matrix above 2200 degrees Fahrenheit. Silica glass
>is pretty non-reactive, but too much of it and the body is overvitrified and
>weak. Free silica is what goes through the quartz inversion phase,
>resulting in about 1% shrinkage/expansion around 1053 degrees Fahrenheit (my
>figure may be off a bit...I don't have any texts here near the computer),
>while cristobalite will goes through a similar phase, but more extreme
>(around 3% (??)) in the 400-500 F range. This excess of cristobalite is
>what causes much clay dunting, I believe.
>
>In order to make sure your body is strong and functional, it's important to
>balance the three types of silica in the fired body. In order to prevent
>too much cristobalite from forming, you should make sure there are adequate
>fluxes in your clay mixture. These fluxes are most often feldspars, and
>most commonly potash feldspar. Sometimes in stoneware clay bodies, most
>particularly in whiteware bodies (why, I'm not sure, but if you check clay
>formulae, you will see that it's mostly in whiteware), talc is added for, as
>I recall reading in Tichane, "tightening the body up." I assume this means
>tightening as in fluxing. One of the particularly nice qualities of Nytal
>is it's approximately 9% calcium content, which added considerably to it's
>fluxing power. Above 2200 F, the kaolinite crystals start to decompose,
>forming mullite (good thing for fired strength) and freeing up silica, which
>will form cristobalite if the temp is held in or above that range for any
>length of time. Having a good glassy melt will take enough of this
>cristobalite into solution so that it will not constitute a structural
>threat when cooling or in repeated heating and cooling. My take on the
>problems of dunting caused by the absence of talc in one clay body in my
>past is that the body was not fluxed properly at high temps, and so not
>enough of the cristobalite was taken into the glassy melt, and therefore was
>free to do it's damage upon cooling.
>
>So, that's my brief take on this issue. I welcome comments from those who
>know more than I.
>
>Best,
>Richard Aerni
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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