Vince Pitelka on mon 4 oct 04
> As you suspected dunting isnt the correct term as it specifically relates
> to thermal shock; i.e. cracking due to sudden change in temperature.
Andrew -
I have always understood that dunting refers to cracking or shattering that
occurs during or upon cooling, and it can be due thermal shock, but is
usually agrivated by poor glaze fit. Shivering is bad enough, but if the
clay-glaze mismatch is more severe, the whole piece can come apart, and that
is dunting. I'd love to know if I have been misrepresenting this
phenomenon.
Thanks -
- 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/
mailtoandrew@FSMAIL.NET on mon 4 oct 04
Hello jklay
As you suspected dunting isnt the correct term as it specifically relates
to thermal shock; i.e. cracking due to sudden change in temperature.
From your description your ware could be under relatively high stress due
to differences in the thermal expansion of the body and glaze. Clearly not
sufficient to cause problems immediately but eventually, or perhaps when
additional stress is applied from the setting down of the hard object you
described. The problem is therefore a mismatch of the respective thermal
expansions and a reformulation of either should help. Based on the
information in your post its a bit difficult to be more specific than that.
Hope thats of a little help.
Regards,
Andrew
mailtoandrew@FSMAIL.NET on tue 5 oct 04
Hello Vince,
Certainly many of the peculiar terms found in ceramics can have slightly
different meanings across the globe, or even between individuals. However
Ive always found dunting is used to refer to thermal shock cracks, which
by the nature of the quartz phase changes are likely to be during cooling
rather than heating. Anyway for an authoritative source the following is
quoted from ASTM C242-01, Standard Terminology of Ceramic Whitewares and
Related Products:
Dunting =96 the cracking that occurs in fired ceramic bodies as a
result of thermally induced stress
A dunt is easily identifiable from other cracks as the exposed body is
shiny with a glassy finish; other cracks leave a rougher texture. Whilst
ware can break solely due to severe mismatch of the glaze/body expansion
perhaps in the cases you describe the combination of a small thermal shock
combined with a slight mismatch of coefficients led to the breaks.
Hope that clarifies,
Regards,
Andrew
Vince Pitelka on tue 5 oct 04
Andrew wrote:
"Anyway for an authoritative source the following is
quoted from ASTM C242-01, Standard Terminology of Ceramic Whitewares and
Related Products: 'Dunting - the cracking that occurs in fired ceramic
bodies as a
result of thermally induced stress.'"
Andrew -
Thanks for doing that legwork. I am glad to have that cleared up. I guess
this just shows that we should each do a Google search on absolutely every
single thing we know. Gee, I wish I had the time . . . . .
- 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/
Louis Katz on tue 5 oct 04
Hi Vince,
Dunting as I understand it is not just cracking from glaze fit but any
cracking on cooling.
Louis
Louis as GQLou:
http://falcon.tamucc.edu/~lkatz/LK/picturesof/pages/b.htm
On Oct 4, 2004, at 9:41 PM, Vince Pitelka wrote:
>> As you suspected dunting isnt the correct term as it specifically
>> relates
>> to thermal shock; i.e. cracking due to sudden change in temperature.
>
> Andrew -
> I have always understood that dunting refers to cracking or shattering
> that
> occurs during or upon cooling, and it can be due thermal shock, but is
> usually agrivated by poor glaze fit. Shivering is bad enough, but if
> the
> clay-glaze mismatch is more severe, the whole piece can come apart,
> and that
> is dunting. I'd love to know if I have been misrepresenting this
> phenomenon.
> Thanks -
> - 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.
>
John Hesselberth on tue 5 oct 04
On Tuesday, October 5, 2004, at 02:09 AM, mailtoandrew@FSMAIL.NET=20
wrote:
> Dunting =96 the cracking that occurs in fired ceramic bodies as a
> result of thermally induced stress
Hi Andrew,
I agree with the above definition, but that does not preclude Vince's=20
use of the word. In fact it validates it. The stress that is thermally=20=
induced results as the piece cools with different COEs of glaze and=20
body. The actual crack can occur days, weeks, or months later. The=20
stress that was responsible for the crack was still thermally induced.
Regards,
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
John Kudlacek on wed 6 oct 04
Thanks John,
Though your broader interpretation of "thermally induced" crossed my mind
I appreciate your validation. I was worried for a while that we might
have to invent a new ceramic term to cover it. Come to think of it, we
still could. It would refer to that form of dunting which occurrs long
after the fire, precipitated by physical shock, change in temperature or
other atmospheric conditions. Any ideas?
By the way, I have experienced this dunting only on thrown forms; the
crack configuration appearing to be related to the throwing induced torque.
John Kudlacek
John Britt on wed 6 oct 04
Hello all,
Hamer says that dunting "is cracking of pottery caused by stesses which
form during firing and cooling."
So you can have bisque dunts, cooling dunts and even early dunts in a
glaze firing that goes up too fast.
John Britt
www.johnbrittpottery.com
mailtoandrew@FSMAIL.NET on wed 6 oct 04
Hello John,
Thanks for your input but, in the nicest way, I disagree.
Dunting does not result from any thermally induced stress but from thermal
shock: sudden change in temperature ... associated volume change...
differential volume change ... resulting in high stress relived by the
formation of a crack.
You stated < stress that is thermally induced results as the piece cools
with different COEs of glaze and body> A properly matched glaze and body
will have different COE! Sure if significantly different problems can
occur, including cracking of the article, but these are not dunts.
As I didnt wish to appear simply perverse I had another quick search in
books:
Dictionary of Ceramics
Third edition
Dodd A and Murfin D
Institute of Materials. 1994
.... Dunt, dunting: A crack, or the formation of cracks which may be
invisible, in ware cooled too quickly after it has been fired
An Introduction To The Technology Of Pottery
Second edition
Rado P
Institute of Ceramics. 1988
.... Dunting: Crack formation in ware having been cooled too quickly
after firing
The Clayworkers Handbook
Searle A.B
Charles Griffin. 1929
.... If cooled too rapidly, the goods will dunt ...
Im not disagreeing that severe clay-glaze mismatch can lead to breakage
but John K.s original post described ware cracking years after firing.
Thermal mismatch could have been the cause but unless he had reheated then
quickly cooled it wasnt thermally shocked, and therefore hadnt dunted.
Bad glaze/body fits can result in cracks straight from the kiln. Such a
system could exasperate a borderline dunting situation. But the term is,
by widespread use and standardised definition, specific to cracks
resulting from stresses caused by rapid changes in volume following sudden
changes in temperature. Unglazed ware can dunt!
Hoping you dont think Im being awkward ....
Regards,
Andrew
Ron Roy on wed 6 oct 04
Hi Andrew,
A couple of questions relating to this - please!
I have always described the cracking that occures in bisque - during
cooling - at 573C - as bisque dunting - this fits with the - "result if
thermally induced stress" - but does not fit with your comment of "body
with shiny and glassy finish." Am I wrong to describe the quartz cracking
as dunting?
Q #2 - If a pot cracks because the inside glaze has too low an
expansion/contraction compared to the body COE - whether during cooling or
in a day or week or years - what do I call that?
Keeping in mind the cracking can be due to the glaze/body COE's alone - or
because of the additional complications of quartz and/or cristobalite.
I suppose what I am asking is - can the term dunting be used to describe
any cracking that occures during cooling and/or after dekilning?
Just to add some more to this - Laurence and West - in Creamic Science for
the Potter - say about dunting "Dunting occurs only after the ware has
cooled to become an elastic solid" and "Some dunts have surfaces that are
glassy smooth and appear to be fire polished. These cracks occure around
the upper temperature range of 600C where the glass is still weak. Cracks
that are irregular at the fracture occure at a lower temperature
..........."
Thanks - RR
>Certainly many of the peculiar terms found in ceramics can have slightly
>different meanings across the globe, or even between individuals. However
>Ive always found dunting is used to refer to thermal shock cracks, which
>by the nature of the quartz phase changes are likely to be during cooling
>rather than heating. Anyway for an authoritative source the following is
>quoted from ASTM C242-01, Standard Terminology of Ceramic Whitewares and
>Related Products:
>
> Dunting =96 the cracking that occurs in fired ceramic bodies as a
>result of thermally induced stress
>
>A dunt is easily identifiable from other cracks as the exposed body is
>shiny with a glassy finish; other cracks leave a rougher texture. Whilst
>ware can break solely due to severe mismatch of the glaze/body expansion
>perhaps in the cases you describe the combination of a small thermal shock
>combined with a slight mismatch of coefficients led to the breaks.
>
>Hope that clarifies,
>
>Regards,
>Andrew
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
=46ax: 613-475-3513=20
Vince Pitelka on wed 6 oct 04
> Hi Vince,
> Dunting as I understand it is not just cracking from glaze fit but any
> cracking on cooling.
Louis -
It is good to have confirmation from you and John Hesselbreth that "dunting"
is what I thought. Periodically through my career in clay I have heard
potters speak in hushed terms about the terrors of dunting, and once I was
at a craft fair in Northern California where another potters wares were
dunting on the shelves. It was awful, and he proceeded to pack up
everything and go home, completely crestfallen.
We have experienced some occurrence of dunting at the Craft Center, always
in the salt kiln, often on wares that have a thick coat of shino on the
inside. I expect that the cause is over-vitrification of the claybody,
causing extreme tension between body and glaze during cooling. It is nice
when we can actually figure out the cause.
- 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 oct 04
Dear Vince,
Never forget that what is presented on the Internet may need to be
double checked to validate its authenticity.
By the way for those people who do not have access to a copy, the
Hamer reference to "Dunting" spread over four pages and includes
several illustrations.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
John Hesselberth on wed 6 oct 04
On Wednesday, October 6, 2004, at 12:04 PM, mailtoandrew@FSMAIL.NET=20
wrote:
> But the term is,
> by widespread use and standardised definition, specific to cracks
> resulting from stresses caused by rapid changes in volume following=20
> sudden
> changes in temperature. Unglazed ware can dunt!
Hi Andrew,
I feel the need to expand on my response of a few minutes ago. The=20
definition you quoted originally was, as you say, from an authoritative=20=
source--the ASTM which is usually the arbiter of technical issues such=20=
as these. Organizations like ASTM do not choose their words lightly. It=20=
was:
> Anyway for an authoritative source the following is
> quoted from ASTM C242-01, Standard Terminology of Ceramic Whitewares=20=
> and
> Related Products:
>
> Dunting =96 the cracking that occurs in fired ceramic bodies =
as a
> result of thermally induced stress
In your current post, you are trying to limit your definition to=20
thermal shock and quote a couple of sources that seem to support that=20
limitation. John Britt just posted the definition from Hamer and Hamer,=20=
another authoritative source:
> "is cracking of pottery caused by stesses which
> form during firing and cooling."
It does not limit the definition to thermal shock. In different words=20
it is almost identical to the ASTM definition you originally quoted.
While I agree that breaks caused by thermal shock are included in the=20
definition of dunting and can occur equally well in bisque as in glaze=20=
firing cycles, I do not agree that dunting is limited to thermal=20
shock--I accept the broader "thermally induced stress" definition. As=20=
I'm sure you know, the stresses induced by thermal shock are usually=20
"through the thickness" stresses and they dissipate when the=20
temperature equalizes throughout the body. Stress induced by a COE=20
mismatch is just as certainly a thermally induced stress but it does=20
not dissipate with time in solid inelastic materials. It sits there=20
like an accident waiting to happen if it approaches the breaking=20
strength of the material.
So it boils down to which "authority" you want to believe. Like many=20
things technical, there are apparently differing views in the=20
literature. I'll stick with ASTM and Hamer and Hamer.
Regards,
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
John Hesselberth on wed 6 oct 04
On Wednesday, October 6, 2004, at 12:04 PM, mailtoandrew@FSMAIL.NET
wrote:
> Hoping you dont think Im being awkward ....
No, but we will have to agree to disagree. Thermally induced stress
includes thermal shock, but thermal shock is not the only form of
thermally induced stress. The definition you quoted was thermally
induced stress (whether it be from thermal shock or slower processes)
and I'll buy that. Pots that crack on the shelf well after they are
cool may well be (but are not always) dunting cracks in my book.
Regards,
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
Louis Katz on thu 7 oct 04
I think its maybe the soda from the shino drying out in the body. I
have had more dunting from ware in salt and soda kilns in my career
than in reductin. I have come to believe it is from a slight increase
in vitrification. Just my best guess.
I was just describing to Ivor how I read everything on ceramics I could
find or at least looked through it from the Detroit Public Library Main
branch in 1972-3. Some things that I "know" turn out to be wrong,
skewed or otherwise inaccurate. We had a load of clay last semester
right near 40% feldspar I think. Unbelieveable problems. Slumpnig
dunting shivering, a mess.
By the way , Bluebird now sells an interlock to turn off the motor on
their pugmills when the hopper is open.
Louis
On Oct 6, 2004, at 8:52 PM, Vince Pitelka wrote:
>> Hi Vince,
>> Dunting as I understand it is not just cracking from glaze fit but any
>> cracking on cooling.
>
> Louis -
> It is good to have confirmation from you and John Hesselbreth that
> "dunting"
> is what I thought. Periodically through my career in clay I have heard
> potters speak in hushed terms about the terrors of dunting, and once I
> was
> at a craft fair in Northern California where another potters wares were
> dunting on the shelves. It was awful, and he proceeded to pack up
> everything and go home, completely crestfallen.
>
> We have experienced some occurrence of dunting at the Craft Center,
> always
> in the salt kiln, often on wares that have a thick coat of shino on the
> inside. I expect that the cause is over-vitrification of the claybody,
> causing extreme tension between body and glaze during cooling. It is
> nice
> when we can actually figure out the cause.
> - 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.
>
Ivor and Olive Lewis on thu 7 oct 04
Dear Ron Roy and Andrew,
I have been trying to imagine your example of a pot with a low CoE
interior glaze and a high CoE clay. This is going to put the interior
into compression and the exterior into tension.
Now it seems to me that fracture can only happen if the tension in the
clay as clay shrinks against the glaze exceeds its tensile strength.
That is not , as I understand "A Dunt". A more likely scenario in this
situation would be for the glaze to spall.
That Internal stresses of this type can remain dormant is a fact which
afflicts many other heat processed materials. Fracture happens when an
instantaneous surge of energy causes stress relaxation.
Did you read the Glossary of L & W. See "Dunting"...I Quote " Dunting.
A crack that occurs while ware is cooling, or bisque ware is reheated,
in the 400=BA to 600=BA C range. It is caused by the nonlinear expansion
and contraction of quartz in this temperature range "
In their discussion these authors warn of Cold Drafts impinging on the
body of a pot causing rapid localised cooling.
I think the following points have to be considered.
1 Dunting is a phenomenon associated with clay in the kiln during
processing, either as Bisque or as a vitrified solid.
2 Dunting is initiated by a rapid change in volume of a constituent of
bisque or vitrified clay.
3 There are two constituents of fired clay (of either variety) which
change their volume. They are Quartz and Cristobalite. both undergo
structural phase changes. Read Hamer on Silica for the details.
4 There is a marked discrepancy between the CoE's of Porcelain and
Quartz
I have experienced Dunting. Pots come from the kiln in two separate
pieces. The fracture was linear. I have experienced delayed stress
relaxation. The fractures are like forked lightning
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
mailtoandrew@FSMAIL.NET on thu 7 oct 04
Hello John, Roy, Ivor, Vince and everyone else,
Looks like a good debate which is keeping our brain cells working!
Just to clarify what I understand the issue is:
1. Dunting is specific to thermal shock cracks
Or
2. Dunting has a wider use that includes the above and cracking due to
mismatched glaze and body CoE, the latter can happen immediately or years
after firing.
Words, and not just jargon or genre specific terms, can have subtly or
widely different meanings but Im still firmly consider dunting to be
interpretation 1 and not 2. This is not based on any arrogance or bloody
mindiness on my part, simply reflecting my experience.
So why the differences in interpretation?
I did initially think maybe variations between countries: many on ClayArt
seem to be from the US whilst Im not. An example would be the difference
in meaning of flint in America than in other countries. However following
a further literature search this morning I found an American reference
that, to me anyway, confirms my understanding of dunting. Two references
are copied at the end of this post, the latter being the US source is
rather lengthy and refers to diagrams which obviously can not be
reproduced here.
Another reason maybe my background in academia and industry, where
certainly dunting is thermal shock only. Many contributors to ClayArt
appear to be from a craft origin, though perhaps Im wrong in that. However
Im simply guessing at possible reasons.
I admit to some hesitancy to describe any single source as being
definitive but I dont think it unreasonable to defer some respect to
organisations such as ASTM, BSI and DIN. Having been involved, albeit on
the periphery, of some of their deliberations I can attest that great care
is taken with the preparations of standards. However how they are used
once published? John Hs reading of the previously quoted ASTM definition
differed to mine. Who=92s right? Again I dont know but I think he is
extending interpretation of the thermally induced too far. It would, of
course, be good to hear from those who sat on the committee.
It would be against the generally friendly nature of ClayArt to be overly
dogmatic but, and risking over repetition: my understanding based on some
widespread practical experience as well as reading of academic books and
papers, is that dunting is thermal shock related alone and not glaze body
mismatch.
Specific terminology is useful but perhaps we should take care to avoid
misunderstanding of jargon.
Kind regards to you all,
Andrew
References mentioned earlier:
Ceramics Physical And Chemical Fundamentals
Samang H, Francis M
Butterworths. 1961
The temperature gradient in the body undergoes marked changes as the
temperature becomes higher, especially at points where thermal expansion
rises sharply and where it halts temporarily, e.g. when water of
constitution is being expelled from the clay substance at temperatures at
which exothermal reactions take place. These sudden changes in gradient
are reflected in volume changes, with the risk of fire cracking or
dunting. Another cause of sudden volume change is the quartz
transformation point at 573oC
Ceramic Microstructures their analysis, significance, and Production
Edited by Fulrath R.M. Pask J.A.
John Wiley. 1966
Proceedings of the Third International Materials Symposium.
University of California, Berkely. June 1966
It is important to analyse and understand the dunting phenomenon of a
whiteware body. Dunting is defined as cracking that occurs in fired
ceramic bodies due to thermally induced stress. In industrial practice
dunting invariably only occurs either in the first fired ware upon cooling
or during subsequent heating and cooling on the glaze firing or refiring.
The temperature at which dunting occurs may be represented as shown in fig
38.18. The thermally induced stress fracture exhibits a surface that
varies widely with the temperature of fracture. Surfaces fractured at or
near the high low quartz inversion temperature range are relatively
smooth, shinning dunt, while surfaces fractured near the cristobalite
inversion range or lower are relatively rough and dull, rough dunt.
Figures 38.19 illustrates the profile of the roughness of the fractured
surface of an early preheat dunt and a late pre heat dunt. The difference
between the preheat and cooling dunt is easily identified by the fact that
cooling dunts are sharp and clear cut at the glaze body interface. Preheat
dunts are more or less contaminated with the glaze seeping into the
fissure. The reason for variations in dunted surfaces has never been
explicitly explained.
Figure 38.20 shows that the surface of a low temperature dunt contains
many large quartz grains, while the fractured path for a high temperature
dunt only goes through small quartz grains and glassy matrix. This implies
that at lower temperatures the dunt links existing large periphery and
matrix cracks surrounding the large quartz grains. A thermally or
mechanically induced stress may immediately join these pre existing
cracked areas to form fractured paths. In the case of high temperature
dunt a much higher stress is necessary to break the piece, because the
internal cracks around the quartz grains are healed or partially heals, so
that the fracture does not interconnect the larger quartz grains. Recent
investigations show that the surface roughness of a fractured commercial
porcelain body may be correlated with the temperature at which breakage
occurs. The roughness profile decreases as temperature increases. Figure
38.21 indicates the statistical average roughness in comparing a
commercial porcelain body containing quartz and that of a similar body
containing alumina. The latter half dos not show variation of surface
roughness on fracture. The overall stresses induced in a whiteware product
may be traced from a number of varied sources. However, the presence in
the complex system of quartz grains, particularly the larger grain sizes,
is the main reason for the presence and propagation of internal crack
systems resulting from microstresses developed within the body on heating
and cooling.
Ivor and Olive Lewis on thu 7 oct 04
Dear John Kudlacek,
<fire, precipitated by physical shock, change in temperature or other
atmospheric conditions. Any ideas?>>
"Catastrophic Stress Relief" seems appropriate.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
John Hesselberth on thu 7 oct 04
On Wednesday, October 6, 2004, at 10:06 AM, John Kudlacek wrote:
> I was worried for a while that we might
> have to invent a new ceramic term to cover it. Come to think of it, we
> still could. It would refer to that form of dunting which occurrs long
> after the fire, precipitated by physical shock, change in temperature
> or
> other atmospheric conditions. Any ideas?
Hi John,
My preference would be to have two (or more) categories of dunting,
e.g. COE mismatch and thermal shock. Of course neither would be pure
and that would probably result in another round of debate at some point
in the future. As you can see from the ongoing debate, I doubt we will
reach resolution on anything in this go-around. These are complicated
issues and I think just having the debate helps clear the air and aids
understanding of the various phenomena we have to learn to deal with as
potters.
Regards,
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
Jim Murphy on thu 7 oct 04
General globally-accepted Thermal Shock Theory jargon:
1. Dunting - associated directly with thermal shock cracking in ceramic
"body" (glazed or unglazed ) formed during cooling stage of firing process.
2. Thermal Shock Fracture Resistance - Encompasses both short & longterm
thermal shock behavior of a ceramic "body" (glazed or unglazed). "Dunting"
falls within the wider scope of Thermal Shock Fracture Resistance.
Best wishes,
Jim Murphy
Louis Katz on fri 8 oct 04
> Hi John,
>
> My preference would be to have two (or more) categories of dunting,
I would prefer to have no dunting at all.
Louis
Mary K on fri 8 oct 04
Dear John,
How about "Post -Fire Traumatic Stress Syndrome" PFTSS ? That could also be an onomatopoeia....... :) Mary K
John Hesselberth wrote:
On Wednesday, October 6, 2004, at 10:06 AM, John Kudlacek wrote:
> I was worried for a while that we might
> have to invent a new ceramic term to cover it. Come to think of it, we
> still could. It would refer to that form of dunting which occurrs long
> after the fire, precipitated by physical shock, change in temperature
> or
> other atmospheric conditions. Any ideas?
Hi John,
My preference would be to have two (or more) categories of dunting,
e.g. COE mismatch and thermal shock. Of course neither would be pure
and that would probably result in another round of debate at some point
in the future. As you can see from the ongoing debate, I doubt we will
reach resolution on anything in this go-around. These are complicated
issues and I think just having the debate helps clear the air and aids
understanding of the various phenomena we have to learn to deal with as
potters.
Regards,
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
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Ivor and Olive Lewis on fri 8 oct 04
Dear Jim Murphy,
I think you owe all of us some additional information on this.
<< General globally-accepted Thermal Shock Theory jargon:
1. Dunting - associated directly with thermal shock cracking in
ceramic"body" (glazed or unglazed ) formed during cooling stage of
firing process.
2. Thermal Shock Fracture Resistance - Encompasses both short & long
term
thermal shock behaviour of a ceramic "body" (glazed or unglazed).
"Dunting"
falls within the wider scope of Thermal Shock Fracture Resistance. >>
To be a valid statement we need access to all of your references and
sources.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
claybair on fri 8 oct 04
"The Potter's Dictionary" (5th addition)
by Hamer & Hamer has 4 pages
(118-121) on dunting.
Gayle Bair
Bainbridge Island, WA
http://claybair.com
John Hesselberth on fri 8 oct 04
On Friday, October 8, 2004, at 10:17 AM, Mary K wrote:
> How about "Post -Fire Traumatic Stress Syndrome" PFTSS ?
Hi Mary,
Don't you think Post-Inflamed Traumatic Stress syndrome would be
better? Then we could just call it The PITS or better yet, Dunting--The
PITS.
Regards,
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
Ivor and Olive Lewis on fri 8 oct 04
Dear John Hesselberth,
Robert Fournier gives a concise appreciation of Dunting in his
Illustrated Dictionary of Practical Pottery.
He gives as the cause "...drafts of cool air striking it when firing
or cooling causing one area to contract more than the rest...."
As I said in an earlier post, Frank Hamer writes extensively on the
topic.
Has anyone mentioned yet that both Cristobalite and Quartz not only
undergo phase changes and that the graphs in L & W show that C of E of
each substance is variable with temperature.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
wjskw@BELLSOUTH.NET on fri 8 oct 04
John:
I thought we already had a name for it.....
I thought we called it "Oops"
Best,
Wayne Seidl
-----Original Message-----
On Friday, October 8, 2004, at 10:17 AM, Mary K wrote:
> How about "Post -Fire Traumatic Stress Syndrome" PFTSS ?
Hi Mary,
Don't you think Post-Inflamed Traumatic Stress syndrome would be
better? Then we could just call it The PITS or better yet,
Dunting--The
PITS.
Regards,
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
Ivor and Olive Lewis on sat 9 oct 04
Dear Wayne,
No, when you see it on a piece that might have been a "Racer" the term
is "S**T"
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
John Hesselberth on sat 9 oct 04
On Friday, October 8, 2004, at 04:31 AM, Ivor and Olive Lewis wrote:
> He gives as the cause "...drafts of cool air striking it when firing
> or cooling causing one area to contract more than the rest...."
> As I said in an earlier post, Frank Hamer writes extensively on the
> topic.
Hello Ivor,
As I noted in an earlier message you can find several definitions that
would support either Andrew's or my view. For example Hamer and Hamer
(at least in the 3rd edition) clearly use the broader definition that I
favor--they talk about dunting occurring well after removal from the
kiln. And they do give the most detailed discussion of the subject I
have run across.
Regards,
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
John Britt on sat 9 oct 04
I don't know the answer to your question but I am certain that we should
all be using the latest edition of the dictionary to quote.
If students in an English class all used different editions of the
Websters there would never be agreement.
Just my opinion,
JOhn Britt
www.johnbrittpottery.com
Ron Roy on sat 9 oct 04
Hi Ivor,
That is not how it works - the low expansion glaze is in compression on the
inside and the outside - because it's bonded to the body.
This works in favour of durable ware because it makes the ware stronger -
if the same or similar expansion glazes are on the inside and outside - up
to a point. At that point the strain becomes to much and either you have
shivering and/or dunting.
Think of it as the glaze becoming too big for the body during cooling -
both inside and outside.
The classic situation - hot water in poured into a tea pot - inside glaze -
already under compression from the body - expands from the heat a micro
second before the body and the pot cracks. Sometimes it happens the first
time tea is made - sometimes weeks, months or years later.
The worst situation is a low expansion on the inside and a crazed glaze on
the outside. The crazed glaze has already started micro crackes in the body
making the failure from a glaze under compression on the inside more
likely.
Pots which are ment to be subject to heat shock will last longer if they
have glazes that are just under compression - or grazed - as a crazing
glaze does not cause failure due to heat shock.
Keeping inside glazes on the thin side in mugs and tea pots helps minimize
the effect the glaze has on the body for example.
Regards - RR
>Dear Ron Roy and Andrew,
>I have been trying to imagine your example of a pot with a low CoE
>interior glaze and a high CoE clay. This is going to put the interior
>into compression and the exterior into tension.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Jim Murphy on sat 9 oct 04
Ivor wrote, "To be a valid statement we need access to all of your
references and sources."
Ivor,
(Smile) Judge Roy Beane would be proud of you! Not quite sure how you define
"valid".
Anyway ... some research focused on studying thermal shock properties in low
porosity (~0 absorption) industrial porcelain bodies (usually unglazed).
Other variables exist in more porous bodies although many thermal shock
studies have been done there as well.
Key to "new" terminology is the broader RANGE of analysis covered by
"Thermal Shock Fracture (& Damage) Resistance".
"Glaze" is often treated as a "laminate" material in Thermal Shock Fracture
(& Damage) Resistance studies, however, the body/glaze combination may still
be analyzed although not all theoretical modeling equations apply.
Here are a just a few references for your reading enjoyment - although 100's
exist "out there":
G. C. Robinson, "Designing Firing Schedules With Thermal Analysis"
(Published in "Science of Whitewares" (1996))
R.E. Moore, "Thermal Shock of Triaxial Porcelains", (Published in "Science
of Whitewares" (1996))
C. Baudin, "Resistance of Refractories to Thermal Shock I: Thermoelastic
Approximation and Energy Balance Criteria." (1993)
R.E. Moore, "Induced Thermal Shock Damage of Refractories and Other Ceramics
Using a Fluidized Bed Apparatus", Proc. of the Pacific Coast Regional
Meeting of the Am. Ceram. Soc., Oct. 29-31, (1984)
D.P.H. Hasselman, "Unified Theory of Thermal Shock Fracture Initiation and
Crack Propagation in Brittle Ceramics", J. Am. Ceram. Soc., 52(11) 60-604
(1969)
Hope this helps.
Best wishes,
Jim Murphy
("Is it safe? ... bzzzzzzzzzzzz ... is it safe?" (The Marathon Man)
mailtoandrew@FSMAIL.NET on sat 9 oct 04
Hello John,
I like the two acronyms PFTSs and PITS!
As your broader use of dunting encompasses a number of mechanisms would
you consider using qualifying terms? Such as coefficient dunting and
thermal shock dunting?
Whilst dont liking these myself, Im still firmly entrenched with the
specific use of the term which in my expereince has universal acceptance,
Im not looking to fall out with anyone!
Regards,
Andrew
Ivor and Olive Lewis on sun 10 oct 04
Dear Ron,
We have differing images in mind. You have put a glaze on the outside
of your example. I restricted my comment to glaze on the interior
surfaces only. In the latter example the glaze will contract during
cooling a lesser amount than the clay ceramic.
Thinking about the former example with identical glaze on the interior
and exterior surfaces. I have no argument with your conclusion. But in
extreme cases I believe the stage is set for the glaze on the outside
to spall, or as you say, shiver.
I am sending you a jpeg image by second post showing what I believe is
the extreme of having a low CoE on the interior and a high CoE n the
exterior.
I thought these were classic examples. Came out of the kiln like this.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
David Beumee on sun 10 oct 04
Ron Roy wrote:
"Pots which are ment to be subject to heat shock will last longer if they
have glazes that are just under compression - or grazed - as a crazing
glaze does not cause failure due to heat shock."
If a lower expansion glaze is put into compression by a higher expansion clay body, wouldn't the body be contracting faster than the glaze? How can such a situation cause a glaze to craze?
"This works in favour of durable ware because it makes the ware stronger -
if the same or similar expansion glazes are on the inside and outside - up
to a point. At that point the strain becomes to much and either you have
shivering and/or dunting."
This is a statement in favor of having the expansion of a body figured by dilitometry, and then creating glazes of enough lower expansion to be put under just the right amount of compression, creating the strongest possible ware. Right?
David Beumee
Earth Alchemy Pottery
Lafayette, CO
> Hi Ivor,
>
> That is not how it works - the low expansion glaze is in compression on the
> inside and the outside - because it's bonded to the body.
>
> This works in favour of durable ware because it makes the ware stronger -
> if the same or similar expansion glazes are on the inside and outside - up
> to a point. At that point the strain becomes to much and either you have
> shivering and/or dunting.
>
> Think of it as the glaze becoming too big for the body during cooling -
> both inside and outside.
>
> The classic situation - hot water in poured into a tea pot - inside glaze -
> already under compression from the body - expands from the heat a micro
> second before the body and the pot cracks. Sometimes it happens the first
> time tea is made - sometimes weeks, months or years later.
>
> The worst situation is a low expansion on the inside and a crazed glaze on
> the outside. The crazed glaze has already started micro crackes in the body
> making the failure from a glaze under compression on the inside more
> likely.
>
> Pots which are ment to be subject to heat shock will last longer if they
> have glazes that are just under compression - or grazed - as a crazing
> glaze does not cause failure due to heat shock.
>
> Keeping inside glazes on the thin side in mugs and tea pots helps minimize
> the effect the glaze has on the body for example.
>
>
> Regards - RR
>
>
> >Dear Ron Roy and Andrew,
> >I have been trying to imagine your example of a pot with a low CoE
> >interior glaze and a high CoE clay. This is going to put the interior
> >into compression and the exterior into tension.
>
> Ron Roy
> RR#4
> 15084 Little Lake Road
> Brighton, Ontario
> Canada
> K0K 1H0
> Phone: 613-475-9544
> Fax: 613-475-3513
>
> ______________________________________________________________________________
> 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.
John Kudlacek on sun 10 oct 04
Regarding responses to my post on Dunting: Thank you all. John and
Andrew, thanks for suggesting the term "COE Dunting". Seems workable to me
and Ron, thanks for your comprehensive yet concise explanation. Now that
that is settled to my satisfaction shall we deal with "How Many Angels can
Dance on the Tip of A Cone" ?
John Kudlacek
wjskw@BELLSOUTH.NET on sun 10 oct 04
John:
That all depends on a few variables:
What kind of dancing... Ballroom dancing is one thing, but if
they're trying line dancing, I would imagine that their wings are
going to get in the way, and knock a few off the tip. I think you
can just about forget clog-dancing, and the tip of a cone is way too
small for a decent rave . I don't recall if angels dance the
Lambada or not. Might piss off the boss! LOL
What type of cone...self-supporting, large or small?
Also, you need to specify if the cone has been fired (bent) or not,
or if it is in the process of being fired. Singed feathers!
Best,
Wayne Seidl
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of John
Kudlacek
Sent: Sunday, October 10, 2004 3:51 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Clay Body and "Dunting"
Regarding responses to my post on Dunting: Thank you all. John and
Andrew, thanks for suggesting the term "COE Dunting". Seems workable
to me
and Ron, thanks for your comprehensive yet concise explanation. Now
that
that is settled to my satisfaction shall we deal with "How Many
Angels can
Dance on the Tip of A Cone" ?
John Kudlacek
Ivor and Olive Lewis on sun 10 oct 04
Dear Jim,
Thanks for all that, including the humour.
I think for once I have asked the right question and have been given a
definitive answer.
If these articles and papers are readily available though our library
services I will certainly read them.
I hope other people who have made contributions have a look as well.
Kind regards,
Ivor Lewis.
Redhill,
S. Australia.
Ivor and Olive Lewis on mon 11 oct 04
Dear John,
I'm sure we can start when we have argued as to the nature of an
"Angel"
Ivor Lewis.
Redhill,
S. Australia.
Ron Roy on mon 11 oct 04
Hi Ivor,
I agree - classic example of what a bad mismatch of glaze and clay can do -
in a way better to see it right away instead of it happening on a pot in
use in somebodies kitchen.
I remember explaining - on ClayArt - about the freeze/boil test for
spotting this problem - some believed that pouring boiling water into a
frozen pot was too extreme a test and that no pottery could survive that.
Gives you some idea about the state of understanding by some potters
producing ware out there.
RR
>We have differing images in mind. You have put a glaze on the outside
>of your example. I restricted my comment to glaze on the interior
>surfaces only. In the latter example the glaze will contract during
>cooling a lesser amount than the clay ceramic.
>Thinking about the former example with identical glaze on the interior
>and exterior surfaces. I have no argument with your conclusion. But in
>extreme cases I believe the stage is set for the glaze on the outside
>to spall, or as you say, shiver.
>I am sending you a jpeg image by second post showing what I believe is
>the extreme of having a low CoE on the interior and a high CoE n the
>exterior.
>I thought these were classic examples. Came out of the kiln like this.
>Best regards,
>Ivor Lewis.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Mary K on mon 11 oct 04
Dear John,
I agree, PITSS is much better. My only objection to "inflammed" is that I first think of infection in that context, and don't know where on the pot to stick the "aspirin/penicillin"
smiling, no hurricanes this week........Mary K
wjskw@BELLSOUTH.NET wrote:
John:
I thought we already had a name for it.....
I thought we called it "Oops"
Best,
Wayne Seidl
-----Original Message-----
On Friday, October 8, 2004, at 10:17 AM, Mary K wrote:
> How about "Post -Fire Traumatic Stress Syndrome" PFTSS ?
Hi Mary,
Don't you think Post-Inflamed Traumatic Stress syndrome would be
better? Then we could just call it The PITS or better yet,
Dunting--The
PITS.
Regards,
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
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Ron Roy on tue 12 oct 04
Hi David,
What I mean is - have the right amount of compression to just keep a glaze
from crazing - or settle for a crazed glaze. Avoid a glaze that is under
too much compression as that situation will lead to failure sooner or
later.
Design of ware helps to cut down the chances of failure from a glaze under
too much compression as well. Rounded forms - as opposed to flat bottomed
forms - transfer strain better. Flat bottoms tend to concentrate strain at
the bottom corners - which explains why those types of forms fracture
around the bottom and the bottoms separate.
Dilatometry is the best way to deal with the problem because it tells you
how much free quartz and cristobalite is in the finished body - the
cristobalite being a crucial issue in ovenware especially.
Using a set of glazes designed to ascertain fit is a reasonable substitute
as they take into consideration the cristobalite with the exception that -
cristobalite is not something you want much of in ovenware because - if one
side of a pot is heated past the cristobalite conversion (approx. 200C)
sooner than the other side - then failure is a real possibility - sooner or
later.
RR
> "Pots which are ment to be subject to heat shock will last longer if they
>have glazes that are just under compression - or grazed - as a crazing
>glaze does not cause failure due to heat shock."
>
> If a lower expansion glaze is put into compression by a higher expansion
>clay body, wouldn't the body be contracting faster than the glaze? How can
>such a situation cause a glaze to craze?
>
> "This works in favour of durable ware because it makes the ware stronger -
>if the same or similar expansion glazes are on the inside and outside - up
>to a point. At that point the strain becomes to much and either you have
>shivering and/or dunting."
>
> This is a statement in favor of having the expansion of a body figured by
>dilitometry, and then creating glazes of enough lower expansion to be put
>under just the right amount of compression, creating the strongest
>possible ware. Right?
>
>David Beumee
>Earth Alchemy Pottery
>Lafayette, CO
>
>
>> Hi Ivor,
>>
>> That is not how it works - the low expansion glaze is in compression on the
>> inside and the outside - because it's bonded to the body.
>>
>> This works in favour of durable ware because it makes the ware stronger -
>> if the same or similar expansion glazes are on the inside and outside - up
>> to a point. At that point the strain becomes to much and either you have
>> shivering and/or dunting.
>>
>> Think of it as the glaze becoming too big for the body during cooling -
>> both inside and outside.
>>
>> The classic situation - hot water in poured into a tea pot - inside glaze -
>> already under compression from the body - expands from the heat a micro
>> second before the body and the pot cracks. Sometimes it happens the first
>> time tea is made - sometimes weeks, months or years later.
>>
>> The worst situation is a low expansion on the inside and a crazed glaze on
>> the outside. The crazed glaze has already started micro crackes in the body
>> making the failure from a glaze under compression on the inside more
>> likely.
>>
>> Pots which are ment to be subject to heat shock will last longer if they
>> have glazes that are just under compression - or grazed - as a crazing
>> glaze does not cause failure due to heat shock.
>>
>> Keeping inside glazes on the thin side in mugs and tea pots helps minimize
>> the effect the glaze has on the body for example.
>>
>>
>> Regards - RR
>>
>>
>> >Dear Ron Roy and Andrew,
>> >I have been trying to imagine your example of a pot with a low CoE
>> >interior glaze and a high CoE clay. This is going to put the interior
>> >into compression and the exterior into tension.
>>
>> Ron Roy
>> RR#4
>> 15084 Little Lake Road
>> Brighton, Ontario
>> Canada
>> K0K 1H0
>> Phone: 613-475-9544
>> Fax: 613-475-3513
>>
>>
>>__________________________________________________________________________
>>____
>> 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.
>
>______________________________________________________________________________
>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
Phone: 613-475-9544
Fax: 613-475-3513
Ivor and Olive Lewis on tue 12 oct 04
Dear Ron,
There were other pots in this firing which were larger and came out
without a blemish. Days later I heard a rifle discharge and I thought
the report sounded rather close. Then the next loud report came and I
realised it was very close. Yes, two pots had shattered, though not
with those delightful patterns.
So stress relief can be delayed and it can be triggered by thermal
shock. Which makes the "Freezer to Boiling Water Transition" a valid
test for the durability of work.
Best regards,
Ivor.
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