Lynn Barnwell on wed 31 mar 04
Help!
I had a batch of salad plates to split in half after they were sitting on
the shelf for 2-4 days. 5 of 9 plates have broken so far and I am afraid
the others might do it at any moment. It is a fairly straight line crack
from edge to edge. The clay body is one I use all the time Standard 112 but
I don't know what is in the glaze. It is a beautiful soft butter yellow
glaze, very glossy and I don't think it is applied too thick. There is no
shivering of the glaze and I rubbed the surface with ink and do not see any
crazing patterns going on the surface.
Don't now if I have given enough info to speculate the cause..
The glaze was dipped for 2-3 seconds.
Fired in cone 6 oxidation
Did not open the kiln until it was at 175 F and let it cool to 90 F before
unloading.
I am doing an boil abosorption test now..
I don't know what I am doing so guide and direct me please.
Lynn Barnwell
Hinkle Creek Pottery
Corinth, MS
Snail Scott on fri 2 apr 04
At 03:10 PM 3/31/04 -0600, you wrote:
>I had a batch of salad plates to split in half after they were sitting on
>the shelf for 2-4 days. 5 of 9 plates have broken so far and I am afraid
>the others might do it at any moment. It is a fairly straight line crack
>from edge to edge.
Eeks! Classic dunting. Has this ever happened with
another glaze on the same clay body and forms? Or
just this combo?
-Snail
Lor & Jas White on fri 2 apr 04
Hi
I had a bowl pretty much crack in half too. It took about a week for it to
happen. It was on porcelain and on the inside I used a low expansion glaze
specifically for porcelain. On the outside I used a more normal expansion
rate glaze. Guess they were fighting each other :( Anyway don't know maybe
something like this happened to your plates. It was a good lesson for me
hope for you too. Wont be able to continue conversation signing off clayart
for a few weeks. Bye everyone!
Loryn White
----- Original Message -----
From: "Lynn Barnwell"
To:
Sent: Wednesday, March 31, 2004 1:10 PM
Subject: Help my plates are splitting in two!
> Help!
>
>
>
> I had a batch of salad plates to split in half after they were sitting on
> the shelf for 2-4 days. 5 of 9 plates have broken so far and I am afraid
> the others might do it at any moment. It is a fairly straight line crack
> from edge to edge. The clay body is one I use all the time Standard 112
but
> I don't know what is in the glaze. It is a beautiful soft butter yellow
> glaze, very glossy and I don't think it is applied too thick. There is no
> shivering of the glaze and I rubbed the surface with ink and do not see
any
> crazing patterns going on the surface.
>
>
>
> Don't now if I have given enough info to speculate the cause..
>
>
>
> The glaze was dipped for 2-3 seconds.
>
>
>
> Fired in cone 6 oxidation
>
>
>
> Did not open the kiln until it was at 175 F and let it cool to 90 F before
> unloading.
>
>
>
> I am doing an boil abosorption test now..
>
>
>
> I don't know what I am doing so guide and direct me please.
>
>
>
> Lynn Barnwell
>
> Hinkle Creek Pottery
>
> Corinth, MS
>
>
____________________________________________________________________________
__
> 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.
>
Craig Dunn Clark on sat 3 apr 04
Lynn, if you have not had any other similar problems with this clay body
and you know for certain that there is not a problem with that particular
batch of clay then the problem is most likely with the glaze. It most
probably does not fit. There is no way to deterimine this really without
knowing what the coefficient of expansion is for the clay body from that
particular batch and whether or not it will fit with that of the glaze.
The problem is that you do not know what is in the glaze. In other words
you are searching around in the dark without a flashlight. You cannot make
any type of substantive determinations without the formula for the glaze
that you are using. Unless you are able to get one then you most probably
ought find another glaze that you know something about.
One thing that you might try and do is to use a glaze that you know the
composition of and that you know works on that batch of clay that is giving
you the problems (assuming you have any of it left.) If the splitting does
not occur then the glaze is one of the most likely culprits. If however it
does then maybe you either got hold of a bad and/or different batch of clay
(this happens occassionally) of the clay was somehow contaminated (is there
any plaster in your studio that may have inadverdently gotten mixed or
wedged in). Is there anyway that someone, other than yourself, could have
gotten into the kiln without your knowlege? Is there any glaze that has run
into breaks. This will tell you if the splitting occured when the kiln was
on the way up or on the way down.
Hope this helps a bit
Craig Dunn Clark
619 East 11 1/2 st
Houston, Texas 77008
(713(861-2083
mudman@hal-pc.org
----- Original Message -----
From: "Lynn Barnwell"
To:
Sent: Wednesday, March 31, 2004 3:10 PM
Subject: Help my plates are splitting in two!
> Help!
>
>
>
> I had a batch of salad plates to split in half after they were sitting on
> the shelf for 2-4 days. 5 of 9 plates have broken so far and I am afraid
> the others might do it at any moment. It is a fairly straight line crack
> from edge to edge. The clay body is one I use all the time Standard 112
but
> I don't know what is in the glaze. It is a beautiful soft butter yellow
> glaze, very glossy and I don't think it is applied too thick. There is no
> shivering of the glaze and I rubbed the surface with ink and do not see
any
> crazing patterns going on the surface.
>
>
>
> Don't now if I have given enough info to speculate the cause..
>
>
>
> The glaze was dipped for 2-3 seconds.
>
>
>
> Fired in cone 6 oxidation
>
>
>
> Did not open the kiln until it was at 175 F and let it cool to 90 F before
> unloading.
>
>
>
> I am doing an boil abosorption test now..
>
>
>
> I don't know what I am doing so guide and direct me please.
>
>
>
> Lynn Barnwell
>
> Hinkle Creek Pottery
>
> Corinth, MS
>
>
____________________________________________________________________________
__
> 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 sat 3 apr 04
Hi Lynn,
I lost the original of your post but - it is a fit mismatch between clay
and glaze.
I need what cone you are firing to, the glaze recipe and any additional
info you have about the clay - like batch number.
You should phone Standard and find out if there have been any changes to
that body lately.
I have measured another type of standard clay and found a large amount of
cristobalite in a cone 6 body - something I have never seen before. I have
offered to help them find out why but I have not heard back from them.
Anyway - once I have calculated the expansion of that glaze I can speculate
more accurately.
RR
>> I had a batch of salad plates to split in half after they were sitting on
>> the shelf for 2-4 days. 5 of 9 plates have broken so far and I am afraid
>> the others might do it at any moment. It is a fairly straight line crack
>> from edge to edge. The clay body is one I use all the time Standard 112
>but
>> I don't know what is in the glaze. It is a beautiful soft butter yellow
>> glaze, very glossy and I don't think it is applied too thick. There is no
>> shivering of the glaze and I rubbed the surface with ink and do not see
>any
>> crazing patterns going on the surface.
>>
>> Don't now if I have given enough info to speculate the cause..
>>
>> The glaze was dipped for 2-3 seconds.
>>
>> Fired in cone 6 oxidation
>>
>> Did not open the kiln until it was at 175 F and let it cool to 90 F before
>> unloading.
>>
>> I am doing an boil abosorption test now..
>>
>> I don't know what I am doing so guide and direct me please.
>>
>> Lynn Barnwell
>> Hinkle Creek Pottery
>> Corinth, MS
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
claybair on sun 4 apr 04
I had 2 platters dunt and could never figure out why.
The I got a tip from a video
(sorry I don't remember which one):
to never sponge the bottom of platters or plates.
Since I have stopped that practice I haven't had
any dunting reoccur. Note, those platters were
glazed on one side.
Gayle Bair
Bainbridge Island, WA
http://claybair.com
-----Original Message-----
From: Snail Scott
Subject: Re: Help my plates are splitting in two!
At 03:10 PM 3/31/04 -0600, you wrote:
>I had a batch of salad plates to split in half after they were sitting on
>the shelf for 2-4 days. 5 of 9 plates have broken so far and I am afraid
>the others might do it at any moment. It is a fairly straight line crack
>from edge to edge.
Eeks! Classic dunting. Has this ever happened with
another glaze on the same clay body and forms? Or
just this combo?
-Snail
Brian Haviland on mon 5 apr 04
That's funny !!!! I had a few bowls and platters do the same thing.
Standard
112.
Also had 3 jars made of standard ^6 porcelain (I know... not real
porcelain) do the same
thing. Nice even walls,slow cooled... no problem w/ any other batches of
clay. Says a lot for learning
to make your own clay. You have more control because you know the
body,expansion
on a more personal level. You can match glaze to clay body. I,m getting
there..Slowly...wish i had waynes
new peter pugger... Jealousy rears it's ugly head...My bad..I've called
standard and they said I'm
stressing the clay body with bad fitting glazes..I know my glazes..matches
clay expansion..who knows !!
Just keep trying till i get it right. Lots of experiments in the kiln when
i fire each week.
Brian Haviland
At 03:10 PM 3/31/04 -0600, you wrote:
>Help!
>I had a batch of salad plates to split in half after they were sitting on
>the shelf for 2-4 days. 5 of 9 plates have broken so far and I am afraid
>the others might do it at any moment. It is a fairly straight line crack
>from edge to edge. The clay body is one I use all the time Standard 112 but
>I don't know what is in the glaze.
>
>Fired in cone 6 oxidation
>Did not open the kiln until it was at 175 F and let it cool to 90 F before
>unloading.
>Lynn Barnwell
>
>
>______________________________________________________________________________
>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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Lois Ruben Aronow on mon 5 apr 04
> Also had 3 jars made of standard ^6 porcelain (I know... not real
> porcelain)
Just a word about Standard's ^6 porcelains - they are indeed are real
porcelains. Standard makes 3 different ^6 porcelains. The 365 is a delight
- nice to throw, warps like hell, and gods beautifully translucent. Super
white grolleg porcelain. Vitreous. #551 also goes translucent, but not as
lusciously as the other. Superior throwing qualities, also very white,
wonderful to throw. Vitreous when fired to temp. Contains grolleg. Lovely.
The #213 is a domestic porcelain - no translucency. Feels to me like
stoneware. Contains ball clay.
#130 has a range of cones 7-9. Looks good at cone 7. Fully mature. No
translucency at that temp.
I don't work for standard, but I do use their porcelain all the time.
...Lo
Stephani Stephenson on mon 5 apr 04
I attended a lecture at NCECA, given by William McCarty.
he has a Ph.D from Alfred and works with the Whiteware Research
Center.
(He mentioned he has taught ceramic science to potters for the last 6
years
he feels like he is just now beginning to bridge the communication gap)
He has a great attitude about the 2 groups, the studio potters and the
scientists, trying to communicate.
and , in deference to potters, he will tell you that he learns as much
from the artists and potters as vice versa.
What comes to mind with regard to your plate problem, is something
McCarty said in the lecture, which stuck in my mind.
During his discussion of the causes of slumping (or pyroplastic
deformation as he put it)
McCarty stated that 'packing uniformity of the body' is one of the 3
main causes of slumping.
some excerpts worth noting, from notes, are as follows:
Packing can be affected during the forming process (via tension and
compression)
Tension is pulling and compression is pushing
Ceramic and glass bodies always fail in tension
Warping is due to compression
Cracking is due to tension
Stresses develop due to non uniform particle packing in the body
Differential stresses also lead to differential shrinkage
He followed up with photos of plates with cracks running from rim
toward the center (density of the rim less than density at the well or
center,) tension :cracking
plates with warped edges where the rim was in compression but stresses
were not sufficient to cause cracking at the center. density
measurement showed the density at the rim was greater than the density
at the well prior to firing. i.e. compression:warping
plates with cracks running straight through the center, almost cutting
the plate in half, but not extending all the way out to the rim,
rim in compression, i.e. density at the rim greater than the density at
the center. Center in tension:cracking
What struck me was that basically this was scientific re-wording of
age old good throwing techniques . We learn to compress the bottom of
a plate, compress the rim of a plate or bowl . What struck me was 1)
a reminder of how, in a plate , consistent density throughout is
important,
2) thinking in terms of uniform packing of the clay body, this also
applies to tilemaking and other handforming, hand pressing tile, etc.
which is something I have never been taught, in so many words, but
which backs up my own dawning observations,
Also was the emphasis, that this is a VERY important factor. So this
was a great reminder: old, very basic information, presented in a new
way and with added emphasis.
BTW, the other 2 main contributers are body composition/chemistry,
primarily as it relates to the 'glass formation boundary': issues of
silica deficient bodies, and the composition of the glass... this is a
topic too in depth for this post, and though I did understand McCarty,
I don't have a sufficient grasp of this part of it to even pretend to
speak knowledgeably.
The third factor is MIXING. I understood McCarty to say that mixing is
critical, and that industry practices, if improved could greatly
increase clay fired strength(was i t by 30% or 50%?) .
He defined good mixing as "mixing is not just to have a uniform
distribution of batch constituents".
What they have found is that felspar particles, or quartz particles
tend to 'cluster', and the clumping persists during dry and wet mixing
of ingredients. When these particles contact each other directly and
persist, they create resulting weak areas in the fired clay..
as I understand it, because the resulting glass in that 'cluster', is
more viscous... McCarty also noted this as a root cause of some
bloating problems
He noted one solution is to premix silica or felspar powder with
clay powder, before any other mixing occurs.. the clay particles
separate and isolate the felspar or silica particles and make for
better mixing... also he followed with other procedures for better
mixing, which I have to admit I am hazy on.... needless to say, McCarty
stressed that suppliers will hopefully improve mixing methods based on
some of this research.
Again I think back to some of the old time information on clay and clay
mixing and clay aging. whether this new research will align with
what the old timers said all along.
If anyone else who heard this lecture or is familiar with this research
has a comment please do! or if I have represented McCarty's statements
in error, please comment .
Anyway , with regard to the plates..I would say take a look at the
forming/throwing process with the image of uniform density /packing in
mind.
Of course this this very general, and perhaps all is well in that
department , in the work of the potter who originally asked!
But I have been having some warping recently in some of my tiles and
am looking at my clay.
I'm usually the first one to say o! it's the clay or o! it's the glaze,
but this talk made me go back and scrutinize my forming process and
skills as well
Stephani Stephenson
steph@revivaltileworks.com
http://www.revivaltileworks.com
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