Vince Pitelka on tue 11 oct 05
Bert Gibson wrote:
"I talked to my instructor and he told me he lost so many large bowls at
cone 10, that he now only makes them for cone 6 firings. So is warpage at
cone 10 a common problem for larger pieces? I can't do anything about the
way they fire at school, but could that be a factor? Since my instructor
mentioned his similar experience, I didn't think it would be anything in my
throwing ."
Bert -
The most common cause of large bowls warping in high-firing is uneven
support under the foot. In academic institutions the shelves are often
slightly warped, or else the surfaces are irregular from accumulated wash
and glaze runs. Are the shelves you used absolutely flat and smooth? If
not, and if you are committed to making large bowls in the academic setting,
you may have to invest in several large 1"-thick high-alumina shelves. Soak
them in water, and then cut them into smaller squares with a masonry blade
on a skill saw, stopping to re-moisten the shelves several times as you make
the cut - not soaking wet, just let the water soak into the shelf before you
start cutting again. Cut the shelves into squares of an appropriate size to
fit under the entire foot of your bowls when you fire them - one square per
bowls. That way you will have absolutely flat support for your bowls every
time.
Make sure that not even a tiny portion of the foot of the bowl is hanging
off the edge of the shelf - that's a surefire warped bowl every time.
Depending on location of the burners and proximity to your bowls, that could
also cause warpage by over-vitrifying part of the bowl, causing greater
shrinkage in that part. The solution is to fire the bowls higher up in the
kiln.
I hesitate to sound critical of your teacher, but if he is taking the same
claybody and firing it to a lower temperature, it will certainly be less
prone to warpage, but it will also be underfired and less durable, and
glazes will be more likely to craze.
The best functional bodies are extremely dense and vitrified when fired to
maturity, and that means that the fluxes in the clay are very active,
forming a fully-developed glassy phase. As a result, the clay becomes quite
pyroplastic, and if it is not supported well, or if heat is applied
unevenly, then warpage is inevitable.
Good luck -
- 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/
Michael Wendt on tue 11 oct 05
Others have already covered the basics but I want to add that many clay
bodies are very sensitive to uneven heating and will go out of round even if
they are perfectly level and well supported. Saggars made of Insulating Fire
Bricks can be installed at the edge of the shelf near the burner ports to
overcome the rapid heating (and cooling) that often lead to odd warping in
larger pots.
YMMV, but at least try it once, you may be surprised how much it helps.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, Idaho 83501
USA
wendtpot@lewiston.com
www.wendtpottery.com
Bert wrote:
I recently had a couple of bowls, about 12 inches in diameter, come out of a
glaze firing (Cone 10 R) warped. I know they weren't warped after my cone 04
bisque. I was told they were on the bottom shelf of the kiln and I know that
is on the same level as the burner ports.
I talked to my instructor and he told me he lost so many large bowls at cone
10, that he now only makes them for cone 6 firings. So is warpage at cone 10
a common problem for larger pieces? I can't do anything about the way they
fire at school, but could that be a factor? Since my instructor mentioned
his similar experience, I didn't think it would be anything in my throwing .
Any thoughts. Thanks.
Bert Gibson
http://home.comcast.net/~lazybpottery/
tsull320@COMCAST.NET on tue 11 oct 05
I regularly fire 17" wide bowls with no warping at cone 10 in a reduction
environment. I found it necessary to build baffle walls around the bowls to
prevent the flame from direct contact with the piece. I fire a single bowl
per shelf, with the shelf above as close as possible (3/8" or so), and
completely surrounded by sections of cylinders slightly taller that the
bowl. I throw the cylinders out of a very groggy clay and cut them into 8"
wide sections, and use as many as necessary to shield the bowls.
Hope this helps.
Tim
Timothy Sullivan
www.creeksidepottery.net
Marietta, GA
tim@creeksidepottery.net
On Tue, 11 Oct 2005 15:49:08 -0700, Bert Gibson
wrote:
>I recently had a couple of bowls, about 12 inches in diameter, come out of
a glaze firing (Cone 10 R) warped. I know they weren't warped after my cone
04 bisque. I was told they were on the bottom shelf of the kiln and I know
that is on the same level as the burner ports.
>
>I talked to my instructor and he told me he lost so many large bowls at
cone 10, that he now only makes them for cone 6 firings. So is warpage at
cone 10 a common problem for larger pieces? I can't do anything about the
way they fire at school, but could that be a factor? Since my instructor
mentioned his similar experience, I didn't think it would be anything in my
throwing . Any thoughts. Thanks.
>
>Bert Gibson
>http://home.comcast.net/~lazybpottery/
>
>______________________________________________________________________________
>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.
Bert Gibson on tue 11 oct 05
I recently had a couple of bowls, about 12 inches in diameter, come out =
of a glaze firing (Cone 10 R) warped. I know they weren't warped after =
my cone 04 bisque. I was told they were on the bottom shelf of the kiln =
and I know that is on the same level as the burner ports.=20
I talked to my instructor and he told me he lost so many large bowls at =
cone 10, that he now only makes them for cone 6 firings. So is warpage =
at cone 10 a common problem for larger pieces? I can't do anything about =
the way they fire at school, but could that be a factor? Since my =
instructor mentioned his similar experience, I didn't think it would be =
anything in my throwing . Any thoughts. Thanks.
Bert Gibson
http://home.comcast.net/~lazybpottery/
danielle smith on tue 11 oct 05
I have thrown many large bowls and have fired them at cone 10 and have not had warping issues. Is it possible that there is a problem with the way the kiln is loaded or fired?
Bert Gibson wrote:I recently had a couple of bowls, about 12 inches in diameter, come out of a glaze firing (Cone 10 R) warped. I know they weren't warped after my cone 04 bisque. I was told they were on the bottom shelf of the kiln and I know that is on the same level as the burner ports.
I talked to my instructor and he told me he lost so many large bowls at cone 10, that he now only makes them for cone 6 firings. So is warpage at cone 10 a common problem for larger pieces? I can't do anything about the way they fire at school, but could that be a factor? Since my instructor mentioned his similar experience, I didn't think it would be anything in my throwing . Any thoughts. Thanks.
Bert Gibson
http://home.comcast.net/~lazybpottery/
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Steve Irvine on tue 11 oct 05
Hi Bert,
The main cause of warped bowls at cone 10 are warped shelves. The clay becomes fairly soft as it
reaches cone 10 and wide bowls will deform to the contours of the shelf.
Shelves fired repeatedly to cone 10 will become much more warped than shelves fired to cone 6,
which is possibly why your instructor had better luck at the lower temperature.
When you put your wide bowl on the shelf, rock it around gently a bit, it should sit firmly on the
shelf without any wobbles to minimize warping.
Steve
http://www.steveirvine.com
Jennifer Boyer on tue 11 oct 05
Hi Bert,
It might just be the shape of your bowls. At cone 10 the clay is almost
a fluid. All parts of the pot are trying to sag towards the shelf. If
you have a small foot on your bowl, warping is a possibility. This is
true especially if there is a a break in the line from rim to foot. I
notice on your web pix that you like bowl shapes where the line of the
bowl is a continuous curve on the upper part, but then cuts in towards
the foot on the lower part of the wall.. You can imagine that there is
alot of gravity pulling on the part where that change of line happens.
The 1st, 2nd and the 4th bowls with Winokur Blue have this shape. Also
the one that says Winokur Blue, but looks beige and mentions Hank's web
site, as well as the shino one. Also if there is alot of cantilevering
caused by a flaring rim there can be warping.....just a few thoughts...
Jennifer
On Oct 11, 2005, at 6:49 PM, Bert Gibson wrote:
> I recently had a couple of bowls, about 12 inches in diameter, come
> out of a glaze firing (Cone 10 R) warped. I know they weren't warped
> after my cone 04 bisque. I was told they were on the bottom shelf of
> the kiln and I know that is on the same level as the burner ports.
>
> I talked to my instructor and he told me he lost so many large bowls
> at cone 10, that he now only makes them for cone 6 firings. So is
> warpage at cone 10 a common problem for larger pieces? I can't do
> anything about the way they fire at school, but could that be a
> factor? Since my instructor mentioned his similar experience, I didn't
> think it would be anything in my throwing . Any thoughts. Thanks.
>
> Bert Gibson
> http://home.comcast.net/~lazybpottery/
>
> _______________________________________________________________________
> _______
> 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.
>
>
************************
Jennifer Boyer
Thistle Hill Pottery
Montpelier, VT
http://thistlehillpottery.com
Ron Roy on wed 12 oct 05
I must concure with most of the answers given and have a couple more
comments to add.
Fast firing can result in parts of the ware melting more than others and
that will cause warping.
One classic way to minimise the problem is to design in stronger rims.
If firing them on a flat surface does not solve the problem it may be worth
making the clay body more refractory - it could be the body is just being
fired too high.
RR
>I recently had a couple of bowls, about 12 inches in diameter, come out of
>a glaze firing (Cone 10 R) warped. I know they weren't warped after my
>cone 04 bisque. I was told they were on the bottom shelf of the kiln and I
>know that is on the same level as the burner ports.
>
>I talked to my instructor and he told me he lost so many large bowls at
>cone 10, that he now only makes them for cone 6 firings. So is warpage at
>cone 10 a common problem for larger pieces? I can't do anything about the
>way they fire at school, but could that be a factor? Since my instructor
>mentioned his similar experience, I didn't think it would be anything in
>my throwing . Any thoughts. Thanks.
>
>Bert Gibson
>http://home.comcast.net/~lazybpottery/
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 wed 12 oct 05
Dear Bert Gibson,=20
Since you do not describe the nature of the warping it is difficult to =
assess the problem and propose a solution.
Briefly, at cone 10 some stoneware clays become highly "Pyroplastic", =
they loose their integrity. If you have a large mass ('Weight") =
supported by a thin wall or a narrow footring then the compressive and =
shear stresses due to Gravity may be sufficient to overcome the hot =
strength of your ceramic. This effect is exaggerated when mass is =
cantilevered out so that it exerts a levering motion against the =
footring.
That being said, are you sure the clay is designed to be fired to mature =
at Cone ten?. If it is then it may need some adjustment to reduce the =
pyroplastic effect. The other thing to think about is the engineering of =
your bowl design.
Another thing to consider is "Clay Memory" which records the correction =
of distortions when a pot shape changes during throwing and is restored =
to its required contour.
Phil in El Vee may wish to comment.
Best regards
Ivor Lewis.
Redhill,
S. Australia.
Earl Brunner on wed 12 oct 05
Vince gave some excellent advice. I would add that I don't see where cone 6
is the answer either, it just brings on different problems.
Earl Brunner
Las Vegas, NV
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Vince Pitelka
Sent: Tuesday, October 11, 2005 8:52 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Cone 10 and warpage
Bert Gibson wrote:
"I talked to my instructor and he told me he lost so many large bowls at
cone 10, that he now only makes them for cone 6 firings. So is warpage at
cone 10 a common problem for larger pieces? I can't do anything about the
way they fire at school, but could that be a factor? Since my instructor
mentioned his similar experience, I didn't think it would be anything in my
throwing ."
Bert -
The most common cause of large bowls warping in high-firing is uneven
support under the foot. In academic institutions the shelves are often
slightly warped, or else the surfaces are irregular from accumulated wash
and glaze runs. Are the shelves you used absolutely flat and smooth?
Tony Ferguson on wed 12 oct 05
For bowl warpage consider the following:
-when forming, make sure you the compress lip and upper wall.
-if you throw the upper lip out of round (when throwing) it may warp--stick a piece of newspaper or plastic on the top before you move it.
-when trimming, use a 2 bat sandwich to flip the bowl over so the rim is left in tact
-after trimming, however you remove the bowl, do not disturb the lip.
--uneven drying will/can also distort a lip--take plastic with holes in it or newspaper and drap it over the top--also drying is a good thing depending on the type and size of bowl
--and as others mentioned, make sure you kiln is heating evenly.
Tony Ferguson
Tony Ferguson
...where the sky meets the lake...
Duluth, Minnesota
Artist, Educator, Web Meister
fergyart@yahoo.com
fergy@cpinternet.com
(218) 727-6339
http://www.aquariusartgallery.com
http://www.tonyferguson.net
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Tony Ferguson on wed 12 oct 05
Bert -
The most common cause of large bowls warping in high-firing is uneven
support under the foot. In academic institutions the shelves are often
slightly warped, or else the surfaces are irregular from accumulated wash
and glaze runs. Are the shelves you used absolutely flat and smooth?
Completely forgot about this one. If you can't replace the shelves, wad your bowls if the shelves are uneven.
Tony Ferguson
...where the sky meets the lake...
Duluth, Minnesota
Artist, Educator, Web Meister
fergyart@yahoo.com
fergy@cpinternet.com
(218) 727-6339
http://www.aquariusartgallery.com
http://www.tonyferguson.net
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steve graber on wed 12 oct 05
in the metal working industry the kiln firing would be called "stress relieving" or heat treating. stress relieving removed stresses induced by the fabrication process. this results in warpped parts sometimes to the point where fixtures are required to heat treat parts.
stresses brought into the pot thru throwing come back out in the kiln fire.
~ so, when you throw be cautious of HOW you are throwing. quick fast strokes & movements put these stresses into the pot. smooth steady throwing keeps the pots uniform in stresses so they are less likely to warp.
the often times shown way of making a pot or plate fast with the stroke of a stick is one way to surly get warped pots after a fire.
see ya
steve
Bert Gibson wrote:
I recently had a couple of bowls, about 12 inches in diameter, come out of a glaze firing (Cone 10 R) warped. I know they weren't warped after my cone 04 bisque. I was told they were on the bottom shelf of the kiln and I know that is on the same level as the burner ports.
I talked to my instructor and he told me he lost so many large bowls at cone 10, that he now only makes them for cone 6 firings. So is warpage at cone 10 a common problem for larger pieces? I can't do anything about the way they fire at school, but could that be a factor? Since my instructor mentioned his similar experience, I didn't think it would be anything in my throwing . Any thoughts. Thanks.
Bert Gibson
http://home.comcast.net/~lazybpottery/
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Bert Gibson on wed 12 oct 05
Thanks to all for taking the time to respond. The bowl that made me curious
and prompted the questions on my part, was close to an oval shape after the
glaze firing. The foot of the bowl seemed to also conform to this altered
shape. I've just never had a piece warp that bad before.
I've only seen the baffle set-up once, but the way I remember, they are set
up about half-way along the side walls and are angled at about 45 degrees to
deflect the flame into the kiln. This was my first thought when I was told
the bowl was on the bottom shelf. I am not there during the week to help
load the kiln, though they certainly need someone more conscientious than
the student help they use now, but I can talk to the instructor who oversees
the firings when I have a large piece.
My instructor likes the bowl form to have a very defined foot. He has held
up a bowl and talked about the way the curve or arc should look continuous
across the bottom of the bowl. Don't know if that makes since, but I think
it is what Jennifer Boyle was pointing out in her post. That may be an
answer
to why he lost so many at cone 10. I plan to talk more about it with him at
class friday night. I'll be curious to know if his only trouble has been
here, or
also at the Kansaa City Art Institute where he got his degree.
The clay was a cone 10 white stoneware from Trinity Ceramics here in Dallas.
I use a cone 5/6 for cone 6 work as does my instructor for his. I imagine
his
better results at cone 6 are like mine, because I do everything myself and
it's
my work, more thought and care is taken at every step.
Thanks again,
Bert Gibson
http://home.comcast.net/~lazybpottery/
Vince Pitelka on thu 13 oct 05
Steve Graber wrote -
"yes, i mean the centering portion of raising & lowering the tower. with
small pots, this is a piece of cake. with large pots, we all seldom do as
complete a centering process. i believe this starts up an S crack issue."
Steve -
I believe that the fact that the crack is in an "S" shape shows that the
person centered very well, but in fact did not compress the bottom enough.
When you widen the bottom of a pot, you are pulling clay away from that
which remains in the bottom, whereas when you raise the walls you are
compressing form both sides. If you leave it at that, the bottom is
uncomrpressed, and when it tries to shrink more than the walls, it tends to
cracks in line with the throwing spiral created during centering. If you
hadn't centered well, the "S" crack wouldn't be in an "S" shape. So you are
talking about two completely separate issues. The "S" crack is not
connected to whether or not you center properly.
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/
Edouard Bastarache Inc. on thu 13 oct 05
Hello,
I believe some of the warping may come from the
way the wet bowls are handled such as getting them
off plaster bats or wheel heads.
It has happened to me already
Later,
"Ils sont fous ces quebecois"
"They are insane these quebekers"
"Están locos estos quebequeses"
Edouard Bastarache
Irreductible Quebecois
Indomitable Quebeker
Sorel-Tracy
Quebec
edouardb@sorel-tracy.qc.ca
www.sorel-tracy.qc.ca/~edouardb/Welcome.html
http://www.flickr.com/photos/potier/
http://perso.wanadoo.fr/smart2000/index.htm
http://www.digitalfire.com/education/toxicity/
William & Susan Schran User on thu 13 oct 05
On 10/11/05 6:49 PM, "Bert Gibson" wrote:
> I recently had a couple of bowls, about 12 inches in diameter, come out of a
> glaze firing (Cone 10 R) warped. I know they weren't warped after my cone 04
> bisque.
Larger bowls can be more susceptible to warping, especially as the
temperature increases and the clay becomes more vitrified. Same can happen
at ^6 if the clay is reaching proper maturation.
Having one edge closer to the flame can certainly cause more warping, but
the same can happen if the edge is too close to an element in an electric
kiln. Make sure the bowls is not hanging off the edge of the shelf. Better,
especially with large bowls, if they are set in the middle of the kiln.
How the bowl is constructed can often lead to warping. I find, especially
with larger/wider bowls, the need for a continuous curve from the rim of one
side to the rim of the opposite side. Think of it as an upside down arch. I
also find terminating the rim with a thicker roll of clay or changing the
direction, flaring out the rim, will often help with the warping issue.
I find bowls with shapes containing angles, instead of curves, and/or thin
straight rims are more prone to warping, both at ^6 oxidation and ^10
reduction.
--
William "Bill" Schran
Fredericksburg, Virginia
steve graber on thu 13 oct 05
i believe that part of why larger bowls may warp is also because the larger mass of clay may not be as thouroughly centered as compared to smaller bowls. ~ leaving those stressed contrasts between layers.
i also attribute the classic S cracks to incomplete centering also.
see ya
steve
William & Susan Schran User wrote:On 10/11/05 6:49 PM, "Bert Gibson" wrote:
> I recently had a couple of bowls, about 12 inches in diameter, come out of a
> glaze firing (Cone 10 R) warped. I know they weren't warped after my cone 04
> bisque.
Larger bowls can be more susceptible to warping, especially as the
temperature increases and the clay becomes more vitrified. Same can happen
at ^6 if the clay is reaching proper maturation.
Having one edge closer to the flame can certainly cause more warping, but
the same can happen if the edge is too close to an element in an electric
kiln. Make sure the bowls is not hanging off the edge of the shelf. Better,
especially with large bowls, if they are set in the middle of the kiln.
How the bowl is constructed can often lead to warping. I find, especially
with larger/wider bowls, the need for a continuous curve from the rim of one
side to the rim of the opposite side. Think of it as an upside down arch. I
also find terminating the rim with a thicker roll of clay or changing the
direction, flaring out the rim, will often help with the warping issue.
I find bowls with shapes containing angles, instead of curves, and/or thin
straight rims are more prone to warping, both at ^6 oxidation and ^10
reduction.
--
William "Bill" Schran
Fredericksburg, Virginia
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William & Susan Schran User on thu 13 oct 05
On 10/13/05 10:52 AM, "steve graber" wrote:
> i also attribute the classic S cracks to incomplete centering also.
Do you mean the coning/wedging portion of centering?
My understanding is that "S" cracks occur due to lack of compression of the
bottom after the clay has been centered and opened and that some clays are
more susceptible to this issue.
--
William "Bill" Schran
Fredericksburg, Virginia
steve graber on thu 13 oct 05
yes, i mean the centering portion of raising & lowering the tower. with small pots, this is a piece of cake. with large pots, we all seldom do as complete a centering process. i believe this starts up an S crack issue.
the "compress the center" we hear of & do - i believe - is a band aid that covers any S cracks that might have happened or might still be there. if one was to section many finished LARGE pots i bet they'll find S cracks are still there.
the S crack likely heads the other direction with a left v right handed thrower. if one was to see an S crack in a black & white colored clay pot i bet you'd even see the stratta of the claybodies showing this separation - meaning the dead center of the pot must NOT have gone thru adequate centering. and if that clay was compressed you'd see the bridge over the S in a sectioned or cut away pot.
what we do with small pots should be scaled up to big pots to get similar results. ~ but the shear scale in relation to our strength means this will not occur. so we develop alternate methods as the scale grows.
but i still say the root cause of S cracks is incomplete centering.
~ and i compress anyway for good measure...
see ya
steve
William & Susan Schran User wrote:On 10/13/05 10:52 AM, "steve graber" wrote:
> i also attribute the classic S cracks to incomplete centering also.
Do you mean the coning/wedging portion of centering?
My understanding is that "S" cracks occur due to lack of compression of the
bottom after the clay has been centered and opened and that some clays are
more susceptible to this issue.
--
William "Bill" Schran
Fredericksburg, Virginia
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William & Susan Schran User on fri 14 oct 05
On 10/13/05 10:00 PM, "steve graber" wrote:
> but i still say the root cause of S cracks is incomplete centering.
>
> ~ and i compress anyway for good measure...
Steve - I agree that creating the tall cone shape and bringing back down
into a centered half spherical shape is important as a wedging and centering
process. I instruct my students to do this process at least three times for
each piece of clay they work with. Larger pieces may require more that three
times. One can feel the clay become more homogeneous and compressed.
I don't have my students slam their clay down on the wheel head. I do have
them spiral wedge the clay, creating a cone shape, then spank the bottom
into a somewhat convex shape. The clay is placed in the center of the wheel
head and the outside of the cone is pressed down against the wheel head
while turning the wheel slowly. The bottom edge is sealed with a finger to
prevent any water from getting under the clay.
But I don't think the raising/lowering process really affects the clay at
the center, where it contacts the wheel head.
For that reason, I believe the compressing after opening is important.
I'd say both parts of the process are important and both are needed to
address the "S" cracking issue.
I can't remember the last time I've gotten an "S" crack and my students, who
follow my instructions, don't get them either.
--
William "Bill" Schran
Fredericksburg, Virginia
steve graber on fri 14 oct 05
no - i don't think so. the "S" crack indicates limited centering. better centering would be a smaller "s" followed by best centering with the basic "dot" or no crack. from a good foundation pulling anything away toward walls & such doesn't affect the center. the compression everyone talks about is really bridging the possible "S" crack undernieth. hiding it.
if you get down to the detail grains of clay they are connected by their little fish scale like platelets. a good centering doesn't disrupt that. the shear size of the "S" crack means the local clay (center spot) was never fully joined & the fundemental flaw shows thru later as the classic "S" crack. that "S" spot is a fracture (like an earthquake fault line) just waiting to be brought visible from the firing. good centering yields no "S". that's why we seldom see it in small pots. they are easy to actually over center if there was such a condition. larger pots are harder to center & we tend to say "good enough" and get rolling with big pots. ~ yet the heart is not fully centered.
you get the "S" shape because of the not-fully-centered clay twisting around like it does thru throwing. the helix that is easy to see when you throw two color claybodies. at the floor it forms an "S". a left handed thrower gets the oposite "S". it looks like an S because that's where the center of the helix starts. center really well & you get a blur at the "S" area rather then a strata of the claybodies.
see ya
steve
Vince Pitelka wrote:
Steve Graber wrote -
"yes, i mean the centering portion of raising & lowering the tower. with
small pots, this is a piece of cake. with large pots, we all seldom do as
complete a centering process. i believe this starts up an S crack issue."
Steve -
I believe that the fact that the crack is in an "S" shape shows that the
person centered very well, but in fact did not compress the bottom enough.
When you widen the bottom of a pot, you are pulling clay away from that
which remains in the bottom, whereas when you raise the walls you are
compressing form both sides. If you leave it at that, the bottom is
uncomrpressed, and when it tries to shrink more than the walls, it tends to
cracks in line with the throwing spiral created during centering. If you
hadn't centered well, the "S" crack wouldn't be in an "S" shape. So you are
talking about two completely separate issues. The "S" crack is not
connected to whether or not you center properly.
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/
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Gene Somdahl on sat 15 oct 05
There are several things that contribute to "S" cracks, but I feel the
most important is uneven drying. When the foot or rim at the base of the
pot dries first a rigid ring is formed. The still wet clay in the
center can only shrink toward the edges causing tension at the center
which fails forming a crack that follows the spiral lines from the
throwing. Things to do to help avoid "S" cracks: compress the bottom,
sponge out any slip that collects at the bottom, keep foot or base rim
moist so it dries at the same rate as the center (particularly important
with a large or deep trimmed foot), turn bowls upside down when you can
to complete drying, and most important DRY SLOWLY.
be patient,
Gene
On 10/14/05 4:36 PM, steve graber wrote:
>no - i don't think so. the "S" crack indicates limited centering. better centering would be a smaller "s" followed by best centering with the basic "dot" or no crack. from a good foundation pulling anything away toward walls & such doesn't affect the center. the compression everyone talks about is really bridging the possible "S" crack undernieth. hiding it.
>
>if you get down to the detail grains of clay they are connected by their little fish scale like platelets. a good centering doesn't disrupt that. the shear size of the "S" crack means the local clay (center spot) was never fully joined & the fundemental flaw shows thru later as the classic "S" crack. that "S" spot is a fracture (like an earthquake fault line) just waiting to be brought visible from the firing. good centering yields no "S". that's why we seldom see it in small pots. they are easy to actually over center if there was such a condition. larger pots are harder to center & we tend to say "good enough" and get rolling with big pots. ~ yet the heart is not fully centered.
>
>you get the "S" shape because of the not-fully-centered clay twisting around like it does thru throwing. the helix that is easy to see when you throw two color claybodies. at the floor it forms an "S". a left handed thrower gets the oposite "S". it looks like an S because that's where the center of the helix starts. center really well & you get a blur at the "S" area rather then a strata of the claybodies.
>
>see ya
>
>steve
>
>
>
>
Ivor and Olive Lewis on sat 15 oct 05
Dear William,
You suggest << But I don't think the raising/lowering process really =
affects the clay at
the center, where it contacts the wheel head.>>
There are two articles about this and "S" cracks in Pottery Making =
Illustrated, Fall issue, 2000.
One of the articles has photographs of the interior structures which =
form inside a billet of clay as it is repeatedly coned in the manner you =
describe.=20
Rather than compressing clay, it flows when pressure is applied. Clay =
from the base is extruded up the centre of the cone as the clay is =
lifted then forced down the outer surface as the cones are pressed back =
to the wheel head. We forget the thixotropic property of many plastic =
clays, especially those which are tempered with bentonite to increase =
their mouldability when the recipe contains high proportions of blocky =
minerals, including grog, and low proportions of Flaky materials.
Best regards,
Ivor
Gail Dapogny on sat 15 oct 05
Hi Vince,
Was interested in your description below. When you make these heavy
(relatively) wasters, do you lay them flat on the shelf or do you wad
them?
Gail Dapogny in Ann Arbor
Vince wrote:
> The most common cause of large bowls warping in high-firing is uneven
> support under the foot. In academic institutions the shelves are often
> slightly warped, or else the surfaces are irregular from accumulated
> wash
> and glaze runs. Are the shelves you used absolutely flat and smooth?
> If
> not, and if you are committed to making large bowls in the academic
> setting,
> you may have to invest in several large 1"-thick high-alumina shelves.
> Soak
> them in water, and then cut them into smaller squares with a masonry
> blade
> on a skill saw, stopping to re-moisten the shelves several times as
> you make
> the cut - not soaking wet, just let the water soak into the shelf
> before you
> start cutting again. Cut the shelves into squares of an appropriate
> size to
> fit under the entire foot of your bowls when you fire them - one
> square per
> bowls. That way you will have absolutely flat support for your bowls
> every
> time.
>
> Make sure that not even a tiny portion of the foot of the bowl is
> hanging
> off the edge of the shelf - that's a surefire warped bowl every time.
>
> Depending on location of the burners and proximity to your bowls, that
> could
> also cause warpage by over-vitrifying part of the bowl, causing greater
> shrinkage in that part. The solution is to fire the bowls higher up
> in the
> kiln.
>
> Good luck -
> - Vince
>
Vince Pitelka on sat 15 oct 05
> Was interested in your description below. When you make these heavy
> (relatively) wasters, do you lay them flat on the shelf or do you wad
> them?
Gail -
I've never wadded them - I guess it never occured to me. If they are made
of 1" high-alumina kiln shelf, there is no chance of them warping, so I
wouldn't even worry about whether they rock a bit on the shelf.
I am curious about calling them "wasters." To me, that term refers to flat
tiles or saucers fired under pots with glazes that might run, with the idea
that you throw them away after use. These kiln shelf tiles get used again
and again indefinitely when firing large bowls, plates, and platters, so
"wasters" doesn't seem the appropriate term.
- 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 sun 16 oct 05
Dear Steve Graber,
On <>Fri, 14 Oct 2005 14:36:06 -0700<> you wrote
"....if you get down to the detail grains of clay they are connected by =
their little fish scale like platelets...."
I am a bit confused by your description of a plastic clay body. How, =
when and where did you observe this sort of effect. I this what your =
teachers have told you? Perhaps you would like to elaborate? Do you have =
any photographs?
Makes for an interesting conversation.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Gail Dapogny on sun 16 oct 05
Vince,
Re. wasters...Guess I never really thought of the meaning. All I know
is that, after years of the usual stacking, firing, unstacking,
reseaching and purchasing kiln shelves, grinding shelves, etc. etc, one
day my patience utterly snapped, and I said "We HAVE to start using
those flat clay cookies for unknown glazes, tests, and our students'
work!! We're trashing too many shelves" Someone said, "Yeah,
wasters." The term stuck. Everyone made communal "wasters" which we
use over and over.
I asked about the wadding wondering if an uneven shelf might crack your
one-inch shelves sitting on top; sounds like it's not a problem,
potential or otherwise.
Gail
On Oct 15, 2005, at 7:08 PM, Vince Pitelka wrote:
>> Was interested in your description below. When you make these heavy
>> (relatively) wasters, do you lay them flat on the shelf or do you wad
>> them?
>
> Gail -
> I've never wadded them - I guess it never occured to me. If they are
> made
> of 1" high-alumina kiln shelf, there is no chance of them warping, so I
> wouldn't even worry about whether they rock a bit on the shelf.
>
> I am curious about calling them "wasters." To me, that term refers to
> flat
> tiles or saucers fired under pots with glazes that might run, with the
> idea
> that you throw them away after use. These kiln shelf tiles get used
> again
> and again indefinitely when firing large bowls, plates, and platters,
> so
> "wasters" doesn't seem the appropriate term.
> - Vince
>
> Vince Pitelka
Earl Brunner on sun 16 oct 05
I've seen this thread for a couple of days now and JUST realized you were talking about "cookies". We use them in the city studio.
Gail Dapogny wrote:Vince,
Re. wasters...Guess I never really thought of the meaning. All I know
is that, after years of the usual stacking, firing, unstacking,
reseaching and purchasing kiln shelves, grinding shelves, etc. etc, one
day my patience utterly snapped, and I said "We HAVE to start using
those flat clay cookies for unknown glazes, tests, and our students'
work!! We're trashing too many shelves" Someone said, "Yeah,
wasters." The term stuck. Everyone made communal "wasters" which we
use over and over.
I asked about the wadding wondering if an uneven shelf might crack your
one-inch shelves sitting on top; sounds like it's not a problem,
potential or otherwise.
Gail
On Oct 15, 2005, at 7:08 PM, Vince Pitelka wrote:
>> Was interested in your description below. When you make these heavy
>> (relatively) wasters, do you lay them flat on the shelf or do you wad
>> them?
>
> Gail -
> I've never wadded them - I guess it never occured to me. If they are
> made
> of 1" high-alumina kiln shelf, there is no chance of them warping, so I
> wouldn't even worry about whether they rock a bit on the shelf.
>
> I am curious about calling them "wasters." To me, that term refers to
> flat
> tiles or saucers fired under pots with glazes that might run, with the
> idea
> that you throw them away after use. These kiln shelf tiles get used
> again
> and again indefinitely when firing large bowls, plates, and platters,
> so
> "wasters" doesn't seem the appropriate term.
> - Vince
>
> Vince Pitelka
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Earl Brunner
e-mail: brunv53@yahoo.com
Earl Brunner on sun 16 oct 05
Yes!!! Ivor is back!!!!
But of course the little platelette story is what we have all heard. It isn't until you have seen a picture of clay from an electron microscope that you see that that we have been told all of our lives isn't really how it is..... I base this on an image from an electron microscope someone pointed us to a few years ago. http://www.cite-sciences.fr/english/web_cite/voir/invisibl/droit_fs.htm.
While looking for the above, I also found this:
http://mineral.galleries.com/minerals/silicate/clays.htm
Which does discribe clay as a material formed from silicate sheets and water...........
Ivor and Olive Lewis wrote:
Dear Steve Graber,
On <>Fri, 14 Oct 2005 14:36:06 -0700<> you wrote
"....if you get down to the detail grains of clay they are connected by their little fish scale like platelets...."
I am a bit confused by your description of a plastic clay body. How, when and where did you observe this sort of effect. I this what your teachers have told you? Perhaps you would like to elaborate? Do you have any photographs?
Makes for an interesting conversation.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
______________________________________________________________________________
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.
Earl Brunner
e-mail: brunv53@yahoo.com
Ivor and Olive Lewis on mon 17 oct 05
Dear Earl Brunner,
I have seen some great images of kaolin clay samples, separated flakes =
as well as "Books" or "Stacks". They were taking pictures of these back =
in the 1940's. The way they made them was to create a replica or to =
shadow them with gold or some other metal that would block the electron =
beams. I think the way to do it is described in the soil science =
research literature.
But, and it is a big BUT. you may recall from earlier conversations =
about clay, someone has yet to image a clay body as used by a potter : =
clay, fluxes, girt, grog and water, something with a bit of "tooth " to =
it
Some of our friends may recall the Magnetic Model that I suggested to =
demonstrate the relationship between Clay mineral particles and Water. =
It is by no means a perfect analogy but it does demonstrate how getting =
an oversupply of water can lead to structural breakdown.
What most folks find hard to swallow is that water will become a solid =
substance under the influence of pure clay above the melting point of =
Ice. Yet Science has been pointing us towards this view since the =
1930's.
Good to hear from you.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Ivor and Olive Lewis on wed 19 oct 05
Dear Antony,
Given that my best library is about 200 Km south if they have them. =
Otherwise it is a 1200 Km drive. What you suggest is not an easy option.
I am asking about SEM or Electron Microscope pictures with a mag of X =
50,000 or greater of commercial plastic clay bodies.
If you have seen these perhaps you would give me the references. I can =
pass them on to our visiting librarian, who I see on alternate Fridays ( =
She is due next week). If they are in the system she will get me =
photocopies.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
skiasonaranthropos@FSMAIL.NET on fri 21 oct 05
Hello Ivor,
If you do get to the library it may be an idea to ask for SEM and TEM
micrographs as both techniques are commonly used to image ceramic raw
materials. Although TEM is perhaps of less value to nonplastic values than
clays it does offer some advantages, mainly more morphological information
although resolution can also be higher. Whilst great caution should be
exercised when quoting magnifications, as more often than not as their use
is erroneous and a scale bar should be shown instead, an appreciation of
what may be possible is TEM could be said to have a power of up to X
800,000 with X 500,000 for SEM; practically however X 500,000 and X
300,000 may be more realistic
When considering imaging a clay body using electron microscopy it must be
remembered that as the sample is contained within a vacuum water can not
be present. Nevertheless plenty of information can be obtained including
particle shape, size, extent of agglomeration ... and yes kaolinite
particles are platelets, and dependent on source are hexagonal
Best regards,
Antony
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