John Rodgers on fri 26 nov 04
Laurie, I don't fire cone 10 or reduction, only C-6 oxidation, but I
wonder if at that temperature shrinkage of the outer ring is greater
than shrinkage in the middle, forcing the center upward, since there is
kiln shelf below center and there is no other direction the center
section can move. If this were true, making the thickness of the outer
ring of a platter thinner might have some effect.
Just a thought.
John Rodgers
Chelsea, AL
Laurie Cowell wrote:
> How can I solve the problem of flat-bottomed platters humping up in the
> middle in a Cone 10 reduction firing? This happens whether I have an
> extra
> foot ring near the centre or just the single outer foot ring. No problem
> in Cone 6 oxidation firings.
>
> Laurie Cowell,
> Waterloo, Ontario, Canada
>
> ______________________________________________________________________________
>
> 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.
>
>
Laurie Cowell on fri 26 nov 04
How can I solve the problem of flat-bottomed platters humping up in the
middle in a Cone 10 reduction firing? This happens whether I have an extra
foot ring near the centre or just the single outer foot ring. No problem
in Cone 6 oxidation firings.
Laurie Cowell,
Waterloo, Ontario, Canada
Craig Clark on fri 26 nov 04
Laurie, it sounds to me like your clay body does not work well for
large patters in cone 10 reduction. Is it possible for you to try
another clay body and see if you have better results?
Craig Dunn Clark
619 East 11 1/2 st
Houston, Texas 77008
(713)861-2083
mudman@hal-pc.org
Vince Pitelka on fri 26 nov 04
> How can I solve the problem of flat-bottomed platters humping up in the
> middle in a Cone 10 reduction firing? This happens whether I have an
> extra
> foot ring near the centre or just the single outer foot ring. No problem
> in Cone 6 oxidation firings.
Laurie -
In high-firing, this phenomenon is usually the result of drying platters too
fast. The rim and the upper surface dry first, and when the bottom dries
and shrinks subsequently, it sets up stresses that are not relieved until
the clay becomes thermoplastic in the high-firing. Then, the rim settles
back down, and the bottom humps up. One good solution is to slowly dry your
platters to trimming consistency in a big damp box, or under a plastic tent,
and then after trimming put them back in the damp box or under the plastic
tent until they are bone dry. I like for the rims of my platters to be very
level, so after trimming I usually flip them over on a large bat, and leave
them there until dry. That way, the later drying takes place from the
bottom, which helps to equalize the forces caused when the earlier drying
takes place from the upper surfaces.
If you make lots of platters, you might consider making some big "washers"
from Masonite or formica (big disks with a large hole in the center - like a
big washer). As soon as the rim is stiff enough, you set one of these giant
"washers" on the rim, and then most of the drying takes place from the
inside center of the bottom and from the outer edges of the foot. And it
keeps the rim perfectly level.
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/
bonnie staffel on sat 27 nov 04
Dear Laurie,
Drying and Firing are very close partners in your case. If you are making a
platter, the raised rim is the first to dry. The center of the platter has
to go somewhere so it humps, perhaps imperceptible in the drying and low
firing. But the same is true in how the heat works on the platter. The
rim, being raised, will react to the heat first, and then when the high fire
gets to the center part, again the clay has no where to go but to hump.
My solution to these problems is to put wax on the rims of the platters as
soon as finished. That will retard the drying of the rims. Then as soon as
the platter is leather hard, I put it on shards which will raise it off the
shelf so that the bottom can dry equally with the top surface. In bisque
firing, again to allow the heat to get to the bottom and top surface I raise
the platter up on shards for the firing. These two places are where the
humping starts.
In the glaze firing, however, one cannot raise the platter, so you can learn
to slow down the rate of heat penetration so that it equalizes. Remember
even the shelf that the platter rests on is also colder than the platter.
Perhaps some clays are less troublesome than others, such as a groggy open
clay body as opposed to porcelain, or a thick or thin body comparison.
Perhaps your rate of heat climb is too fast after Cone 6 to Cone 9-10.
My advice is to educate yourself by reading several books on drying and
firing practices so that you learn the reasons why something happens. If
your platter didn't react as it does for you, it could also have a big S
crack as the clay seeks release from the tension of drying unevenly.
Keeping the platter under plastic for a day or two does not solve the
problem. It faces open drying after the plastic is removed and the above
problems then arise if you are not aware.
Good luck.
Bonnie Staffel
http://webpages,charter.net/bstaffel
http://vasefinder.com/bstaffelgallery1.html
Potters Council Charter member
Victoria E. Hamilton on sat 27 nov 04
Ron -
I'm confused. Is this a description of what happens, or a prescription for
what one should do?
This is probably a silly question, but then, I did say I was confused,
didn't I?
Thanks,
Victoria Hamilton
Millennia Antica Pottery
Seattle, WA
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG]On Behalf Of Ron Roy
Sent: Saturday, November 27, 2004 16:48
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Hump in middle of platters
The bottom must be concave - any bottom will move during drying - if
it's concave to start with it will move down instead of up.
RR
____________________________________________________________________________
__
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.
Ann Brink on sat 27 nov 04
John, and Laurie: In line with what John said, another reason the middle
can't shrink quite as much is that we generally compress the bottom more
than the sides, to prevent s-cracks. Maybe you are compressing just a bit
more than you need to.
Ann Brink in Lompoc CA
----- Original Message -----
John Rodgers wrote:
> Laurie, I don't fire cone 10 or reduction, only C-6 oxidation, but I
> wonder if at that temperature shrinkage of the outer ring is greater
> than shrinkage in the middle, forcing the center upward, since there is
> kiln shelf below center and there is no other direction the center
> section can move. If this were true, making the thickness of the outer
> ring of a platter thinner might have some effect.
>
>
> Laurie Cowell wrote:
>
> > How can I solve the problem of flat-bottomed platters humping up in the
> > middle in a Cone 10 reduction firing? This happens whether I have an
> > extra
> > foot ring near the centre or just the single outer foot ring. No
problem
> > in Cone 6 oxidation firings.
>
David Beumee on sat 27 nov 04
Laurie,
Could you give me more information as to what type of clay you're using, how thin you are trimming your platters, and how large a span beween your footring.
I'm sure I can be of assistance.
David Beumee
Earth Alchemy Pottery
Lafayette, CO
-------------- Original message ----------------------
From: Laurie Cowell
> How can I solve the problem of flat-bottomed platters humping up in the
> middle in a Cone 10 reduction firing? This happens whether I have an extra
> foot ring near the centre or just the single outer foot ring. No problem
> in Cone 6 oxidation firings.
>
> Laurie Cowell,
> Waterloo, Ontario, Canada
>
> ______________________________________________________________________________
> 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 27 nov 04
The bottom must be concave - any bottom will move during drying - if
it's concave to start with it will move down instead of up.
RR
Vince Pitelka on sun 28 nov 04
Ron Roy said:
> The bottom must be concave - any bottom will move during drying - if
> it's concave to start with it will move down instead of up.
Ron -
I beg to differ my friend. If I am not mistaken, it was in an exhibition at
University of Nevada during NCECA Las Vegas that I first saw one of your
temmoku platters in person. It was exquisite, and it featured a broad,
gentle curve from rim to rim. Your platters are better than mine, in many
ways, but during my ten years as a full-time potter I made hundreds of
platters up to 24" in diameter, and they all had wide, flat bottoms. They
never humped up in the center unless I tried to dry them quickly. It didn't
take me long to figure out that fast drying platters simply was not an
option. Once we figure out something like that, it is amazing how we can
always accommodate slower drying.
Laurie Cowell was the one who posted the original question about this. I
have been communicating with her off-list, and I think we have it figured
out. As she mentioned, she throws the platters on a vinyl disk, and as soon
as they are stiff enough, she turns them over on their rims and peels off
the vinyl disk. So, as the platters are shrinking through that first
period, they are still stuck to the vinyl disk. They are drying and
shrinking at the upper surface, but are unable to shrink on the lower
surface where they are stuck to the disk. That sets up the uneven stresses
I spoke of earlier. In the firing, the stresses are equalized, and the
center humps up as the bottom surface "catches up" in shrinkage.
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/
Laurie Cowell on sun 28 nov 04
Ron:
Follow-up questions! Do I assume the concave is very subtle and you tool
the bottom convexly to keep the thickness even? Would you need a pretty
deep foot to prevent the glazed bottom from dropping to the kiln
shelf? Or, does it not drop that much?
Laurie
At 11/27/2004 07:47 PM, you wrote:
>The bottom must be concave - any bottom will move during drying - if
>it's concave to start with it will move down instead of up.
>
>RR
>
>______________________________________________________________________________
>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 mon 29 nov 04
If the cross section of a plate or platter is treated as an
engineering problem which can be solved using principles of Mechanical
Physics the solution comes from appreciating this structure is a
Double Cantilever Bridging System supported on Piers.
Strength of the various parts is dictated by the distribution of Mass.
If Mass is assymetrically allocated then there will be movement when
the ceramic becomes pyroplastic. This motion will be aggravated by the
distribution of accumulated residual stresses develped during
throwing, trimming and drying.
Excess mass at the extremes of the cantilevers will create a turning
force. If the interior of the well rise you get a "Hump", if it falls,
a "Spinner"
Do Intellegently designed plates and platters really need an interior
footring?
Remember what Mel tells you about Science.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Lili Krakowski on mon 29 nov 04
Because the latest bit I read confused me, I reread just now in the =
Archives the series of messages.=20
It appears that the platters are thrown on a non-absorbent bat, and then =
inverted on (I guess) another bat and then trimmed. I this correct?
If so: I think that hump is a clandestine sag. When a big platter is =
thrown on a non-absorbent bat and then inverted, moisture is trapped on =
the inside. So you throw the platter first, let it set up, with the =
bottom sitting on a nonabsorbent surface which OF COURSE delays its =
drying, except on its top surface. Then you flip the poor dear and trap =
air underneath again keeping the top part damper. Then, I assume, you =
trim the platter and OF COURSE in the proces of trimming out your =
footrings you press DOWN, ever so slightly. Who would not sag under =
these conditions. =20
As the pot is turned right side up the sag shows up as a =
hump--accentuated by firing.
Add these to all the suggestions already made:
Cover the inside of the platter with some plastic to delay the drying of =
the top of the bottom (oh, gee) while it still sits on the throwing =
bat.=20
Cut out some foam rubber disks the size, or very close to the size of =
the inside of your platter. Use fairly stiff foam rubber. Then when =
you invert the poor dear it will have support on the inside, AS WELL AS =
some absorption of the moisture. Trim with this support. =
Alternativelty you can make a solid clay support that you cover with =
plastic and tip the platter onto that for trimming.
You also might try some more absorbent bats--maybe the canvas so often =
suggested?
And remember that the Dutchess of Windsor's potter sister always said: =
"One cannot be too rich, too thin, nor dry one's pots too slowly."
=20
Lili Krakowski
Be of good courage
Kate Johnson on mon 29 nov 04
> Do Intellegently designed plates and platters really need an interior
> footring?
> Remember what Mel tells you about Science.
Hi Ivor--I had to laugh about this one, because the type of plates and
platters I make, modeled after the 18c ones, didn't have them! Whether they
were intelligently designed or not is anyone's guess, though--
I have had a couple of them warp in drying/firing, but it was caused from
handling them too quickly before they had firmed up completely, so I ended
up with a lovely Rocking Platter. Might make a good wall-hanger strictly
for decor, but you'd get seasick trying to eat from it.
Best--
Kate
Ron Roy on mon 29 nov 04
Hi Laurie,
I like the curve to be very subtle - to the point of the viewer not being
particularly aware of it.
Yes - with rounded bottoms you need to trim the bottoms convexly to get the
thickness even - or wind up with a heavy pot. I make the bottoms thinner
than the sides as well - to get the plate to "feel" the right weight.
On a wide platter the foot ring needs to be quite high - depending on the
inside curve. I remember when I did extended throwing - I used to make
slabs to add to the pots instead of coils. Had to get the slabs the same
thickness - so developed a skill - seeing how even (or not even) clay was
when looking down on it - a wonderful skill to have when throwing platters.
My eyes are not as good anymore so I do the seeing with my finger tips - a
very good way to perceive slight curves and undulations.
I have a way of dealing with the bottoms - I leave a small "button" in the
middle during trimming - about 1/4 inch - and it has a very small nipple on
that button - and I use a straight edge to see that it is just a little
below the foot ring.
That button is waxed before glazing - after firing the nipple is ground off
- so there is no chance of the plate rocking.
Porcelain does want to drop during a firing - and kiln shelves are not
always flat - so having a system that takes care of all the possible
eventualities pay off. I spend hours on my platters and dinnerware -
getting them all out is a paying proposition.
RR
>Ron:
>
>Follow-up questions! Do I assume the concave is very subtle and you tool
>the bottom convexly to keep the thickness even? Would you need a pretty
>deep foot to prevent the glazed bottom from dropping to the kiln
>shelf? Or, does it not drop that much?
>
>Laurie
>
>At 11/27/2004 07:47 PM, you wrote:
>>The bottom must be concave - any bottom will move during drying - if
>>it's concave to start with it will move down instead of up.
>>
>>RR
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on mon 29 nov 04
Hi Vince,
First - thanks for saying that about my platter - what a wonderful thing
for me to hear. The best compliments are from potters of course.
I like making platters - like them to have lots of what I call "gimmy" -
emptiness - asking to be filled. If the bottoms hump up you lose it.
One of the fortunate situations where the technical and the aesthetic
solutions coincide.
I'm not sure we disagree on this but I remember writing that and knowing it
was not going to be a good enough explanation.
I have made a lot of platters - mostly in porcelain - design and drying are
formidable problems.
As the sides shrink - before the inside dries and shrinks - the bottom will
move up or down. If the bottom is straight - flat - or humped - the bottom
will move up - if the bottom is concave it will move down. It is just a
matter of imagining what happens when a disk of plastic material does when
you compress the edges - the inside moves up or down depending on where it
has to start from.
I can remember - as my platters dry - after they are off the bats - the
bottoms do move down- they rock a bit before they are trimmed in other
words.
Even drying becomes the foremost problem - especially with porcelain
because it tends to dry unevenly - and - unlike stoneware - it does not
want to get even again.
So - as you observed in my platter - a gentle curve from the middle is the
trick. I make a fair amount of dinnerware - plates and such - again - leave
a hump or even dead flat and the bottoms will rise. If that happens the
inside space is not empty anymore - and with a glaze like tenmoku - the
fault is glaring.
So - do we still disagree?
Best regards - RR
>Ron Roy said:
>> The bottom must be concave - any bottom will move during drying - if
>> it's concave to start with it will move down instead of up.
>
>Ron -
>I beg to differ my friend. If I am not mistaken, it was in an exhibition at
>University of Nevada during NCECA Las Vegas that I first saw one of your
>temmoku platters in person. It was exquisite, and it featured a broad,
>gentle curve from rim to rim. Your platters are better than mine, in many
>ways, but during my ten years as a full-time potter I made hundreds of
>platters up to 24" in diameter, and they all had wide, flat bottoms. They
>never humped up in the center unless I tried to dry them quickly. It didn't
>take me long to figure out that fast drying platters simply was not an
>option. Once we figure out something like that, it is amazing how we can
>always accommodate slower drying.
>
>Laurie Cowell was the one who posted the original question about this. I
>have been communicating with her off-list, and I think we have it figured
>out. As she mentioned, she throws the platters on a vinyl disk, and as soon
>as they are stiff enough, she turns them over on their rims and peels off
>the vinyl disk. So, as the platters are shrinking through that first
>period, they are still stuck to the vinyl disk. They are drying and
>shrinking at the upper surface, but are unable to shrink on the lower
>surface where they are stuck to the disk. That sets up the uneven stresses
>I spoke of earlier. In the firing, the stresses are equalized, and the
>center humps up as the bottom surface "catches up" in shrinkage.
>Best wishes -
>- Vince
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Vince Pitelka on mon 29 nov 04
> So - as you observed in my platter - a gentle curve from the middle is the
> trick. I make a fair amount of dinnerware - plates and such - again -
> leave
> a hump or even dead flat and the bottoms will rise. If that happens the
> inside space is not empty anymore - and with a glaze like tenmoku - the
> fault is glaring.
> So - do we still disagree?
Ron -
No, we don't disagree. You discovered a way to make platters that are a
beautiful shape and are not nearly as subject to drying flaws. It makes
perfect sense. At the time, 25 years ago, I liked flat-bottom platters with
a wide flange rim. It gave nice opportunities for slip decoration on both
the bottom and the flange. Those ones required very slow drying to avoid
problems, but I got used to it. It is easy to see how a platter with a
gently curved cross-section would be far less troublesome. And it is such a
beautiful shape.
In grad school, when I started making large-diameter colored clay platters
(go to http://iweb.tntech.edu/wpitelka/gallery/early%20work/early_work.htm
to see them), I did all the inlaid imagery on flat slabs, and then slumped
them upside-down over a huge gently domed plaster bat cast from a giant
restaurant wok. I love a platter with a broad, smoothly-convex surface, and
as you may have noticed, when hanging on the wall, they reflect and magnify
sound in very strange ways.
- 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/
Scott Paulding on tue 30 nov 04
Hi Laurie,
I was just talking to a friend about this last night. She fires till cone
8 just starts to bend, in oxidation. We unloaded her kiln and found one
platter that had humped in the middle. When I looked over the platter, I
noticed that she had trimmed her foot ring too far towards the center.
Even though she had a second ring in the middle, when the rim settled (and
it settled a lot) in the firing, it forced everything in the center up.
For her, I suggested trimming a larger (circumference-wise) foot ring so
that her rim is smaller and wouldn't settle as much. I imaginge it would
also help her if she left a little extra meat around the ring (ie, not
trim it so thinly).
Hope this helps.
-scott
rochester, ny
=====
"I should have been a plumber."
-Albert Einstein
__________________________________________________
Do You Yahoo!?
Tired of spam? Yahoo! Mail has the best spam protection around
http://mail.yahoo.com
Roly Beevor on thu 2 dec 04
Lili Krakowski wrote 'Duchess of Windsor's potter sister always said: "One
cannot be too rich, too thin, nor dry one's pots too slowly."'
Lili
Tell us more. What was she called? Was she any good? Pictures anywhere?
Intrigued
Roly
| |
|
