Melissa Bridgman on fri 14 nov 03
So, at 9 months pregnant, I just opened my kiln to unload my last pre-baby load,
which included 24 dinner and dessert plates requested by a friend of the family.
They were molded/hand-built, white stoneware, about 13" across and a bit shy of 1/4"
thick. Fired at 04, they were fine. Glazed, fired at 6 (slowly) every single one of the plates
warped at the rims. Are they too thin? My thrown plates don't ever do this- but
I needed to get this order finished up as I doubt I'll be doing pottery again until spring.
Advice?? I buy my clay moist from Nashville and used commercially available glazes,
so I'm not much in the notion (especially now) for mixing chemicals or fire clay.
Several platters (also handbuilt, but slightly thicker) and cups/bowls in the same load
were fine. Thanks for any and all help.
Melissa
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Snail Scott on sat 15 nov 03
At 08:44 AM 11/14/03 -0800, you wrote:
>They were molded/hand-built, white stoneware...plates
>warped at the rims. Are they too thin? My thrown plates don't ever do
this...
In my experience, handbuilt work is more prone to
warping at thicknesses equivalent to thrown work.
I believe this is because the clay is usually less
compressed and aligned in handbuilt work, compared
with similar forms made by throwing. If you break
a thrown form, you can see what appears to be a
directionality to the particles at the break. A
handbuilt form of similar configuration will show
a much more random arrangement of particles, and
(I believe) a looser, less compacted structure.
Although it might be theoretically possible to
attain an equally tight and aligned structure by
handbuilding, I don't see how it might be done
practically. I simply make handbuilt work thicker.
The hard part, for me, is remembering to thin out
my occasional thrown stuff!
-Snail
William Sheppard on sun 16 nov 03
Snail,
All of my work is both hand built AND very thin (1/4" or less). I
have found that I need to turn and flip my clay as I pass it through my
slab roller EVERY time I make the next pass to get to the thickness I need.
This might take 4-5 passes. If I keep running the clay through in the same
direction I was at times experienceing a plate or platter that would warp
in the second glaze firing even though it was fine when it came out of the
bisque firing.
As with all of our troubleshooting this revelation took some time to
discover. For so many years we have used a wooden rolling pin with out
that problem comming up. Got a slab roller a couple of years ago and had
to learn new things about the simple art of rolling a slab.
I agree that alignment of clay platelets plays a big role in how well
our art turns out. Since I have never seen a clay platelet I just believe
this has to be true.
William Sheppard
Owner/Artist of Pat Young Ceramic Arts
See my work on www.greatpottery.com
Hank Murrow on sun 16 nov 03
On Nov 16, 2003, at 6:47 PM, William Sheppard wrote:
> All of my work is both hand built AND very thin (1/4" or less).
> I
> have found that I need to turn and flip my clay as I pass it through my
> slab roller EVERY time I make the next pass to get to the thickness I
> need.
> This might take 4-5 passes. If I keep running the clay through in the
> same
> direction I was at times experienceing a plate or platter that would
> warp
> in the second glaze firing even though it was fine when it came out of
> the
> bisque firing.
Dear William;
Does your slab roller have only a top roller, or does it have two
rollers (upper & lower)?
Cheers, Hank
Vince Pitelka on mon 17 nov 03
> I agree that alignment of clay platelets plays a big role in how well
> our art turns out. Since I have never seen a clay platelet I just believe
> this has to be true.
William -
This is so true in so many of the forming methods we use, but most of all in
slab work. The grain in a unidirectional slab is just like the grain in a
green board. Green wood shrinks across the grain much more than along the
grain, and the same is true of a slab. Drying shrinkage occurs as the water
layers between the platelets wick out to the surface and evaporate. Because
of the way a slab roller compresses the clay, there are tighter connections
between the platelets along the length of the slab than across the slab, and
thus there is more water to evaporate between platelates across the slab,
thus the greater shrinkage in that direction.
The only way to really eliminate adverse effects of clay memory in slab work
is to roll the slab several times at 90 degrees to the previous pass. I
just roll my slabs once to the thickness I want, and then I roll them
agressively with a rolling pin. With a little practice, you can thin your
slabs as accurately with a rolling pin as you can wiht a slab roller, and
the rolling pin does a much better job of eliminating clay memory.
Another critical thing with slabs is that bent or folded shapes must be
established while the slabs are very wet, or else clay memory will make the
slab want to flatten out again. It is quite easy to handle freshly rolled
slabs, using the broad surfaces of the hands, and the slabs can be slumped
over or into a wide variety of forms. Letting the slabs stiffen even
slightly before forming is almost a guarantee of problems in drying and
firing.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Office - wpitelka@tntech.edu
615/597-6801 x111, FAX 615/597-6803
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Veena Raghavan on tue 18 nov 03
Hi Ingeborg,
Although I am not Vice (!), I thought I would answer your post. Some days=
ago, there was a post, or there were posts on using Roofing Felt with
slabs. I had read about this before and have used the method to created
slab built vases. I cut out the roofing felt, which you can get at Home
Depot (I used the one that is not too thick), cut out templates of the
shapes I wanted, put them on the clay slab, cut out the slabs, keeping th=
e
felt in place, attached the slabs to form the vase, and then removed the
felt. The felt kept the slabs from flopping around, so I could stand them=
up and attach them. I did not wait for the slabs to firm up, as I thought=
they would attach better if soft. Maybe this would work for you.
All the best.
Veena
Message text written by Clayart
>How does one manage slabs and construct things without having them
bend, break, fall apart etc.Unless I have them fairly stiff, I find them
unmanageable.<
Veena Raghavan
75124.2520@compuserve.com
Valice Raffi on tue 18 nov 03
Vince Said:
"Letting the slabs stiffen even slightly before forming is almost a
guarantee of problems in drying and firing"
Almost, but not neccessarily so. I've done a whole lot of slab vessels
(some 3'x2') that I let stiffen slightly before forming, otherwise they
wouldn't be able to hold their shape. Rolled with a rolling pin, flipping
and turning 90 degrees. I take them down from about 1/2" slabs to a little
over 1/4". After forming into a curve, I let them stiffen up enough to
stand them, and then join. It's been working for me. This is with a
grogged sculpture clay.
Valice
in Sacramento
Michael Wendt on tue 18 nov 03
Three days ago a woman came into my studio and wanted me to make her an oven
bake tile 16" x 18". I wedged the clay using my "laminar wedging" technique
which consists of cutting the piece, stacking the two halves and slamming
them onto the wedging table, first one face, then the other. I did this 30
times creating 2 to the 30th power layers. I placed the 10 lbs of clay in
the slab roller ( photos at www.wendtpottery.com/supply.htm ). By
alternating the direction and sides rolled, I created a tile 20" x 17" which
I left out to dry on the original cloth over night. In the morning, I loaded
the dry tile into the glaze load, fired it to cone 10 and today we unloaded
the tile ( photo at the bottom of the page).
Coddling slabs or drying them between sheetrock is not needed with every
clay body. Mine likes to be dried quickly and I suspect most clay bodies
have way more clay than needed for plasticity. Plasticity is a two edged
sword. Sure, more plastic clays are easier to form, but they are harder to
dry without warping or cracking.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
wendtpot@lewiston.com
William Sheppard on tue 18 nov 03
Hank, I made my slab roller from a machine that was used to put a
photographic, light sensitive coating on aluminum printing plates. It is
motorized with a very slow gear reduction and has TWO rubber rollers, one
above the other. Hey I could never afford one of the real things. Works
great for me.
Vince, As I said before, and am agreeing with you, I rotate and flip my
slab between passes. I also use the rolling pin to do the final touching up
ans smoothing out of the nearly finished slab. And yes I work with very
flexable, damp slabs to create the very contorted pieces I make. Take a
look at www.greatpottery.com under William Sheppard to see what I can do
with a simple slab of clay.
William
ccpottery@BELLSOUTH.NET on tue 18 nov 03
Vince -
So, assuming I have rolled the slab with a rolling pin after it is rolled through a slab
roller ..... if I had a long, thin shape that I absolutely did not want to curl would I cut it with
the length of the slab or at a right angle to the length?
My best guess is across the slab.
Thanks
Chris Campbell - in North Carolina
Ingeborg Foco on tue 18 nov 03
Vince Said:
"Letting the slabs stiffen even slightly before forming is almost a
guarantee of problems in drying and firing"
.
Vince,
With the above statement do you have any suggestions for working with large
slabs? How does one manage slabs and construct things without having them
bend, break, fall apart etc.Unless I have them fairly stiff, I find them
unmanageable. (large items, even stiff, are quite frankly often
unmanageable) Thanks in advance for the info.
Ingeborg
the Potter's Workshop & Gallery
P.O. Box 510
3058 Stringfellow Road
St. James City, Florida 33956
239-283-2775
David Beumee on wed 19 nov 03
Excellent Michael! Please share the recipe with us all. I want to modify it
for my own tile making from a Grolleg porcelain body.
David Beumee
Lafayette, CO
> Three days ago a woman came into my studio and wanted me to make her an oven
> bake tile 16" x 18". I wedged the clay using my "laminar wedging" technique
> which consists of cutting the piece, stacking the two halves and slamming
> them onto the wedging table, first one face, then the other. I did this 30
> times creating 2 to the 30th power layers. I placed the 10 lbs of clay in
> the slab roller ( photos at www.wendtpottery.com/supply.htm ). By
> alternating the direction and sides rolled, I created a tile 20" x 17" which
> I left out to dry on the original cloth over night. In the morning, I loaded
> the dry tile into the glaze load, fired it to cone 10 and today we unloaded
> the tile ( photo at the bottom of the page).
> Coddling slabs or drying them between sheetrock is not needed with every
> clay body. Mine likes to be dried quickly and I suspect most clay bodies
> have way more clay than needed for plasticity. Plasticity is a two edged
> sword. Sure, more plastic clays are easier to form, but they are harder to
> dry without warping or cracking.
> Regards,
> Michael Wendt
> Wendt Pottery
> 2729 Clearwater Ave
> Lewiston, ID 83501
> wendtpot@lewiston.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.
Michael Wendt on thu 20 nov 03
David Beumee asked for the recipe for my non warping slab. Before I answer,
here is a perfect example of how useful a clayart firmness standard would
be. All the clay I use is rock hard ( how hard is that?) by most people's
standards. I am used to it and never have had any hand or wrist problems nor
trouble centering it.
I sent samples to Tony Hansen at Plainsman and he said they would never be
able to sell clay this firm to potters. So how firm is it? I will build a
meter this weekend and test it. Earl suggested plywood where possible and
that sounds like a good idea to me. He also posed the question of wedging or
not and I think this tool would allow us to measure the role of wedging by
trying samples straight from the bag and again after wedging to compare the
two.
Back to the slab...
I save all my bisque that cracks or gets damaged. When I have a few hundred
pounds, I grind it in the hammer mill and screen it to 60 mesh. This gives
me very porous fine grog with the same chemistry as the clay body. Such
grog, when added to an existing body (mine for the slab was 35% soft bisque
grog) allows moisture to wick out easily. Coupled with the stiffness of the
clay , the wedging method and the very low shrinkage on drying (less than
3%), slabs just don't curl. I did burnish both faces with a 12" sheetrock
knife because they want a smooth surface to bake on.
David, you could do this too even if you don't have a hammer mill. Use a
rolling pin to crush up trim scrap to a fine powder, screen it, put it in
clay jars and bisque fire it to cone 09. Then you can add it to your clay
body in varying amounts until you find the amount required when you need
hand building slabs with less shrinkage and warping.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
wendtpot@lewiston.com
www.wendtpottery.com
David asked:
Excellent Michael! Please share the recipe with us all. I want to modify it
for my own tile making from a Grolleg porcelain body.
David Beumee
Lafayette, CO
> Three days ago a woman came into my studio and wanted me to make her an
oven
> bake tile 16" x 18". I wedged the clay using my "laminar wedging"
technique
> which consists of cutting the piece, stacking the two halves and slamming
> them onto the wedging table, first one face, then the other.
BILL LEE on thu 20 nov 03
Valice and Vince, Ditto on the handling of slightly stiff slabs. I have
an old Bailey , the kind with cables and a single roller that rolls over the
slab. I turn and flip the slab several times to get the thickness I need.
As mentioned in a previous post, this compresses the clay particles and
helps prevent thin slabs from warping. Even if there is some warping in my
vessels, it adds to the asymetrical designs and gives them more "movement"!
I also use canvas mats under the cut out clay pieces to aid in tranporting
from roller to drywall shelves to stiffen. In this way I can lay out a whole
day's production in the morning. The first slab is ready to handle wnen I
cut the last in the series. I use a white earthenware that is well kneaded
to improve the plasticity. Bill
Bill in Knoxville, Tennessee
Proclaim the good news,
when necessary use words.
----- Original Message -----
From: "Valice Raffi"
To:
Sent: Wednesday, November 19, 2003 12:00 AM
Subject: Re: handbuilt/warping at glaze firing
> Vince Said:
> "Letting the slabs stiffen even slightly before forming is almost a
> guarantee of problems in drying and firing"
>
> Almost, but not neccessarily so. I've done a whole lot of slab vessels
> (some 3'x2') that I let stiffen slightly before forming, otherwise they
> wouldn't be able to hold their shape. Rolled with a rolling pin, flipping
> and turning 90 degrees. I take them down from about 1/2" slabs to a
little
> over 1/4". After forming into a curve, I let them stiffen up enough to
> stand them, and then join. It's been working for me. This is with a
> grogged sculpture clay.
>
> Valice
> in Sacramento
>
>
____________________________________________________________________________
__
> 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 thu 20 nov 03
When slabs are rolled - no mater how you do it you build in stresses that
are bound to come out during drying and firing.
It's because the slabs are stretched around the edges and compressed in the
middle. The stretched clay wants to shrink more then the compressed clay
when drying and during the shrinkage that comes with vitrification.
If you want to see this happen roll out a slab and cut it into 3 inch
squares - number the squares and let them dry - note the deformation in
each square and then fire to glaze temp.
Put them back together in the order of the original slab - you will see
what happens - the middle squares will deform the least.
You can find ways to recompress the stretched clay around the edges either
during the slab making - using rails - or paddle it in and re role a bit.
If a slab is round then compressing the edge on the wheel would be a
perfect way to combat the problem.
Throwing the slab between rolls is a way of evening out the compression as
well - thats why lifting and turning the slab helps.
RR
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Snail Scott on fri 21 nov 03
At 09:46 PM 11/18/03 -0800, Michael W. wrote:
>I suspect most clay bodies
>have way more clay than needed for plasticity. Plasticity is a two edged
>sword.
Too true! If you are using the same clay to
throw AND make tiles, one of them is not going
to be getting the optimum properties for that
purpose. I really like a short, gritty clay for
large handbuilding, and for flat pieces like
tiles and wall reliefs.
-Snail
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