Ellen Currans on sun 2 sep 07
I make a lot of slab square and rectangular plates and trays formed in
or over glass, plastic, metal or wood recycled found objects. Warping
is always lurking about ready to pounce, or maybe I should say slump.
Besides the good suggestions already made, I would suggest that maybe
the clay you are using is not suitable for slab work. If you are
serious about slab work, test the available clays. A good throwing
clay won't necessarily work very well for slabs. I happen to use the
same body for throwing as slab work but we make our own clay and we
worked very hard to formulate one that would work for both. One pug
mill and two clays doesn't work very well!. We pug the slab clay much
firmer than the throwing clay. If I happen to use soft clay for slab
work I always have more warping and cracking. The piece simply has
more moving to do as it dries and more opportunity to move around. The
slabs I work with can be picked up off the canvas without distortion,
but they are not so firm that the surface cracks when they are pressed
into a form.
I absolutely agree that the clay must be very uniformly wedged, pugged,
slammed - whatever you do to prepare it. In my case, if a 4 inch pug
(l4 inches long) has been sitting in the studio for more than a week
(wrapped in a heavy plastic bag) it will get repugged before I use it.
Moisture migrates in the bag, or maybe it is not completely closed - if
your slab is softer on one edge than another you will have a warping
problem. You will even have a thinner slab where the clay is softer.
You have my sympathy if you are trying to do slab work without a pug
mill. Preparing firm clay for the slab roller is hard work. Even with
a pug mill, I slam the pug down on the wedging table a couple of times
to get it to the approximate shape I need, and then use a sock covered
mallet to really work it over before I take it to the slab roller. My
slab roller is electric and works wonderfully, except it will jump on
its chain if I try to feed it too thick stiff clay. I inch is about
the limit. I always pass the clay through twice - the second time back
on itself, and I sqeegee the surface with a window sqeegee firmly in a
third direction before I put on my pattern.
Some forms just contribute to warping more than others. The square
plate you mentioned might be such. I find that a flat bottomed plate
with a fairly narrow rise will warp less than a plate with a smaller
bottom and a wider deeper rise to the rim. It doesn't warp in bisque
because it hasn't gotten nearly hot enough to begin to deform. But,
when you do the glaze fire, and especially if the clay you use is maybe
cone 9 and you fire to cone 10, it will begin to slump and especially
on those pieces which don't have enough support for a wider rim. (Ok.
I am not using the technical jargon for what happens when clay turns
into ceramic in a fire, but you know what I mean.)
I also have found that glass slumped tray forms about 6 by 12 warp much
more than 12 by 12 or larger rectangles. The bigger the piece the less
warping. Bigger tiles do not warp as easily as smaller ones. You may
have to experiment with clay thickness for different forms.
Drying is very important. I spread newspapers or old sheeting over the
pieces as they are made (while still in the forms) to prevent drafts
from drying one edge more than another. I turn them out and finish the
other side as soon as they are firm enough to stand without distortion
and then dry them on sheetrock bats in a damp closet ( rack of shelves
enclosed in plastic.) Some dry upside down and some on their bottoms
(you learn from experience.) Some are put back in their forms to dry
in the damp closet. Some have a light sheet of plastic over them. They
are only moved to open shelves to continue drying when they have begun
to look dry on the surface. Dryer than leather hard.
I fire everything flat on the shelves. I had some cracking of flat
large trays early on until I learned that I had to make them thick
enough to withstand the very strong glaze that covered only one
surface. (Tom Coleman's No Craze White ) Other glazes did not cause
dunting.
When you have done everything possible you can think of to do, you
still may get a bit of warping now and then. If anyone has completely
solved the problem of warping slab work, I'd love to hear about it.
Ellen
Dundee, Oregon
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Vince Pitelka on sun 2 sep 07
When I make slumped slab plates and platters, I use very soft clay slabs, I
slum them immediately, and I leave them slumped over the mold until they are
stiff enough to pick up. The theory of using very soft plastic clay is that
there will be minimal clay memory from the slumping process, and therefore
the clay will be less inclined to return to its flat slab form.
The most prevalent problem causing warping in slab forms is grain structure
from the rolling process. Whenever you form clay, the platelets are going
to line up along the lines of force in the forming process. That happens in
all throwing and handbuilding processes, but tends to be more of a problem
in certain processes, and slab construction is perhaps the most problematic.
When there is a linear grain structure due to platelets being lined up in a
directional fashion, the clay always shrinks more across the grain than it
does along the grain. When you roll a slab with a slab-roller, you create a
very distinct linear grain structure in line with the movement of the slab
through the roller. Furthermore, if you roll slabs with a single-roller
slab roller, the clay is compressed more on the surface contacting the
roller, because the pressure is more concentrated in a smaller area, and
thus the slab will tend to curl.
What's the solution? It's very simple. If you have a wide-bed slab-roller,
just turn the slab 90 degrees, decrease the thickness setting slightly, and
run the slab through the roller again. If you have a single-roller machine,
like a Brent, turn the slab 90 degrees AND flip it over before running it
through at a thinner setting.
To me, that's much more trouble than simply re-rolling with a large rolling
pin (go to amazon.com and enter Vic Firth Maple Rolling Pin 15" in the
search box). If the slab is stuck to the canvas, you can roll it very
agressively without thinning it at all. Be careful not to apply too much
pressure to the edges, however, or it will thin out a little in those areas.
Whenever I prepare slabs, I initially roll them a little thicker than I want
them with the slab roller, and then roll them to the thickness I want with
the rolling pin, flipping them and rotating them frequently. That
eliminates all directional grain structure, and almost all problems with
warping. You still have to protect the forms from drafts or sunlight, and
it is best to dry them slowly in a damp box or plastic bins.
When making slab plates, I always slump them upside-down on rigid foam-board
molds that are cut with a saber-saw set on a 45-degree angle and then sanded
to the desired profile with very coarse sandpaper (30-mesh). I add a coil
foot ring as soon as I slump the plate.
In my experience, clay memory and subsequent warping are far more
problematic with stiffer clay. The softer the clay when you bend it, the
less problems with cracking (then or later) and the less it wants to return
to its previous shape.
You can get any claybody to work for slumped forms, but a claybody
containing fine grog or sand will dry and fire with less shrinkage and less
inclination to warp.
- 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/
Ron Roy on mon 3 sep 07
Hi Lisa,
Ellen makes a good point in that some clays shrink a lot and that can be
very difficult to deal with.
I suggest you find out the dry and fired shrinkage of your clay - if it's
over 12% you may need to find another type.
Let me know if you need instructions on how to find shrinkage.
RR
>I make a lot of slab square and rectangular plates and trays formed in
>or over glass, plastic, metal or wood recycled found objects. Warping
>is always lurking about ready to pounce, or maybe I should say slump.
>
>Besides the good suggestions already made, I would suggest that maybe
>the clay you are using is not suitable for slab work. If you are
>serious about slab work, test the available clays. A good throwing
>clay won't necessarily work very well for slabs. I happen to use the
>same body for throwing as slab work but we make our own clay and we
>worked very hard to formulate one that would work for both. One pug
>mill and two clays doesn't work very well!. We pug the slab clay much
>firmer than the throwing clay. If I happen to use soft clay for slab
>work I always have more warping and cracking. The piece simply has
>more moving to do as it dries and more opportunity to move around. The
>slabs I work with can be picked up off the canvas without distortion,
>but they are not so firm that the surface cracks when they are pressed
>into a form.
>Ellen
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Lisa Elbertsen on tue 4 sep 07
Thank you all for your tips and tricks regarding warping! It is
appreciated greatly!
Regards,
www.LisaElbertsen.com
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