Vince Pitelka on tue 22 sep 09
Gene at Mudduck Pottery wrote:
"Is either method less likely to produce warping?? I have been making some
large 18" bowls using the hump method over a plaster mold and then adding a
foot ring. When fired they are folding up like a taco, but not all the bowl=
s
fold some stay straight. More warp than not."
"The slabs are rolled on a slab roller in multiple passes down to about 3/8
inch, maybe even a little thicker. When they come off the hump mold the
bowls are dried under newspaper and plastic until dry. I don't see any sign=
s
of warping in the drying or the bisque. Just seems to be showing up in the
glaze firing. ( cone 10 reduction ) A ring of sand is placed under the foot
ring to allow the bowl to slide rather than sticking to the shelf. I use a
24 X 24 shelf to fire them on so there is no over hanging. Some bowls early
on were fired on a 12 X 24 shelf with the rim hanging over and it didn't
seem to be any different on either shelf. I even switched clays from
standard 119 to phoenix, same problem with both."
Gene -
Others have suggested rotating the clay 90 degrees with each pass, but no
one has explained why. The slab roller produces unidirectional grain
structure in the clay, aligning the platelets in one direction. Since the
flat, disk-like platelets are aligned in the direction of rolling (along th=
e
length of the slab-roller table), there are far more water layers separatin=
g
platelets across the width of the slab than along its length, and thus the
slab shrinks far more across its width, just like a green wood plank. You
can eliminate the grain structure by turning the slab 90 degrees with each
pass, but you can also do this by simply rolling across the slab (90 degree=
s
to the rolling direction) with a large rolling pin after you finish on the
slab roller. If the slab is still stuck to the piece of canvas under it,
rolling it with a rolling pin will not thin it out at all, but will
effectively eliminate the grain structure on a thin slab. To be really sur=
e
on a thicker slab or on a large slab piece like this, cross roll it on one
side with a rolling pin, smooth a sheet of canvas over it and roll it again
to adhere the canvas, flip it over, remove the canvas from the other side,
and cross roll it from that side.
When I do stiff-slab construction with my students I emphasize the
importance of cross-rolling to eliminate grain structure, but occasionally
someone forgets and builds a slab box without taking this critical step.
Those boxes sometimes completely rip themselves apart in the glaze firing.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net; wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka
Michael Wendt on tue 22 sep 09
Gene,
I hate to sound like a broken record but since I am a broken
record, here goes:
I believe warping is not clay memory but other forces at
work, most notably uneven moisture content in the clay
causing differential shrinkage and variations in finished
density that result in slightly different shrinkage rates
when fired to maturity.
I reached these conclusions as the result of work I did for
the Idaho Research Foundation during the early 70s. The clay
research grant was headed by Frank Cronk at the University
of Idaho .
We needed a way to create many small samples of clay body
formulations to test for properties. I began experimenting
with stack and slam wedging where the number of iterations
went from 2^30 to 2^40 doublings. When clay is subjected to
around 2^30 doublings, compositional uniformity and moisture
differences approach a limit where further doublings seem
not to change the character of the clay further. Most
surprising to me was that warping in slabs and wheel thrown
pieces virtually disappeared.
Now days I advocate doing what we would call a controlled
experiment:
Take the clay for one of your bowls and wire wedge it by the
stack and slam method 30 doublings, then make the slab and
bowl in the usual manner and label the piece, finish it and
fire it in your usual manner.
Make a second bowl using the same methods but stack and slam
wire wedge it only 20 times.
Do a third as well wire wedging only 10 times.
The last bowl should be made in the manner you currently use
as a control.
Be sure to fire the bowls in as close to identical
conditions as possible to try to eliminate firing
differences as cause of the warping.
Check your results and see if stack and slam wire wedging
reduced or eliminated the tendency to warp. Others who have
reported to me on their experimental results tend to support
the notion that warping is reduced by careful stack and slam
wire wedging.
If wire wedging does not solve the problem, saggaring can
often help eliminate warping on such large pieces. I use
insulating fire brick splits as saggars with great success.
Regards,
Michael Wendt
Gene wrote:
Is either method less likely to produce warping?? I have
been making some large 18" bowls using the hump method over
a plaster mold and then adding a foot ring. When fired they
are folding up like a taco, but not all the bowls fold some
stay straight. More warp than not.
The slabs are rolled on a slab roller in multiple passes
down to about 3/8 inch, maybe even a little thicker. When
they come off the hump mold the bowls are dried under
newspaper and plastic until dry. I don't see any signs of
warping in the drying or the bisque. Just seems to be
showing up in the glaze firing. ( cone 10 reduction ) A ring
of sand is placed under the foot ring to allow the bowl to
slide rather than sticking to the shelf. I use a 24 X 24
shelf to fire them on so there is no over hanging. Some
bowls early on were fired on a 12 X 24 shelf with the rim
hanging over and it didn't seem to be any different on
either shelf. I even switched clays from standard 119 to
phoenix, same problem with both.
Does anyone have any ideas???
Thanks for any help???
Gene
mudduck on tue 22 sep 09
Is either method less likely to produce warping?? I have been making =3D
some large 18" bowls using the hump method over a plaster mold and then =3D
adding a foot ring. When fired they are folding up like a taco, but not =3D
all the bowls fold some stay straight. More warp than not.
The slabs are rolled on a slab roller in multiple passes down to about =3D
3/8 inch, maybe even a little thicker. When they come off the hump mold =3D
the bowls are dried under newspaper and plastic until dry. I don't see =3D
any signs of warping in the drying or the bisque. Just seems to be =3D
showing up in the glaze firing. ( cone 10 reduction ) A ring of sand is =3D
placed under the foot ring to allow the bowl to slide rather than =3D
sticking to the shelf. I use a 24 X 24 shelf to fire them on so there is =
=3D
no over hanging. Some bowls early on were fired on a 12 X 24 shelf with =3D
the rim hanging over and it didn't seem to be any different on either =3D
shelf. I even switched clays from standard 119 to phoenix, same problem =3D
with both.
Does anyone have any ideas???
Thanks for any help???
Gene
mudduck@mudduckpottery.net
www.mudduckpottery.net
bill lee on tue 22 sep 09
Gene,=3DA0=3DA0=3DA0 when you pass the slabs through the roller, do you tur=
n the =3D
slab 90 degrees to the previous pass? If you lay the clay on a piece of can=
=3D
vas, you easily rotate the slab each time through the roller. Doing this wi=
=3D
ll eliminate most warping. Everything else you're doing is fine.=3DA0 I wor=
k =3D
primarily with slabs all the time.
Bill Lee, Clay Artist=3D20
www.billleeclay.com=3D20
www.southernhighlandguild.org/billlee=3DA0
--- On Tue, 9/22/09, mudduck wrote:
From: mudduck
Subject: slump or hump???
To: Clayart@LSV.CERAMICS.ORG
Date: Tuesday, September 22, 2009, 6:14 PM
Is either method less likely to produce warping?? I have been making some l=
=3D
arge 18" bowls using the hump method over a plaster mold and then adding a =
=3D
foot ring. When fired they are folding up like a taco, but not all the bowl=
=3D
s fold some stay straight. More warp than not.
The slabs are rolled on a slab roller in multiple passes down to about 3/8 =
=3D
inch, maybe even a little thicker. When they come off the hump mold the bow=
=3D
ls are dried under newspaper and plastic until dry. I don't see any signs o=
=3D
f warping in the drying or the bisque. Just seems to be showing up in the g=
=3D
laze firing. ( cone 10 reduction ) A ring of sand is placed under the foot =
=3D
ring to allow the bowl to slide rather than sticking to the shelf. I use a =
=3D
24 X 24 shelf to fire them on so there is no over hanging. Some bowls early=
=3D
on were fired on a 12 X 24 shelf with the rim hanging over and it didn't s=
=3D
eem to be any different on either shelf. I even switched clays from standar=
=3D
d 119 to phoenix, same problem with both.
Does anyone have any ideas???
Thanks for any help???
Gene
mudduck@mudduckpottery.net
www.mudduckpottery.net
=3D0A=3D0A=3D0A
William & Susan Schran User on wed 23 sep 09
On 9/22/09 6:14 PM, "mudduck" wrote:
> The slabs are rolled on a slab roller in multiple passes down to about 3/=
8
> inch, maybe even a little thicker. When they come off the hump mold the b=
owls
> are dried under newspaper and plastic until dry. I don't see any signs of
> warping in the drying or the bisque. Just seems to be showing up in the g=
laze
> firing. ( cone 10 reduction ) A ring of sand is placed under the foot rin=
g to
> allow the bowl to slide rather than sticking to the shelf. I use a 24 X 2=
4
> shelf to fire them on so there is no over hanging. Some bowls early on we=
re
> fired on a 12 X 24 shelf with the rim hanging over and it didn't seem to =
be
> any different on either shelf. I even switched clays from standard 119 to
> phoenix, same problem with both.
>
> Does anyone have any ideas???
Multiple passes in one direction of stretching through the slab roller may
be the problem..
Try running through the slab roller and rolling out at a 90 degree angle to
more evenly orient the clay.
Bill
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
Marcia Selsor on wed 23 sep 09
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Besides the 90 degree passes on a slab roller,
removing the piece from either hump or slump may strain the lip of the
piece. If you place a board on top of the slump mold and flip the clay
onto the board...mold and all, then pull the mold off, you can avoid
stress on the lip. For a hump mold, I use Linda Blossom's plywood
sandwich system which I apply to oval hump molds 22-26 inches.
You can see the article in PMI last year. I am emailing you jpgs from
a workshop at Dix Hills last Nov.
They show the hump mold which sits inside the rim of the plywood oval.
Another oval goes on over the clay and bottom oval. Sandwiched
together, I can lift the clay off the hump, flip it and suspend on
brick or whatever supports. This avoids disturbing the lip. You can
then cut or alter the lip as you wish.
> On 9/22/09 6:14 PM, "mudduck" wrote:
>
>> The slabs are rolled on a slab roller in multiple passes down to
>> about 3/8
>> inch, maybe even a little thicker. When they come off the hump mold
>> the bowls
>> are dried under newspaper and plastic until dry. I don't see any
>> signs of
>> warping in the drying or the bisque. Just seems to be showing up in
>> the glaze
>> firing. ( cone 10 reduction ) A ring of sand is placed under the
>> foot ring to
>> allow the bowl to slide rather than sticking to the shelf. I use a
>> 24 X 24
>> shelf to fire them on so there is no over hanging. Some bowls early
>> on were
>> fired on a 12 X 24 shelf with the rim hanging over and it didn't
>> seem to be
>> any different on either shelf. I even switched clays from standard
>> 119 to
>> phoenix, same problem with both.
>>
>> Does anyone have any ideas???
Marcia Selsor
http://marciaselsor.com
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KATHI LESUEUR on wed 23 sep 09
After more than twenty years of doing slabwork I've come to the
conclusion that the problems with warping and slumping are not in the
methods of construction but with the clay itself and then the drying.
I do large platters on styrofoam forms. I roll the slabs out in one
direction. I do large bowls with a foot thrown on on bisque forms,
and roll the slabs in just one direction. I have a warping problem
with neither. The large platter forms are dried slowly. I cover them
with a very light plastic for a while then remove the plastic. If
they start to warp I put the plastic back on. It's a plastic dance
until they are dry. The slab bowls are dried on their rim until
completely dry. They don't have to be babied like the large
platters. Any warping after that in the firing I would blame on the
clay itself. I use Balcones clay at cone 10 from Armadillo clay in
Austin, TX. It's the best clay I've ever used for slab ware. I live
in Michigan and ship it up here 5000# at a time. It's worth it.
KATHI LESUEUR
http://www.lesueurclaywork.com
Des & Jan Howard on wed 23 sep 09
Gene
Some possibilities.
The slab:
Turn slab 90o each height reduction pass.
The footring:
Is it a thrown ring applied?
Is it a coil joined & smoothed?
You didn't mention footring diameter. We've found the
slightest undulation in a footring will bring about
warping in dishes & bowls, also the smaller the
footring &/or larger the diameter of the pot, the
greater the warping. Use a metal straight edge during
base trimming.
The firing:
Instead of sand use a FLAT cookie ~1/4" thick, just a
bit bigger than footring diameter, same clay as bowl,
biscuit fire, kiln wash, place under bowl. One use only.
If edge of pot is facing flame blast use a shield.
We use part cylinders 3/16"-1/4" thick, just a bit
bigger than height & diameter of bowl/dish/plate rim,
usual clay. Can be re-used.
Des
mudduck wrote:
> I have been making some large 18" bowls using the
> hump method over a plaster mold and then adding a
> foot ring. When fired they are folding up like a taco,
> but not all the bowls fold some stay straight.
--
Des & Jan Howard
Lue Pottery
Lue NSW
Australia
2850
02 6373 6419
www.luepottery.hwy.com.au
-32.656072 149.840624
ivor & olive lewis on thu 24 sep 09
Dear Vince,
I agree. Not only is it necessary to rotate the clay between each pass so
that the stored stress becomes omnidirectional, inverting the clay slab so
that the top and under surface change places ensures that both surfaces are
allowed to extend freely.
This cannot happen if the slab roller has a single roll and a moving
platform.
It is customary, when reducing sheet thickness by rolling to have an upper
and a lower roll so that the vertical stress is applied equally both upward=
s
and downwards.
With a single roll and a canvas sheet it is not unusual for the clay to
adhere to the fabric. This leads to horizontal shear stress in the clay, th=
e
relief of which may lead to warping.
Best regards,
Ivor Lewis,
Redhill,
South Australia
paul gerhold on thu 24 sep 09
Gene,
When you put a slab over a hump you stretch the outer edge. When you slump
the outer edge is compressed and the center is stretched. Clay that is
stretched is more likely to warp then clay that is compresses so if you are
making a slab bowl and are worried about warping on the edges you want to
slump if possible. The best approach is to slump, let dry somewhat and the=
n
invert over a matching form to continue drying, add a thrown foot , etc.
Second best is to slump and then dry inverted on a bat while adding foot.
Paul
On Tue, Sep 22, 2009 at 6:14 PM, mudduck wrote=
:
> Is either method less likely to produce warping?? I have been making some
> large 18" bowls using the hump method over a plaster mold and then adding=
a
> foot ring. When fired they are folding up like a taco, but not all the bo=
wls
> fold some stay straight. More warp than not.
>
> The slabs are rolled on a slab roller in multiple passes down to about 3/=
8
> inch, maybe even a little thicker. When they come off the hump mold the
> bowls are dried under newspaper and plastic until dry. I don't see any si=
gns
> of warping in the drying or the bisque. Just seems to be showing up in th=
e
> glaze firing. ( cone 10 reduction ) A ring of sand is placed under the fo=
ot
> ring to allow the bowl to slide rather than sticking to the shelf. I use =
a
> 24 X 24 shelf to fire them on so there is no over hanging. Some bowls ear=
ly
> on were fired on a 12 X 24 shelf with the rim hanging over and it didn't
> seem to be any different on either shelf. I even switched clays from
> standard 119 to phoenix, same problem with both.
>
> Does anyone have any ideas???
>
> Thanks for any help???
> Gene
> mudduck@mudduckpottery.net
> www.mudduckpottery.net
>
KATHI LESUEUR on thu 24 sep 09
On Sep 24, 2009, at 10:13 AM, paul gerhold wrote:
> Gene,
> When you put a slab over a hump you stretch the outer edge. When
> you slump
> the outer edge is compressed and the center is stretched. Clay
> that is
> stretched is more likely to warp then clay that is compresses so if
> you are
> making a slab bowl and are worried about warping on the edges you
> want to
> slump if possible. The best approach is to slump, let dry somewhat
> and then
> invert over a matching form to continue drying, add a thrown foot ,
> etc.
> Second best is to slump and then dry inverted on a bat while adding
> foot.
>
> Paul
>
>
I have to respectfully disagree with this advice. I make many large
slab bowls which are done on hump molds. I believe the key is to be
sure that the slab you roll out is oversized enough that you can coax
it down onto the form without stretching. And, since Gene said he is
adding a foot ring to the piece I would say it is essential to do it
on a hump mold. Then the piece can be put on the wheel while still on
the form and the foot thrown on it. I've always had trouble with
pieces that I did in a slump mold and then tried to add a foot ring.
They always slumped around the ring in the glaze firing, but never
when I threw the foot onto the piece while it was still on the form.
KATHI LESUEUR
http://www.lesueurclaywork.com
KATHI LESUEUR on thu 24 sep 09
On Sep 24, 2009, at 6:06 PM, Des & Jan Howard wrote:
> Kathi
> An alternative to throwing a footring on a slab form.
> Short video.
> http://www.onetreehillpottery.com.au/Studio/Rolling.htm
> Des
>
>
An interesting way to do it. However, my bowls are designed to hang
on the wall so I throw a foot ring similar to the ones that Richard
Aerni does.
Thanks,
Kathi
Vince Pitelka on thu 24 sep 09
Paul Gerhold wrote:
When you put a slab over a hump you stretch the outer edge. When you slump
the outer edge is compressed and the center is stretched. Clay that is
stretched is more likely to warp then clay that is compresses so if you are
making a slab bowl and are worried about warping on the edges you want to
slump if possible. The best approach is to slump, let dry somewhat and the=
n
invert over a matching form to continue drying, add a thrown foot , etc.
Second best is to slump and then dry inverted on a bat while adding foot.
Paul -
In both kinds of slumping, whether into a recessed mold or over a raised
mold, the outer edge gets compressed because it becomes smaller in
circumference as you slump it downwards over or into the mold. In either
process the inside surface gets compressed, but when you slump into a mold
you have no way to compress the outer surface. When you slump over a hump
mold it is easy to lightly rib the outer surface to even the compression. =
I
always add the coiled foot ring immediately when the clay is soft, and that
minimizes any problems with cracking or separation of the foot ring. Once
the slumped plate or platter has begin to stiffen up, I take it off the
slump mold and place it directly on a flat board or bat so that the rim
remains level as it dries.
All my plates and platters are slumped over raised molds made of rigid
insulating foamboard - the pink or blue kind, never the white stuff, becaus=
e
it crumbles too easily and is difficult to cut. In my "slab-built
tableware" workshop I teach this process, and as long as the slabs are
cross-rolled to eliminate grain structure and are slumped and dried as
described above, we have very little problems with warpage. With the
foamboard process, you can go from initial drawn posterboard template to
finished, usable slump mold in less than fifteen minutes. The molds last
indefinitely if you take good care of them.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net; wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka
Michael Wendt on thu 24 sep 09
Gene,
A minor point:
Regardless of whether you slump into a cavity or hump over a
positive form, the inside of the artifact is compressed and
the outer surface is stretched. To see this, take a deck of
cards and bend it into a curve. Notice how the cupped side
end cards protrude out farther than the convex side cards.
The same applies to any plastic medium be it metal or clay.
This subject is studied at length in an engineering course
called "Strength and Materials". It is a real eye opener and
worth the time to take if they will let you. The point is:
Whenever you bend a plastic material, you set up horizontal
shear forces from the centroidal axis outward to both faces.
Since clay is self healing, it would not be expected to see
those shear forces preserved as residual stresses (read
"clay memory") after drying has occurred.
By itself, being stretched or compressed cannot account for
the warping. One of the problems with firing to the point of
maturity is the clay begins to soften. Soft materials slump
given time. Minute variations in thickness are sufficient to
alter the rate at which a material slumps and in this way,
create warping. Variation comes in part if the clay moisture
content is not even throughout the entire piece since softer
sections shrink more and are thus thinner.
I still say stack and slam wire wedging trials are worth at
least a look.
Regards,
Michael Wendt
Paul wrote:
Gene,
When you put a slab over a hump you stretch the outer edge.
When you slump
the outer edge is compressed and the center is stretched.
Clay that is
stretched is more likely to warp then clay that is
compresses so if you are
making a slab bowl and are worried about warping on the
edges you want to
slump if possible. The best approach is to slump, let dry
somewhat and then
invert over a matching form to continue drying, add a thrown
foot , etc.
Second best is to slump and then dry inverted on a bat while
adding foot.
Paul
Johanna San Inocencio on fri 25 sep 09
--=3D=3D=3D=3D=3D=3D=3DAVGMAIL-14287859=3D=3D=3D=3D=3D=3D=3D
This is a multi-part message in MIME format.
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so you combine hump molding and throwing?
Johanna
"A man is as free as he chooses to make himself,
never an atom freer."
The Raven, Lillith by George MacDonald
KATHI LESUEUR wrote:
> On Sep 24, 2009, at 10:13 AM, paul gerhold wrote:
>
>> Gene,
>> When you put a slab over a hump you stretch the outer edge. When
>> you slump
>> the outer edge is compressed and the center is stretched. Clay
>> that is
>> stretched is more likely to warp then clay that is compresses so if
>> you are
>> making a slab bowl and are worried about warping on the edges you
>> want to
>> slump if possible. The best approach is to slump, let dry somewhat
>> and then
>> invert over a matching form to continue drying, add a thrown foot ,
>> etc.
>> Second best is to slump and then dry inverted on a bat while adding
>> foot.
>>
>> Paul
>>
>>
>
> I have to respectfully disagree with this advice. I make many large
> slab bowls which are done on hump molds. I believe the key is to be
> sure that the slab you roll out is oversized enough that you can coax
> it down onto the form without stretching. And, since Gene said he is
> adding a foot ring to the piece I would say it is essential to do it
> on a hump mold. Then the piece can be put on the wheel while still on
> the form and the foot thrown on it. I've always had trouble with
> pieces that I did in a slump mold and then tried to add a foot ring.
> They always slumped around the ring in the glaze firing, but never
> when I threw the foot onto the piece while it was still on the form.
>
> KATHI LESUEUR
> http://www.lesueurclaywork.com
> ------------------------------------------------------------------------
>
>
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> Checked by AVG - www.avg.com
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09 18:00:00
>
>
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KATHI LESUEUR on fri 25 sep 09
Yes, I do on these pieces. The foot is just a very minor part of the
piece, but throwing it gives the look and lift that I want.
KATHI LESUEUR
http://www.lesueurclaywork.com
On Sep 25, 2009, at 7:31 AM, Johanna San Inocencio wrote:
> so you combine hump molding and throwing?
> Johanna
> "A man is as free as he chooses to make himself,
> never an atom freer."
> The Raven, Lillith by George MacDonald
>
>
> KATHI LESUEUR wrote:
>>
>> On Sep 24, 2009, at 10:13 AM, paul gerhold wrote:
>>
>>> Gene,
>>> When you put a slab over a hump you stretch the outer edge. When
>>> you slump
>>> the outer edge is compressed and the center is stretched. Clay
>>> that is
>>> stretched is more likely to warp then clay that is compresses so if
>>> you are
>>> making a slab bowl and are worried about warping on the edges you
>>> want to
>>> slump if possible. The best approach is to slump, let dry somewhat
>>> and then
>>> invert over a matching form to continue drying, add a thrown foot ,
>>> etc.
>>> Second best is to slump and then dry inverted on a bat while adding
>>> foot.
>>>
>>> Paul
>>>
>>>
>>
>> I have to respectfully disagree with this advice. I make many large
>> slab bowls which are done on hump molds. I believe the key is to be
>> sure that the slab you roll out is oversized enough that you can coax
>> it down onto the form without stretching. And, since Gene said he is
>> adding a foot ring to the piece I would say it is essential to do it
>> on a hump mold. Then the piece can be put on the wheel while still on
>> the form and the foot thrown on it. I've always had trouble with
>> pieces that I did in a slump mold and then tried to add a foot ring.
>> They always slumped around the ring in the glaze firing, but never
>> when I threw the foot onto the piece while it was still on the form.
>>
>> KATHI LESUEUR
>> http://www.lesueurclaywork.com
>>
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Des & Jan Howard on fri 25 sep 09
Kathi
An alternative to throwing a footring on a slab form.
Short video.
http://www.onetreehillpottery.com.au/Studio/Rolling.htm
Des
KATHI LESUEUR wrote:
> And, since Gene said he is
> adding a foot ring to the piece I would say it is essential to do it
> on a hump mold. Then the piece can be put on the wheel while still on
> the form and the foot thrown on it.
--
Des & Jan Howard
Lue Pottery
Lue NSW
Australia
2850
02 6373 6419
www.luepottery.hwy.com.au
-32.656072 149.840624
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