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torquing/warping coil question

updated sat 18 mar 06

 

Dave Finkelnburg on wed 15 mar 06


Lauren,
Here's "Warping 101." :-)
ALL warping of ceramics is caused by SHRINKAGE
when one part of a piece shrinks more than another
part. When a clay body is still sufficiently wet and
plastic it is possible for particles of the body to
slip past other particles and for the piece to
rearrange itself enough to relieve some of the forming
and/or drying stresses. Once a piece dries beyond a
certain point, however, warping or cracking becomes
the only way to relieve stresses caused by
differential shrinkage. Drying at different rates in
different parts of a piece is the most common cause of
warping in the drying process, but density difference
can also be a factor.
The DENSER ware is from forming, the LESS it will
shrink in firing. If a portion of the piece is denser
than the rest, that dense portion will shrink less
than the rest of the piece. Only warping or cracking
can relieve the resulting stress. Of course, you
could fire the ware so hot that enough of it is liquid
to allow parts of the piece to rearrange, but that
usually results in slumping--not a controlled release
of shrinkage stress.
What does this mean to you? The way you are
forming your handles or possibly how they are drying
(though less likely) is creating some shrinkage
stress, and the attachment isn't strong enough to
resist that stress. If the crack is right at the
point of attachment, then your joining method, however
good it has been, is still not good enough for this
situation. If thicker handles had more uniform
density, that would be better, but it sounds like
thicker handles are worse in your case.
I hope this helps, and that you are able to solve
the warping problem. Good potting!
Dave Finkelnburg


--- Lauren Bellero wrote:

> need some understanding (or science!) here...
>
> this is the problem: i made handles for the ends of
> an oblong platter. they are twisted coils that are
> then
> manipulated into a shape similar to elephant ear
> pastry,
> if you can picture it. i score and slip with a paper
> slip/magic water conncoction for joining to the rim
> of the
> platter. (i use this joining slip alot and know it
> is not
> the problem, nor is it the clay). the first batch i
> made
> came out fine. the next couple of batches, i made
> the
> handles bigger meaning i started with fatter coils
> (about 1/2").
> one side of these handles torqued right off the rims
> by the
> time they came out of the glaze firing.
>
> my theory is that it is the clay memory thing at
> work:
> the twisted coils want to untwist and that for the
> smaller/
> thinner version, the join was enough to hold it in
> place
> adequately. however, when they got heftier there was
> more
> force at work to overcome the join. (my plan is to
> weight
> them down as best i can.)
>
> when i consulted a potter friend, she responded:
> > Heavier should make it better not worse.


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Lauren Bellero on wed 15 mar 06


need some understanding (or science!) here...

this is the problem: i made handles for the ends of
an oblong platter. they are twisted coils that are then
manipulated into a shape similar to elephant ear pastry,
if you can picture it. i score and slip with a paper
slip/magic water conncoction for joining to the rim of the
platter. (i use this joining slip alot and know it is not
the problem, nor is it the clay). the first batch i made
came out fine. the next couple of batches, i made the
handles bigger meaning i started with fatter coils (about 1/2").
one side of these handles torqued right off the rims by the
time they came out of the glaze firing.

my theory is that it is the clay memory thing at work:
the twisted coils want to untwist and that for the smaller/
thinner version, the join was enough to hold it in place
adequately. however, when they got heftier there was more
force at work to overcome the join. (my plan is to weight
them down as best i can.)

when i consulted a potter friend, she responded:
> Heavier should make it better not worse.

i'm again confused and searching for answers.
what say you, oh wise ones?

thanks!
lauren
--
Lauren Bellero, Mudslingers Pottery
http://mudslingerspottery.net
Red Bank, NJ

Dave Finkelnburg on thu 16 mar 06


Linda,
You are very right about the amount of moisture
versus amount of shrinkage. As you say, the wetter
the clay the more the body shrinks.
In Lauren's case, though, regardless of whether
the handles she made are the same as, wetter than or
dryer than the platter, they are still uncoiling.
That is an indication the handles are not uniformly
dense. The forming process is making parts of the
handle dense, and parts less dense.
I think a good analogy is to wheel throw and open
too fast. The result is ware that shrinks a lot in
the center, less at the walls. The visible result
tends to be cracking in the center of the foot. For
small forms you can sometimes cure this by pressing
the center of the piece from top and bottom just after
trimming. If the clay is soft enough you increase the
density a little by pressing out some of the air, and
that is enough to prevent "S" cracking.
The trouble with Lauren's handles is they are a
more complex form and I suspect wouldn't look the same
if she pinched them or pressed them to make them
uniformly dense. I liked the suggestion that she
overtwist them, then unwind them a bit while attaching
them.
I think there are many creative ways to deal with
warping. What's important is to simply understand
that non-uniform density of the clay is the root cause
of warping. As you point out %moisture affects clay
density, so uniform drying is also important to
prevent warping.
With your plates, the slab is uniformly dense
provided it's been turned as it's rolled so it isn't
stretched more in one direction than another.
However, when you lift the rim you stretch the outside
a little (making it less dense), compress the inside a
little (making it more dense), where you bend the
clay. You probably haven't made the rim less dense.
If it cracks there, radially, something else is going
on, possibly drying stress, perhaps due to winter
heating there.
Ball clay will increase shrinkage in the body, and
that can add to cracking problems. Bodies with more
nonplastics like grog can resist cracking and warping
because they are open (dry more uniformly) and because
they don't shrink so much, but such bodies, being less
plastic, can be hard to form without cracking. I
favor a more plastic body because it's rugged in
forming, and then you have to just handle and dry it
carefully. As always, YMMV (your mileage may vary!).
:-)
Good potting!
Dave Finkelnburg

--- Linda Blossom wrote:

> Hi Dave and Lauren,
>
> I have two things: one a question for Dave and the
> other my own observation.
>
> First my observation, shrinkage of a clay body has
> everything to do with how wet that clay is. After
> all, the wetter the clay, the more water will have
> to evaporate out and that water took up space. If I
> make a bar out of a very wet piece of clay it will
> appear to have greater shrinkage than if I were
> using a less wet batch. So when I make a countertop
> to fit a site, it is important that I take the
> wetness of the clay into consideration.
>
> Now for my question:
>
> Dave could you, would you, put this information into
> a specific example? It would help if I could picture
> the particles in their alignment and closeness to
> each other in the context of a specific item.
>
> In Lauren's example I can picture what is happening
> - the handles were shrinking more and went their own
> way. I wonder Lauren, if your clay was of the same
> consistency (wetness)?
>
> I make some sort-of sushi plates which occasionally
> have rim cracking. They didn't in the past but at
> these higher temperature at which I am working, it
> is more common. I start with a slab of clay off the
> slab roller, cut out a piece about 8 x 10 and put
> this piece of clay on a board with as little
> stretching as possible. I try to use a clay that is
> not too wet and if it is, I leave it on the board
> until it is firm enough to retain the shape I will
> impose on it. Then I raise the edges using my
> fingers to bring the edges up in a curl. I use very
> little water to do this and have tried to do it with
> a sliding motion and also at other times just by
> bringing it up and pressing gently from inside to
> outside rather than along the length.
>
> According to the shrinkage theory, my rim or raised
> edge has more shrinkage than the flat area even
> though I have been compressing it (especially when I
> use the sliding motion) and causing more density. I
> do take a small roller and gently roll the slab
> toward its center to prevent stretching it and
> making it thinner. Am I correct that the greater
> area of shrinkage is where the cracks are? If I
> could picture the particles in the different areas
> of one of these plates it would help to understand
> it better.
>
> I am considering add more ball clay to my body - do
> you see clay body as a culprit?


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Snail Scott on thu 16 mar 06


At 10:00 PM 3/15/2006 +0000, you wrote:
>...these handles torqued right off the rims by the
>time they came out of the glaze firing...


Could you 'over-rotate' them a bit, before returning
them to their final torque and attaching them?

-Snail

Linda Blossom on thu 16 mar 06


Hi Dave and Lauren,

I have two things: one a question for Dave and the
other my own observation.

First my observation, shrinkage of a clay body has
everything to do with how wet that clay is. After
all, the wetter the clay, the more water will have
to evaporate out and that water took up space. If I
make a bar out of a very wet piece of clay it will
appear to have greater shrinkage than if I were
using a less wet batch. So when I make a countertop
to fit a site, it is important that I take the
wetness of the clay into consideration.

Now for my question:

Dave could you, would you, put this information into
a specific example? It would help if I could picture
the particles in their alignment and closeness to
each other in the context of a specific item.

In Lauren's example I can picture what is happening
- the handles were shrinking more and went their own
way. I wonder Lauren, if your clay was of the same
consistency (wetness)?

I make some sort-of sushi plates which occasionally
have rim cracking. They didn't in the past but at
these higher temperature at which I am working, it
is more common. I start with a slab of clay off the
slab roller, cut out a piece about 8 x 10 and put
this piece of clay on a board with as little
stretching as possible. I try to use a clay that is
not too wet and if it is, I leave it on the board
until it is firm enough to retain the shape I will
impose on it. Then I raise the edges using my
fingers to bring the edges up in a curl. I use very
little water to do this and have tried to do it with
a sliding motion and also at other times just by
bringing it up and pressing gently from inside to
outside rather than along the length.

According to the shrinkage theory, my rim or raised
edge has more shrinkage than the flat area even
though I have been compressing it (especially when I
use the sliding motion) and causing more density. I
do take a small roller and gently roll the slab
toward its center to prevent stretching it and
making it thinner. Am I correct that the greater
area of shrinkage is where the cracks are? If I
could picture the particles in the different areas
of one of these plates it would help to understand
it better.

I am considering add more ball clay to my body - do
you see clay body as a culprit?

Thanks,

Linda




Here's "Warping 101." :-)
ALL warping of ceramics is caused by SHRINKAGE
when one part of a piece shrinks more than another
part. When a clay body is still sufficiently wet
and
plastic it is possible for particles of the body to
slip past other particles and for the piece to
rearrange itself enough to relieve some of the
forming
and/or drying stresses. Once a piece dries beyond a
certain point, however, warping or cracking becomes
the only way to relieve stresses caused by
differential shrinkage. Drying at different rates
in
different parts of a piece is the most common cause
of
warping in the drying process, but density
difference
can also be a factor.
The DENSER ware is from forming, the LESS it
will
shrink in firing. If a portion of the piece is
denser
than the rest, that dense portion will shrink less
than the rest of the piece. Only warping or
cracking
can relieve the resulting stress. Of course, you
could fire the ware so hot that enough of it is
liquid
to allow parts of the piece to rearrange, but that
usually results in slumping--not a controlled
release
of shrinkage stress.
What does this mean to you? The way you are
forming your handles or possibly how they are drying
(though less likely) is creating some shrinkage
stress, and the attachment isn't strong enough to
resist that stress. If the crack is right at the
point of attachment, then your joining method,
however
good it has been, is still not good enough for this
situation. If thicker handles had more uniform
density, that would be better, but it sounds like
thicker handles are worse in your case.
I hope this helps, and that you are able to
solve
the warping problem. Good potting!
Dave Finkelnburg


--- Lauren Bellero wrote:

> need some understanding (or science!) here...
>
> this is the problem: i made handles for the ends
of
> an oblong platter. they are twisted coils that are
> then
> manipulated into a shape similar to elephant ear
> pastry,
> if you can picture it. i score and slip with a
paper
> slip/magic water conncoction for joining to the
rim
> of the
> platter. (i use this joining slip alot and know it
> is not
> the problem, nor is it the clay). the first batch
i
> made
> came out fine. the next couple of batches, i made
> the
> handles bigger meaning i started with fatter coils
> (about 1/2").
> one side of these handles torqued right off the
rims
> by the
> time they came out of the glaze firing.
>
> my theory is that it is the clay memory thing at
> work:
> the twisted coils want to untwist and that for the
> smaller/
> thinner version, the join was enough to hold it in
> place
> adequately. however, when they got heftier there
was
> more
> force at work to overcome the join. (my plan is to
> weight
> them down as best i can.)
>
> when i consulted a potter friend, she responded:
> > Heavier should make it better not worse.

Lauren Bellero on fri 17 mar 06


dave, linda, snail, steve: thank you for all your helpful responses.

dave, i appreciate the 'warping 101' lesson. i agree that my joining
technique in this case is just not good enough. whatever made it
work the first time was apparently something i wasn't conscience of.

regarding state of the clay used at time of the join:
the rim and the handles were made at the same time from the
same batch of clay (ie, same wetness). the rim and handles were put
on immediately -- no stiffening time. i will try the mudslinger's
suggestion of letting it set up some. i will also try 'over-rotating'
before applying (since they all seem to torque in the same
direction (that of the UNtwist), i read over-rotate to mean:
torque it the opposite way, return it to it's correct shape, and
then apply)

one more question: somewhere (a mag article by a clayarter??)
i remember that too much slip is bad (can't remember the reason why),
so it's possible i may be stingy when applying it (among other technique
implementation problems!).
so the question is: is it possible to use "too much" joining slip?

thanks again!
lauren

--
Lauren Bellero, Mudslingers Pottery
http://mudslingerspottery.net
Red Bank, NJ

Lynn Goodman Porcelain Pottery on fri 17 mar 06


Yes, it is absolutely possible to use too much slip. I had a problem
where my handles were cracking length-wise; I had been trying to
improve the joint, and kept adding different stuff to the joining slip
and also, just more slip. When I switched to using vinegar only (I
score, apply just a tiny bit of vinegar, then twist the joint a little
until it sticks) or a drop of water, my handles stopped cracking.
Apparently, it can form a barrier and stop a good joint from being
formed. This may be different from your specific problem, but it should
apply to some extent. (I use ^6 porcelain.)


On Mar 17, 2006, at 11:39 AM, Lauren Bellero wrote:

> dave, linda, snail, steve: thank you for all your helpful responses.
>
> dave, i appreciate the 'warping 101' lesson. i agree that my joining
> technique in this case is just not good enough. whatever made it
> work the first time was apparently something i wasn't conscience of.
>
> regarding state of the clay used at time of the join:
> the rim and the handles were made at the same time from the
> same batch of clay (ie, same wetness). the rim and handles were put
> on immediately -- no stiffening time. i will try the mudslinger's
> suggestion of letting it set up some. i will also try 'over-rotating'
> before applying (since they all seem to torque in the same
> direction (that of the UNtwist), i read over-rotate to mean:
> torque it the opposite way, return it to it's correct shape, and
> then apply)
>
> one more question: somewhere (a mag article by a clayarter??)
> i remember that too much slip is bad (can't remember the reason why),
> so it's possible i may be stingy when applying it (among other
> technique
> implementation problems!).
> so the question is: is it possible to use "too much" joining slip?
>
> thanks again!
> lauren
>
> --
> Lauren Bellero, Mudslingers Pottery
> http://mudslingerspottery.net
> Red Bank, NJ
>
> _______________________________________________________________________
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>
>
Lynn Goodman
Fine Porcelain Pottery
548 Court St.
Brooklyn, NY 11231
718-858-6920
Cell 347-526-9805
www.lynngoodmanporcelain.com

Stephen Mills on fri 17 mar 06


Dear Lauren,

Try making them well in advance out of really soft clay, pre-form them,
and leave to stiffen a bit before applying to the vessel.

This is what I do with the more *off-the-wall* of my handles....works
every time.

Steve
Bath
UK




In message , Lauren Bellero writes
>need some understanding (or science!) here...
>
>this is the problem: i made handles for the ends of
>an oblong platter. they are twisted coils that are then
>manipulated into a shape similar to elephant ear pastry,
>if you can picture it. i score and slip with a paper
>slip/magic water conncoction for joining to the rim of the
>platter. (i use this joining slip alot and know it is not
>the problem, nor is it the clay). the first batch i made
>came out fine. the next couple of batches, i made the
>handles bigger meaning i started with fatter coils (about 1/2").
>one side of these handles torqued right off the rims by the
>time they came out of the glaze firing.
>
>my theory is that it is the clay memory thing at work:
>the twisted coils want to untwist and that for the smaller/
>thinner version, the join was enough to hold it in place
>adequately. however, when they got heftier there was more
>force at work to overcome the join. (my plan is to weight
>them down as best i can.)
>
>when i consulted a potter friend, she responded:
>> Heavier should make it better not worse.
>
>i'm again confused and searching for answers.
>what say you, oh wise ones?
>
>thanks!
>lauren
>--
>Lauren Bellero, Mudslingers Pottery
>http://mudslingerspottery.net
>Red Bank, NJ

--
Steve Mills
Bath
UK

Snail Scott on fri 17 mar 06


At 04:39 PM 3/17/2006 +0000, you wrote:
>...is it possible to use "too much" joining slip?


Ideally, you want to use just enough joining slip
to fill the gaps caused by scoring, so no air pockets
remain to be weak spots. Air pockets don't cause
explosions - only moisture can do that - but air
pockets ARE weak spots, which are never good things
in a joint already vulnerable to stress.

Slip (unless deflocculated) has more water in it
that the surrounding clay does. That's why it's
squishier than the clay. This lets it fill all
those little textured gaps so nicely, but any
clay with more water will have...(uh, oh!) more
shrinkage. When the water evaporates, and the clay
particles snug up close together because they're no
longer being held apart by their thin cushion of
water, that's wet-to-dry shrinkage. The more water,
the thicker that cushion, and the more shrinkage
you get.

If you have a joint with a bunch of wet slip between
the clay parts being joined, that slip will shrink
up and cause cracking at the joint. It's just as if
you'd joined a really wet piece of clay to a drier
one, except that it's one really wet piece of clay
sandwiched between two drier ones. Not much different,
except now you've got TWO interfaces of unequal
moisture instead of one. Doesn't matter whether the
clay is thin (a slip layer) or thick (an attachment),
the stress of unequal shrinkage is there, proportional
the the inequity in the moisture levels.

So, use just enough slip to fill any gaps and scoring.
No more, and no less. No squishy gobs of slip in the
joint, but no air pockets either. When you seat the
attachment onto the main body of the piece, give it
a little shimmy, to snug it down and help force the
slip into the gaps, and squish any excess out the side.

An excessively long explanation of a simple phenomenon;
sorry. Still, a great many people approach scoring and
slipping as if it were a religious ritual to be
followed by rote, exempt from logical scrutiny and
experimentation. Remember the 'Far Side cartoon where
the scientists are writing a long equation on the
chalk board, and one step in the middle is is 'now a
miracle happens'? 'Magic Water' aside, it ain't magic,
it's still clay. We can predict much of its behavior,
and test even more.

-Snail