Susan Setley on sun 27 apr 03
In a message dated 4/27/03 10:16:24 PM, john@FROGPONDPOTTERY.COM writes:
<< I know compressing the bottom is centuries-old potter lore, but I am
coming to believe it is just another 'old-potter's-tale'. Rebuttals?? >>
It's certainly intriguing and worth a test or two!!
Lily Krakowski on sun 27 apr 03
I can only offer Mother Krakowski's theory.
When we throw a bowl directly on the wheel (or on a bat, same idea) the
bottom of the sides, the part I call "the heel" is relatively thin in
relation to the sides.
When we throw a bowl off a hump, the "heel" tends to be much much thicker.
In a regular bowl the thinning from bottom to rim may be 1/2 inch. With a
hump thrown bowl is most likely is a 3/4 to 1 inc difference. Obviously
with a big bowl the effect is minimized. With a little bowl it is
maximized.
The theory is that the difference in thickness creates stresses which
"resolve themselves" in cracks.
I have had good luck with:
1. Pretrimming little bowls on the hump itself to reduce heel thickness.
Throw dry and get ALL the water out.
2. Cutting off the hump with a very wide, sharpened very wet spackling
knife. The support helps. It works better for me than wires etc.
3. INVERT the bowls as soon as possible. And allow bottom to dry. Ideally
after inversion keep them overnight in a damp box.
4. Trim them over a hump. If you are not familiar with this technique:
center a small lump of clay,and trim or push it in till at the wheelhead end
it is the same diameter as the inside diameter of your bowl. Cover the hump
with a bit of fine plastic or plastic wrap. Seat your bowl-to-be-trimmed on
it. Trim. Reduces stress. The key here is to lign up the bowls to be
trimmed by size so you start with the biggest, reduce the hump size as you
go to fit smaller and smaller.
Lili Krakowski
P.O. Box #1
Constableville, N.Y.
(315) 942-5916/ 397-2389
Be of good courage....
Ilene Mahler on sun 27 apr 03
Thanks will try..Ilene
----- Original Message -----
From: "Lily Krakowski"
To:
Sent: Sunday, April 27, 2003 6:09 PM
Subject: Throwing off hump/S cracks
> I can only offer Mother Krakowski's theory.
>
> When we throw a bowl directly on the wheel (or on a bat, same idea) the
> bottom of the sides, the part I call "the heel" is relatively thin in
> relation to the sides.
>
> When we throw a bowl off a hump, the "heel" tends to be much much thicker.
>
> In a regular bowl the thinning from bottom to rim may be 1/2 inch. With a
> hump thrown bowl is most likely is a 3/4 to 1 inc difference. Obviously
> with a big bowl the effect is minimized. With a little bowl it is
> maximized.
>
> The theory is that the difference in thickness creates stresses which
> "resolve themselves" in cracks.
>
> I have had good luck with:
>
> 1. Pretrimming little bowls on the hump itself to reduce heel thickness.
> Throw dry and get ALL the water out.
>
> 2. Cutting off the hump with a very wide, sharpened very wet spackling
> knife. The support helps. It works better for me than wires etc.
>
> 3. INVERT the bowls as soon as possible. And allow bottom to dry.
Ideally
> after inversion keep them overnight in a damp box.
>
> 4. Trim them over a hump. If you are not familiar with this technique:
> center a small lump of clay,and trim or push it in till at the wheelhead
end
> it is the same diameter as the inside diameter of your bowl. Cover the
hump
> with a bit of fine plastic or plastic wrap. Seat your bowl-to-be-trimmed
on
> it. Trim. Reduces stress. The key here is to lign up the bowls to be
> trimmed by size so you start with the biggest, reduce the hump size as you
> go to fit smaller and smaller.
>
>
> Lili Krakowski
> P.O. Box #1
> Constableville, N.Y.
> (315) 942-5916/ 397-2389
>
> Be of good courage....
>
>
____________________________________________________________________________
__
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
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>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
>
Jennifer F Boyer on sun 27 apr 03
This thread has come through Clayart a bunch and I got a great
tip awhile back that has reduced my s cracking :
Scrounge some sheet rock and cut pieces the size of your ware
boards. Set freshly thrown pots on p piece of sheet rock resting
on your ware board. It gets the bottoms dry faster....I get my
sheet rock from a friend who's a contractor. He always has small
pieces left from jobs.
I like clay that has fine grog, so shrinkage can be a bit high,
leading to more s cracking
Take Care
Jennifer
Lily Krakowski wrote:
> I can only offer Mother Krakowski's theory.
>
> When we throw a bowl directly on the wheel (or on a bat, same idea) the
> bottom of the sides, the part I call "the heel" is relatively thin in
> relation to the sides.
>
> When we throw a bowl off a hump, the "heel" tends to be much much thicker.
>
> In a regular bowl the thinning from bottom to rim may be 1/2 inch. With a
> hump thrown bowl is most likely is a 3/4 to 1 inc difference. Obviously
> with a big bowl the effect is minimized. With a little bowl it is
> maximized.
>
> The theory is that the difference in thickness creates stresses which
> "resolve themselves" in cracks.
>
> I have had good luck with:
>
> 1. Pretrimming little bowls on the hump itself to reduce heel thickness.
> Throw dry and get ALL the water out.
>
> 2. Cutting off the hump with a very wide, sharpened very wet spackling
> knife. The support helps. It works better for me than wires etc.
>
> 3. INVERT the bowls as soon as possible. And allow bottom to dry. Ideally
> after inversion keep them overnight in a damp box.
>
> 4. Trim them over a hump. If you are not familiar with this technique:
> center a small lump of clay,and trim or push it in till at the wheelhead end
> it is the same diameter as the inside diameter of your bowl. Cover the hump
> with a bit of fine plastic or plastic wrap. Seat your bowl-to-be-trimmed on
> it. Trim. Reduces stress. The key here is to lign up the bowls to be
> trimmed by size so you start with the biggest, reduce the hump size as you
> go to fit smaller and smaller.
>
>
> Lili Krakowski
> P.O. Box #1
> Constableville, N.Y.
> (315) 942-5916/ 397-2389
>
> Be of good courage....
>
> ______________________________________________________________________________
> 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.
>
--
~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*
Jennifer Boyer mailto:jboyer@adelphia.net
Thistle Hill Pottery Montpelier VT USA
http://www.thistlehillpottery.com/
Never pass on an email warning without checking out these sites
for web hoaxes and junk:
http://urbanlegends.about.com/
http://snopes.com
~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*~*
John Hesselberth on sun 27 apr 03
I agree with Lili's theory, but would state it more simply. I am coming
to believe that preventing s-cracks has nothing to do with compressing
the bottom but rather with uniform moisture levels and uniform/slow
drying. I think compressing the bottom mostly serves to mop up the
excess water. I am also convinced some clay bodies are more prone to
s-cracking than others. I suspect a modest amount of grog in the body
helps.
Many of the things Lili mentions below just assure more uniform
moisture levels as the piece is cut off the hump and during drying.
I know compressing the bottom is centuries-old potter lore, but I am
coming to believe it is just another 'old-potter's-tale'. Rebuttals??
Regards,
John
On Sunday, April 27, 2003, at 07:25 PM, Ilene Mahler wrote:
>
>> I can only offer Mother Krakowski's theory.
>>
>> When we throw a bowl directly on the wheel (or on a bat, same idea)
>> the
>> bottom of the sides, the part I call "the heel" is relatively thin in
>> relation to the sides.
>>
>> When we throw a bowl off a hump, the "heel" tends to be much much
>> thicker.
>>
>> In a regular bowl the thinning from bottom to rim may be 1/2 inch.
>> With a
>> hump thrown bowl is most likely is a 3/4 to 1 inc difference.
>> Obviously
>> with a big bowl the effect is minimized. With a little bowl it is
>> maximized.
>>
>> The theory is that the difference in thickness creates stresses which
>> "resolve themselves" in cracks.
>>
>> I have had good luck with:
>>
>> 1. Pretrimming little bowls on the hump itself to reduce heel
>> thickness.
>> Throw dry and get ALL the water out.
>>
>> 2. Cutting off the hump with a very wide, sharpened very wet spackling
>> knife. The support helps. It works better for me than wires etc.
>>
>> 3. INVERT the bowls as soon as possible. And allow bottom to dry.
> Ideally
>> after inversion keep them overnight in a damp box.
>>
>> 4. Trim them over a hump. If you are not familiar with this
>> technique:
>> center a small lump of clay,and trim or push it in till at the
>> wheelhead
> end
>> it is the same diameter as the inside diameter of your bowl. Cover
>> the
> hump
>> with a bit of fine plastic or plastic wrap. Seat your
>> bowl-to-be-trimmed
> on
>> it. Trim. Reduces stress. The key here is to lign up the bowls to
>> be
>> trimmed by size so you start with the biggest, reduce the hump size
>> as you
>> go to fit smaller and smaller.
http://www.frogpondpottery.com
http://www.masteringglazes.com
Dave Finkelnburg on mon 28 apr 03
John,
You wrote, "> I know compressing the bottom is centuries-old potter
lore, but I am
> coming to believe it is just another 'old-potter's-tale'. Rebuttals??"
I guess this qualifies as a rebuttal. :-)
Wet clay is, as you know, essentially incompressible. So the term
"compressing" is inaccurate. However, applying pressure to clay,
particularly the bottom of a piece while it turns on the wheel, does
accomplish, in my experience, an important function.
First, try to visualize the appearance of the end of a thick coil if you
pull it in two. The surface is torn and irregular. The same sort of
surface can be created with some clay bodies by the process of opening a
lump of clay on the wheel. The fact that we don't see the tear in the clay
is because our hand, or fingertips, smooth clay over it as the wheel turns.
But the tear is present, and it shows up in the drying process as an "S"
crack.
Al Tennant mentioned in a workshop a week ago that he has solved
S-cracking problems in his hump-thrown tea bowls by applying pressure to the
trimmed bottom of each bowl from the foot side and the inside
simultaneously, right at the center of the piece. This marks the clay (he
uses his thumbs) but it solves a problem.
Michael Wendt burnishes the bottoms of his ware after trimming, and has
virtually eliminated s-cracking from his studio. I am convinced the
burnishing serves to smooth clay over any point on the foot that would tend
to tear apart from tensile forces during drying. The burnishing is making
the ware stronger on the foot, and more resistant to tearing apart and
leaving a crack.
The process of applying pressure to the bottom, inside of a
wheel-thrown pot, while it doesn't compress the clay, does effectively
"burnish" it and basically patches up any tears that may have been started
from vigorously opening the ball of clay to make the pot.
Of course, you can't do this with hump-thrown ware, so some other
solution, such as Michael and Al apply, is helpful.
S-cracking is always worse when the foot is thicker than the wall of the
ware. Proper throwing or trimming so the foot isn't too thick is critical
to avoid s-cracks. However, what most potters call compressing the bottom
of a pot is also helpful to reduce this problem.
Counter rebuttals?? :-)
Dave Finkelnburg
Lee Love on mon 28 apr 03
----- Original Message -----
> Counter rebuttals?? :-)
Clay is the single most important factor in eliminating "S" cracks. I
never get "S" cracks from Mashiko clay. Not at the workshop and not at my own
workshop. I don't do any kind of "special compression" either. I think
there are two reasons: Mashiko clay is not refined (no air floated clay), so
there is variation in particle size. It also has a lot of sand in it.
I had good luck throwing off the hump with Continental Raku clay. It
has a lot of grog. I liked the color I got with this raku fired at cone 10,
especially in wood.
--
Lee Love
Mashiko JAPAN Ikiru@hachiko.com
"Life is an expression not so much of matter as of its informing
pirit." --Joe Campbell
Michael Wendt on mon 28 apr 03
I had "s" cracks in off the hump wares when I first started and observed
that I was applying so much drag to the clay while raising the next pot that
I sometimes tore the pot completely off. When this happened, it became clear
that a heavy touch torques the clay and sets up a laminar flow that has
small cavitation areas ( areas where the clay has sheared apart with a void
created). Since these areas are essentially at near vacuum, "recompressing"*
can heal them, but a lighter touch does not create them in the first place.
I recommend a little lighter touch while throwing off the hump and I use
lots of water when throwing small pieces. Bowls like you describe take less
than a minute each if the clay is right and they are nice and wet.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Avenue
Lewiston, Idaho 83501
1-208-746-3724
wendtpottery.com
*I earlier claimed that clay is incompressible and Vince so rightly pointed
out that the term "compress" is a specialized pottery term describing a
specific set of actions, not the narrow sense of the word as used in
physics.
>John wrote:
I agree with Lili's theory, but would state it more simply. I am coming
to believe that preventing s-cracks has nothing to do with compressing
the bottom but rather with uniform moisture levels and uniform/slow
drying. I think compressing the bottom mostly serves to mop up the
excess water. I am also convinced some clay bodies are more prone to
s-cracking than others. I suspect a modest amount of grog in the body
helps.
Many of the things Lili mentions below just assure more uniform
moisture levels as the piece is cut off the hump and during drying.
I know compressing the bottom is centuries-old potter lore, but I am
coming to believe it is just another 'old-potter's-tale'. Rebuttals??
Regards,
John
Chris Morgan on tue 29 apr 03
Dave, I agree, compressing is an inaccurate term. The function of
pressing the bottom of a vessel and throwwing the lump down on the
wheel head is to horizontaly align the clay crystal. I'm conviced most
s-cracks are the result of differential shrinking. Un-aligned clay
crystals shrink more across the surface than aligned particles would,
creating a gap. I'm skeptical that differences in drying rates between
the walls and base is anything more han a contributing factor. I bet
you can create s-cracks in a flat slab if it prepared in such a way that
the particles are not aligned across its length. I'll give it a shot tonight
and post on it soon. For more information check out Ceramic
Science for the Potter by W.G. Lawrence. ISBN 0-8019-5728-1. I
never throw of the hump, so I don't really have a good solution, bt
hopefully this is at least an accurate description of nature of the
problem.
Chris Morgan
iandol on fri 2 may 03
Dear John Hesselberth,=20
Sounds plausible. Now I have to try and imagine that as a three =
dimensional model. I like the idea of trillions of relatively big bits =
not cracking but providing places where the clay paste can crack. This =
would prevent the single fracture line propagating as it seems to in =
very smooth fine uniform clay bodies.
Thanks for putting that forward four us all to think about.
Best regards,
Ivor Lewis.
iandol on thu 8 may 03
Dear Ron,
You say <>=20
Have you procured images of this phenomenon? If so, I, for one, would =
like to see them
How are you able to distinguish between this phenomenon and the =
breakdown between flock structures?
If you analyse the forces imposed in application of vertical pressure on =
the inner base of the pot you may find that this is parallel to the =
crack surface not normal to it. Pressure applied to the side will assist =
in bringing the faces of the facture towards each other. Several people =
have commented on their success applying a squeeze from the side.
Function of Grog or other coarse non plastic materials is an interesting =
one. But that is another story.
Best regards,
Ivor Lewis.=20
Ruth Ballou on fri 9 may 03
John,
I don't think there is just one reason for an S crack. Different pots
crack for different reasons, some based on clay body, wedging, kneading
or throwing techniques, others due to drying factors. Early on, I never
had a problem with s cracks. Then suddenly, I was plagued with the
problem when I switched to a different clay body. Color wise, it was
the nicest body around at the time for ^6, so I didn't want to switch.
One particular s crack was revealing. The crack was so bad that it
nearly broke the pot in two, an extreme example. I finished the job and
could see that the spiral twisting was so pronounced that the clay had
physically separated into two distinct pieces. After that, I made sure
that I slapped the clay vigorously before centering and strongly
narrowed the mound beneath the pot to join the twists from kneading and
centering back together. Whether this is compression as people have
been referring to it is debatable. I see it more as closing a hole. The
s-cracks greatly decreased. Drying rates, evenness, water in the bottom
of the pot, etc., did not seem to make a difference for these
particular cracks. The crack was a structural pot of the pot. At the
time (about 20 years ago) I consulted a clay expert who advertised in
CM... He was no help. We've come a long way in researching, analyzing
and sharing information about technical problems, thanks in large part
to Clayart.
I've had other problems with s cracks with different clay bodies. In
these other cases, making sure I mop up excess water has been the
solution. I think every potter has to look carefully at his or her
process, from clay all the way through to drying.
Ruth Ballou
Silver Spring, MD
On Sunday, April 27, 2003, at 09:19 PM, John Hesselberth wrote:
> I agree with Lili's theory, but would state it more simply. I am coming
> to believe that preventing s-cracks has nothing to do with compressing
> the bottom but rather with uniform moisture levels and uniform/slow
> drying. I think compressing the bottom mostly serves to mop up the
> excess water. I am also convinced some clay bodies are more prone to
> s-cracking than others. I suspect a modest amount of grog in the body
> helps.
>
> Many of the things Lili mentions below just assure more uniform
> moisture levels as the piece is cut off the hump and during drying.
>
> I know compressing the bottom is centuries-old potter lore, but I am
> coming to believe it is just another 'old-potter's-tale'. Rebuttals??
>
> Regards,
>
> John
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