L TURNER on wed 15 oct 08
Michael wrote:
"Burnishing plays a role because
I believe cracks start from micro tears that
result from the cut off wire passing through
the clay. Burnishing heals these cracks if
it is done soon enough.
I am doubtful that clay is very compressible
unless it contains a lot of entrained air so the
old time admonition to compress the bottom
well makes a lot of sense, but if your clay is
thoroughly deaired, there are no voids in the
clay to compress."
Michael,
Burnishing is a form of surface comression. Among other things it forces
the clay into a smoother surface and, it helps equilize the surface stresses
that have occurred during the drying from wet to leather hard, and perhaps
in the cutting away from the board. This step is vital in components made
"off the hump". Sealing the surface cracks also helps to reduce their
propogation.
I have no problem calling the technique that we are discussing either
burnishing or compression.
As regards the compressibility of clay itself, and what effect a deairing
pug mill might have, that is a subject for a separate discussion.
Regards,
L. Turner
The Woodlands, TX
Paul Borian on wed 15 oct 08
I throw several different bowls in the 1 - 1 =BE lb range and often get =
base cracks, especially on the smaller ones for reasons I have yet to =
figure out. However, I have never seen a crack on a bowl in that range =
with a foot ring carved out - and I make hundreds, if not thousands per =
year. Sometimes I get behind on things and cut them off the bats too =
late and the rims are bone dry by the time I flip them (and I have =
always thought that to be a cause of base cracks) but if they have a =
foot ring they never crack, no matter what. So for now I am footing all =
those bowls just so I know they won't crack but I see that as a =
temporary fix. The only other thing I can mention is that, for the =
non-footed pots, it does not seem to matter if the base is thick or thin =
- either way they are prone to cracking.
=20
Has anyone figured this one out? If so I would like to hear the =
technical explanation.
=20
Thanks,
Paul
Lee Love on wed 15 oct 08
Paul,
Have you tried different clays?
--
Lee Love in Minneapolis
http://heartclay.blogspot.com/
http://mashikopots.blogspot.com/
http://claycraft.blogspot.com/
"Let the beauty we love be what we do.
There are hundreds of ways to kneel and kiss the ground." --Rumi
L TURNER on wed 15 oct 08
Paul,
your question on bottom cracking:
"Has anyone figured this one out? If so I would like to hear the technical
explanation."
Speculation:
compression of the bottom of the pot is what reduces the cracking tendency.
When you turn the foot, you usually compress the bottom in the process even
if unintentionally.
I have been told by pottters that I trust that if you will compress the
bottom of pots cut directly off the bat, either with you fingers, palm, or a
soft rubber rib, you will reduce you cracking problems.
It works for me. When I have bottom cracks it is when I get in a hurry and
don't compress.
Regards,
L. Turner,
The Woodlands, TX
Michael Wendt on wed 15 oct 08
Paul,
From my own experience, I seldom if ever
foot anything, yet loss rates due to bottom
cracks are nearly nil unless I fail to invert
the piece and burnish the foot as soon as
practical. Burnishing plays a role because
I believe cracks start from micro tears that
result from the cut off wire passing through
the clay. Burnishing heals these cracks if
it is done soon enough. Footing carves these
nucleation points away and if you do as I do,
the trimming out of the foot is usually followed
by surface burnishing to make the foot more
attractive and smooth for table contact.
I am doubtful that clay is very compressible
unless it contains a lot of entrained air so the
old time admonition to compress the bottom
well makes a lot of sense, but if your clay is
thoroughly deaired, there are no voids in the
clay to compress.
Have you tried plastic bats and not cutting the pots
off at all? The super smooth surface releases
the pot without cracking! It's amazing.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave.
Lewiston, Id 83501
U.S.A.
208-746-3724
wendtpot@lewiston.com
http://www.wendtpottery.com
http://UniquePorcelainDesigns.com
Paul wrote:
I throw several different bowls in the 1 - 1 =BE lb range
and often get base cracks, especially on the smaller
ones for reasons I have yet to figure out. However, I
have never seen a crack on a bowl in that range with a
foot ring carved out - and I make hundreds, if not
thousands per year. Sometimes I get behind on things
and cut them off the bats too late and the rims are
bone dry by the time I flip them (and I have always
thought that to be a cause of base cracks) but if they
have a foot ring they never crack, no matter what. So
for now I am footing all those bowls just so I know
they won't crack but I see that as a temporary fix. The
only other thing I can mention is that, for the
non-footed pots, it does not seem to matter if the base
is thick or thin - either way they are prone to
cracking.
Has anyone figured this one out? If so I would like to
hear the technical explanation.
Thanks,
Paul
Ivor and Olive Lewis on thu 16 oct 08
Dear Paul Borian,
Seems as though you may be consolidating the plastic clay as you apply
pressure to your turning tool.
Another thought is that rotation of a pot during turning the base
will be contra to the original throwing direction. This might lead to
a form of physical stress relief in strained clay.
I look forward to reading comments from others.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Lee Love on thu 16 oct 08
Actually, footed pots do crack. It depends upon your claybody.
--
Lee Love in Minneapolis
http://heartclay.blogspot.com/
http://mashikopots.blogspot.com/
http://claycraft.blogspot.com/
"Let the beauty we love be what we do.
There are hundreds of ways to kneel and kiss the ground." --Rumi
Michael Wendt on thu 16 oct 08
L,
Paul did not say if he was throwing off the hump.
Certainly some cut off techniques contribute to
cracking more than others but off the hump wares
usually have thicker bottoms which create higher
dryness differentials and further increase the chance
of cracking.
A few years back when this discussion came up,
I made a heavy bisque pestle with the inside curve
to match my standard off the hump 6" soup bowl.
At soft leather, I patiently pummeled the inside of
half of the bowls thrown off the hump while they
were on a very firm cloth covered surface.
I would certainly say this constituted compression.
I noted no change in loss rates due to bottom
cracking when compared to the ones I flipped and
quickly burnished with a rib.
Not until I built the new twin drive deairing pug
mill did I see a reduction in off the hump losses.
My conclusion is that "S" cracking is caused by
the cavitation effect* which is easier to induce
if the clay has appreciable discontinuities in the
form of air bubbles. This further leads me to believe
that cracks in wet clay can come from:
1. High moisture differentials on some areas
resulting from either letting water stand on
some portion of the pot or lands and grooves
that are wetter because they are difficult to get
very dry at cleanup.
2. Stress concentrations caused by either sharp
bends or sharp cuts.
3. Unrepaired tiny cracks created by the passage
of some tool like a cutoff wire.
Vince Pitelka suggested cloth bats which I tried
and liked for the large walk tile project at Salmon,
Idaho's Sacajawea Memorial a few years ago.
I found lower loss rates because the tile were free
to shrink with the cloth bottom which later simply
peeled off and no wire ever tore the clay.
There may be other cracking causes in wet clay
and others may well list them but for me, these
have been the three big problems to watch for.
*cavitation occurs in many systems. Propellers
in water can spin fast enough to create momentary
voids in water that look like air bubbles but are
in fact near vacuum voids that collapse under the
weight of the water. In clay, due to its thick paste
like nature, these induced voids due to torque do
not necessarily close up perfectly which I suggest
accounts for the classis "S" cracks in the bottoms
of some pots and which truly does allow
compression of a normally incompressible
material like clay.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave.
Lewiston, Id 83501
U.S.A.
208-746-3724
wendtpot@lewiston.com
http://www.wendtpottery.com
http://UniquePorcelainDesigns.com
L wrote:
Michael,
Burnishing is a form of surface comression. Among
other things it forces
the clay into a smoother surface and, it helps equilize
the surface stresses
that have occurred during the drying from wet to
leather hard, and perhaps
in the cutting away from the board. This step is vital
in components made
"off the hump". Sealing the surface cracks also helps
to reduce their
propogation.
I have no problem calling the technique that we are
discussing either
burnishing or compression.
As regards the compressibility of clay itself, and what
effect a deairing
pug mill might have, that is a subject for a separate
discussion.
Regards,
L. Turner
The Woodlands, TX
Sherron & Jim Bowen on thu 16 oct 08
"In clay, due to its thick paste
like nature, these induced voids due to torque do
not necessarily close up perfectly which I suggest
accounts for the classis "S" cracks in the bottoms
of some pots and which truly does allow
compression of a normally incompressible
material like clay."
That's pretty much what Clyde Tullis (AKA Mudlark) said on this list about
ten years ago.
JB
----- Original Message -----
From: "Michael Wendt"
To:
Sent: Thursday, October 16, 2008 8:18 AM
Subject: Re: footed pots never crack - anyone have an explanation?
DRB Seattle on thu 16 oct 08
This seems like the right direction...
Another thought is that=A0 rotation of a pot during turning the base
will be contra to the original throwing direction. This might lead to
a form of physical stress relief in strained clay.
A friend had the routine of carving out the inside of T-pot spouts.=A0 This=
"cutting the tendons" released the torque and the spouts didn't unwind in =
the kiln.
=A0
At one time I went through a phase of throwing untrimmed forms off the hump=
.
when I opended I did it in a down and out way to leave a cone of clay in th=
e middle which I then compressed. I had no cracks but the friend whose pots=
cracked all over crabbed that it was just hte sandy clay I used. As I didn=
't run a control to check it my theory belongs to the class of some many th=
ings that we do...
I eat my peas with honey
I've done it all my life
It does sound rather funny
But it keeps them on the knife
=A0
DRB=0A=0A=0A
Steve Slatin on thu 16 oct 08
Michael --
I hesitate to dispute what you say, as you
are very generally correct, but I have one
thought that is not consistent with your
suggestion, but does relate to the trimmed/
untrimmed foot experience.
I got a few boxes of B-Mix last year from
a friend who was getting out of cone 6 work --
I started getting quite a few s-cracks,
though other clay I was working with at
the same time didn't give me any. (This
if anything supports your theory of
nucleation from voids.)
But I did get the hammer and smash each
of the flawed pots. I saw a consistent
flaw in the trimming -- thicker bottoms
than sides. I found two pots thrown from
B-Mix that had glaze flaws but no s-cracks,
and smashed them as well -- they had bottoms
slightly thinner than the walls of the sides
of the pots. All of these pots were trimmed
in about the same way, making a compression
explanation somewhat less likely.
As I recall, the original post was about
some pots that would have been trimmed with
foot rings, but were not because they were
too dry when checked for trimming. These
would almost certainly have had thicker
bottoms than sides (unless the original
poster was planning on trimming very small
foot rings with an extremely thin base).
What we seem to know is that some clays
are more susceptible to s-cracking than
others, and there seems to be a correlation
between bottom thickness and s-cracks.
The vortex/nucleation idea may explain
where a crack develops, but it doesn't
seem to me to be an explanation of
why a crack exists.
Best wishes -- Steve S
--- On Wed, 10/15/08, Michael Wendt wrote:
> Paul,
> From my own experience, I seldom if ever
> foot anything, yet loss rates due to bottom
> cracks are nearly nil unless I fail to invert
> the piece and burnish the foot as soon as
> practical. Burnishing plays a role because
> I believe cracks start from micro tears that
> result from the cut off wire passing through
> the clay. Burnishing heals these cracks if
> it is done soon enough. Footing carves these
> nucleation points away and if you do as I do,
> the trimming out of the foot is usually followed
> by surface burnishing to make the foot more
> attractive and smooth for table contact.
> I am doubtful that clay is very compressible
> unless it contains a lot of entrained air so the
> old time admonition to compress the bottom
> well makes a lot of sense, but if your clay is
> thoroughly deaired, there are no voids in the
> clay to compress.
> Have you tried plastic bats and not cutting the pots
> off at all? The super smooth surface releases
> the pot without cracking! It's amazing.
> Regards,
> Michael Wendt
Vince Pitelka on fri 17 oct 08
Michael Wendt wrote:
"Burnishing plays a role because
I believe cracks start from micro tears that
result from the cut off wire passing through
the clay. Burnishing heals these cracks if
it is done soon enough."
L. Turner wrote:
"I am doubtful that clay is very compressible
unless it contains a lot of entrained air so the
old time admonition to compress the bottom
well makes a lot of sense, but if your clay is
thoroughly deaired, there are no voids in the
clay to compress."
L. -
We are not being strictly literal when we use the term "compression" in
reference to wheel-thrown pots. As has been discussed in previous Clayart
threads over the years, it is not a case of mechanically compressing clay,
and has nothing to do with whether or not the clay has been deaired. It is
all about applying compressive force, which helps to reduce the water
content and increase the contact points between platelets, thus reducing
shrinkage of the bottom of the pot. When throwing, the walls are compressed
from both sides, while the base is compressed only from the top. Often,
when widening the inside bottom of a pot, clay is actually pulled away from
that which remains in the bottom, and there is no compressive force at all.
Those pots are almost certain to develop S-cracks. The bottom-compression
that we normally do in throwing pots serves simply to equalize the treatment
of the clay in the bottom and the clay in the walls, so that both areas
shrink compatibly during drying and firing.
The familiar S-cracks are of course most often caused by lack of compression
during throwing and/or trimming, or from leaving water in the bottom of the
pot after throwing. If the cracks pass across the bottom of the pot in
straight lines, or if there are many small cracks or tears, it is usually
because the potter pulled the cutoff wire under the pot without it spinning
on the wheel, or because the potter pulled the cutoff wire under the pot too
quickly, and these kinds of cracks most often appear in porcelain bodies.
Burnishing is specifically the act of compressing and/or polishing a surface
by rubbing with a hard, unyielding object. There are lots of way we can
compress the surface that do not qualify as burnishing, so "compressing" is
the more appropriate term.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net; wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka
Paul Borian on mon 20 oct 08
Thanks for the replies so far.
First of all, just to clarify, I am throwing on bats (wood, masonite,
hardy backer board and sometimes formica) not off the hump (I couldn't
throw a single pot of the hump with any amount of practice and I find it
hard to believe anyone can do that - how in the world do you throw like
40 mugs the same size when you have no way to weigh out the clay for
each one first??).
=20
I think Michael really explained it well because I used to burnish all
the non-footed pots like small bowls, vases and mugs and I did not get
as many cracks. The reason I did that was because I was using mullite
grog and the stuff left the bottoms really sharp (even with burnishing
them I had to smooth them out again before selling them) so I would
smooth the leather hard pots out with an old credit card. When I
switched to flint-clay grog I stopped doing this (except on plates,
platters, etc) and it was on the pots I stopped doing it on that cracked
the most. Plus it is possible that the mullite was holding the clay
together a little better than the flint clay grog - it has some kind of
needle-like structure that provides strength. Plus all this started at
the beginning of summer when I started using the a/c everyday so the
faster drying could not have helped much either (even though I take a
lot of care to dry pots evenly, cover with plastic to slow down the
drying, etc I can't always keep up with things when they are drying so
fast.)
=20
I will go back to burnishing them and report the results in a few weeks.
=20
One other thing that goes along with the idea - I burnish the plates by
spinning them on the wheel and the motion with credit card is just like
when trimming, and as someone else mentioned that provides stress relief
since it goes against the direction during throwing. And those plates
and other similar forms almost never crack.
The mugs, etc, were not burnished in such a way because it is too hard
with their small size so I just swipe the card across them - and even
when I did this they cracked sometimes, and with more frequency than
plates, etc.
=20
Thanks,
Paul
Michael Wendt on mon 20 oct 08
Paul,
If you can find a springy stainless steel rib,
I suggest you try that. It will smooth the
bottoms of even groggy clay if you use
it at the right time.
We fan dry our pots until the rims are soft
leather, invert onto smooth, cloth covered
carry boards and burnish every piece to a
shine. Loss rates to cracking when this is
done at the right time approach zero.
As to off the hump, I throw mass quantities
of small items off the hump and am working
on an article that details my measuring
system that makes it fast and easy. I'll try
to speed up the process.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave.
Lewiston, Id 83501
U.S.A.
208-746-3724
wendtpot@lewiston.com
http://www.wendtpottery.com
http://UniquePorcelainDesigns.com
Paul wrote:
Thanks for the replies so far.
First of all, just to clarify, I am throwing on bats
(wood, masonite,
hardy backer board and sometimes formica) not off the
hump (I couldn't
throw a single pot of the hump with any amount of
practice and I find it
hard to believe anyone can do that - how in the world
do you throw like
40 mugs the same size when you have no way to weigh out
the clay for
each one first??).
I think Michael really explained it well because I used
to burnish all
the non-footed pots like small bowls, vases and mugs
and I did not get
as many cracks. The reason I did that was because I was
using mullite
grog and the stuff left the bottoms really sharp (even
with burnishing
them I had to smooth them out again before selling
them) so I would
smooth the leather hard pots out with an old credit
card. When I
switched to flint-clay grog I stopped doing this
(except on plates,
platters, etc) and it was on the pots I stopped doing
it on that cracked
the most. Plus it is possible that the mullite was
holding the clay
together a little better than the flint clay grog - it
has some kind of
needle-like structure that provides strength. Plus all
this started at
the beginning of summer when I started using the a/c
everyday so the
faster drying could not have helped much either (even
though I take a
lot of care to dry pots evenly, cover with plastic to
slow down the
drying, etc I can't always keep up with things when
they are drying so
fast.)
I will go back to burnishing them and report the
results in a few weeks.
One other thing that goes along with the idea - I
burnish the plates by
spinning them on the wheel and the motion with credit
card is just like
when trimming, and as someone else mentioned that
provides stress relief
since it goes against the direction during throwing.
And those plates
and other similar forms almost never crack.
The mugs, etc, were not burnished in such a way because
it is too hard
with their small size so I just swipe the card across
them - and even
when I did this they cracked sometimes, and with more
frequency than
plates, etc.
Thanks,
Paul
KATHI LESUEUR on mon 20 oct 08
On Oct 20, 2008, at 1:59 AM, Paul Borian wrote:
> Thanks for the replies so far.
>
> First of all, just to clarify, I am throwing on bats (wood, masonite,
> hardy backer board and sometimes formica) not off the hump (I couldn't
> throw a single pot of the hump with any amount of practice and I
> find it
> hard to believe anyone can do that - how in the world do you throw
> like
> 40 mugs the same size when you have no way to weigh out the clay for
> each one first??).>>
I routinely throw off of the hump. I never get s-cracks. I think it
has more to do with the clay than technique. I don't throw mugs, but
do throw lots of bowls. How do you get the same size? It has to do
with "centering" a certain size of lump from the hump to be made into
the piece. Once you've done this for awhile your hands learn how to
measure the proper amount. Variations are no different than weighing
clay and throwing bowls all the same size. They all have slight
variations.
Kathi
>
>
>
>
Lee Love on mon 20 oct 08
On Mon, Oct 20, 2008 at 6:44 AM, KATHI LESUEUR
wrote:
> I routinely throw off of the hump. I never get s-cracks. I think it
> has more to do with the clay than technique. I don't throw mugs, but
> do throw lots of bowls. How do you get the same size? It has to do
> with "centering" a certain size of lump from the hump to be made into
> the piece. Once you've done this for awhile your hands learn how to
> measure the proper amount. Variations are no different than weighing
> clay and throwing bowls all the same size. They all have slight
> variations.
>
You hit the nail on the head. Stuff made of Mashiko clay rarely
crack. It is very sandy, being found between layers of sand. It is
also "natural" clay and is of mixed particle size.
One thing I do with my unfooted, wirecut pattern work, to
make it smoother for table surfaces, is inlay the bottom with
contrasting slip, and then I scrape to reveal the pattern. It
smooths the bottom and you move grog or sand that can scratch
surfaces.
You can also put terra sig on bottoms and feet to give a smooth
bottom. In Japan, they are more aware of these kinds of finishing.
All pots are smoothed with a wet stone and you often hear the ringing
of the stone at the festival announcing a pottery sale.
--
Lee Love in Minneapolis
http://heartclay.blogspot.com/
http://mashikopots.blogspot.com/
http://claycraft.blogspot.com/
"Let the beauty we love be what we do.
There are hundreds of ways to kneel and kiss the ground." --Rumi
Veena Raghavan on mon 20 oct 08
Hi Paul,
Someone, ages ago, suggested burnishing with the silicon slider you can get
in a hardware store. I have used these ever since, and do not get cracks. I do
it by holding the piece by hand, not on the wheel, and it only take a minute
or so.
Hope this helps.
Veena
In a message dated 10/20/2008 5:55:49 AM Eastern Daylight Time,
Paul@DOCRET.COM writes:
>
> The mugs, etc, were not burnished in such a way because it is too hard
> with their small size so I just swipe the card across them - and even
> when I did this they cracked sometimes, and with more frequency than
> plates, etc.
>
VeenaRaghavan@cs.com
Paul Borian on mon 20 oct 08
Veena,
What is a silicon slider? Could you explain what it looks like so I can
see if it would work for my purposes? I also have the flexible metal rib
tool Michael refers to and use it as well. But I am always looking for
new ideas - especially when it comes to preventing cracking.
=20
Thanks,
Paul
Veena Raghavan on mon 20 oct 08
Sorry for lack of detail, Paul.
They are sliders used for putting under furniture to slide easily. they are
gray on one side and black on the other. They come in various shapes and sizes,
round and rectangular for the most part. I use the round ones and find them
great.
Veena
In a message dated 10/20/2008 3:07:41 PM Eastern Daylight Time,
Paul@DOCRET.COM writes:
>
> Veena,
>
> What is a silicon slider? Could you explain what it looks like so I can
> see if it would work for my purposes? I also have the flexible metal rib
> tool Michael refers to and use it as well. But I am always looking for
> new ideas - especially when it comes to preventing cracking.
>
>
>
> Thanks,
>
> Paul
VeenaRaghavan@cs.com
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