Jessica Morton on fri 13 apr 01
New to throwing but already an expert in creating s-cracks - and
trying to move on to more interesting classic pitfalls - I've been
told they're related to compression and also to clay's 'memory'.
An article in PMI suggested reversing the wheel now and then, to
compensate for the one-way torque on the clay when forming flat
pieces like plates. I'd guess the lower incidence of cracks with
slabs (if true) could be at least partly because the rolled clay's
been compressed like crazy, but without a twist that will "want" to
untwist once it's released (as in thrown ware).
Meanwhile, I just compress like the dickens when I throw plates and
bowls lately, and seem to be having better luck. So far. This week.
"It's not that children are little scientists but that
scientists are big children."
---"The Scientist in the Crib" by Gopnik, Meltzoff, and Kuhl
Dave Finkelnburg on sat 14 apr 01
Jessica,
I have found with large plates, and much more so with platters, other
forms wide at the base, I need to be sure to get the piece separated from
the bat and keep it separated. Large, wide forms tend to sit back down onto
the bat and stick, thus making it easier for them to crack as the base of
the piece shrinks.
A good twisted pair of wires for a cutoff wire lets air under the piece
better and helps avoid this problem. Otherwise, cut the piece off, and a
few hours later, do it again if it appears to have settled back down from
it's own weight.
Good throwing!
Dave Finkelnburg
Tony Ferguson on sat 14 apr 01
Jessica,
Don't waste your time reversing the wheel--just
develop your throwing skills. Make sure you go back
and forth from the center of the bottom of whatever
your throwing to your right compressing your bottom.
Tony Ferguson, Duluth, MN
--- Jessica Morton wrote:
> New to throwing but already an expert in creating
> s-cracks - and
> trying to move on to more interesting classic
> pitfalls - I've been
> told they're related to compression and also to
> clay's 'memory'.
> An article in PMI suggested reversing the wheel
> now and then, to
> compensate for the one-way torque on the clay when
> forming flat
> pieces like plates. I'd guess the lower incidence of
> cracks with
> slabs (if true) could be at least partly because the
> rolled clay's
> been compressed like crazy, but without a twist that
> will "want" to
> untwist once it's released (as in thrown ware).
>
> Meanwhile, I just compress like the dickens when I
> throw plates and
> bowls lately, and seem to be having better luck. So
> far. This week.
>
>
>
>
>
>
>
>
>
>
>
> "It's not that children are little
> scientists but that
> scientists are big children."
> ---"The Scientist in the Crib" by
> Gopnik, Meltzoff, and Kuhl
>
>
______________________________________________________________________________
> 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.
=====
--Tony Ferguson, fergyart@yahoo.com315 N. Lake Ave. Apt 401Duluth, MN 55806(218) 727-6339Looking to see, buy or barter artwork go to:http://acad.uwsuper.edu/www/aferguso/fergyart.htm
mudlark on sat 14 apr 01
There's really not much mystery as to what causes S-cracks. Not many potters can
take the time to reverse the wheel and which way the clay was wedged as to which
way it is thrown is alot to keep track of. And.... for most people these things
just go away as their skill developes. If the rim goes through most of it's drying
shrinkage before the base/bottom/floor then you willlllll get S-cracks. Like when
your pots dry too fast in the sun. If you don't compress the bottom of a piece you
willlllll get S-cracks. If the bottom is too thick compared to the walls you
willlllll get S-cracks. When trowing off the hump, if you push in on the base of
the piece and don't compress down on the bottom you willlllll get S-cracks. If you
take forever to throw a piece, leaving water set in the bottom, turning the clay
there into mush you willlllll get S-cracks. I'm sure there are more.
In Hammer's Dictionary there are many pages dealing with cracks, every potter
should have a copy.
Jessica Morton wrote:
> New to throwing but already an expert in creating s-cracks - and
> trying to move on to more interesting classic pitfalls - I've been
> told they're related to compression and also to clay's 'memory'.
> An article in PMI suggested reversing the wheel now and then, to
> compensate for the one-way torque on the clay when forming flat
> pieces like plates. I'd guess the lower incidence of cracks with
> slabs (if true) could be at least partly because the rolled clay's
> been compressed like crazy, but without a twist that will "want" to
> untwist once it's released (as in thrown ware).
>
> Meanwhile, I just compress like the dickens when I throw plates and
> bowls lately, and seem to be having better luck. So far. This week.
>
> "It's not that children are little scientists but that
> scientists are big children."
> ---"The Scientist in the Crib" by Gopnik, Meltzoff, and Kuhl
>
> ______________________________________________________________________________
> 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.
--
Clyde Tullis
Mudlark Pottery
320 G Street
Salida, CO 81201
719-539-1299
mudlark@chaffee.net
http://www.mudlarkpottery.com
iandol on sun 15 apr 01
Dear Mudlark,
An interesting post. Got to agree, all of those things you suggest will =3D
alleviate the pesky problem. I like to invert my pots as soon as their =3D
rims will support their weight without folding. Lets the free water =3D
drain downward from the base
But not one of those solutions tells us why the clay cracks. Cracks =3D
result from stress relief and have to have a point from which they =3D
propagate to dissipate energy. That is the mystery which must be =3D
explained.
Best regards,
Ivor
Nikki Simmons on sun 15 apr 01
Ivor,
I am glad you are looking in the microscope for additional information on
the s-crack problem. I use a clay that has does not seem to s-crack. I
don't compress my bottoms, and I also have lots of newbies and children
using this clay. (That translates into thick bottoms, lots of water on the
clay, walls of varying thicknesses.)
However, I should mention that the only time I have trouble with cracking is
when I have not wire cut the pot from the masonite bat. If I try and remove
the pot from the bat at the wrong time, it has difficulty breaking its seal.
A little drier than leatherhard, but not bone dry, as bone dry releases
itself.) Those pots, unless they have a thicker base, usually crack while
drying.
I look forward to hearing if you find any conclusive results.
Sincerely,
Nikki Simmons
nsimmons@mid-mo.net
iandol on mon 16 apr 01
Dear Chris Clarke,
I wrote an article which illustrated the way clay behaved as it was =
centred, coned and opened. The illustrations showed the degree to which =
clay from below was lifted and sheared then recycled out and down during =
the coning and pressing cycle. One of the results of this is that when =
the clay is firmer than usual it looses adhesion and a small cavity is =
created between the clay and the wheelhead or bat. Using that sweeping =
inward radial motion to press the clay back down again in the base of an =
opened pot reduces the incidence of future cracks. It is my opinion that =
it is impossible to compress clay. The story about aligning the clay =
crystals is untrue unless several tons per square inch of pressure is =
applied. Under those conditions the plastic fraction is squeezed past =
the non plastic fraction to give a slick surface. This occurs during =
extrusion and is one cause of laminations in pugged clay.
Best regards,
Ivor Lewis. Redhill, South Australia
Chris Clarke on mon 16 apr 01
I recently read an article on s-cracks in, I think, Pottery Making
Illustrated. Anyway, it stated that coning was the answer. I had always
been lazy about coning, but I was getting lots of s-cracks so I tried it.
Haven't had one since, I really cone the hell out of the clay. Seems it
kind of wedges it on the wheel head.
chris
who trims wet, dries in the sun, never slows the drying on anything. And
never gets too attached to any piece as clay is a wicked mistress.
chris@ccpots.com
www.ccpots.com
----- Original Message -----
From: mudlark
To:
Sent: Saturday, April 14, 2001 8:59 AM
Subject: Re: "S" Crack musings
> There's really not much mystery as to what causes S-cracks. Not many
potters can
> take the time to reverse the wheel and which way the clay was wedged as to
which
> way it is thrown is alot to keep track of. And.... for most people these
things
> just go away as their skill developes. If the rim goes through most of
it's drying
> shrinkage before the base/bottom/floor then you willlllll get S-cracks.
Like when
> your pots dry too fast in the sun. If you don't compress the bottom of a
piece you
> willlllll get S-cracks. If the bottom is too thick compared to the walls
you
> willlllll get S-cracks. When trowing off the hump, if you push in on the
base of
> the piece and don't compress down on the bottom you willlllll get
S-cracks. If you
> take forever to throw a piece, leaving water set in the bottom, turning
the clay
> there into mush you willlllll get S-cracks. I'm sure there are more.
> In Hammer's Dictionary there are many pages dealing with cracks, every
potter
> should have a copy.
>
>
> Jessica Morton wrote:
>
> > New to throwing but already an expert in creating s-cracks - and
> > trying to move on to more interesting classic pitfalls - I've been
> > told they're related to compression and also to clay's 'memory'.
> > An article in PMI suggested reversing the wheel now and then, to
> > compensate for the one-way torque on the clay when forming flat
> > pieces like plates. I'd guess the lower incidence of cracks with
> > slabs (if true) could be at least partly because the rolled clay's
> > been compressed like crazy, but without a twist that will "want" to
> > untwist once it's released (as in thrown ware).
> >
> > Meanwhile, I just compress like the dickens when I throw plates and
> > bowls lately, and seem to be having better luck. So far. This week.
> >
> > "It's not that children are little scientists but that
> > scientists are big children."
> > ---"The Scientist in the Crib" by Gopnik, Meltzoff, and
Kuhl
> >
> >
____________________________________________________________________________
__
> > 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.
>
> --
> Clyde Tullis
> Mudlark Pottery
> 320 G Street
> Salida, CO 81201
> 719-539-1299
> mudlark@chaffee.net
> http://www.mudlarkpottery.com
>
>
____________________________________________________________________________
__
> 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.
Tommy Humphries on mon 16 apr 01
Anytime you let the bottom center of the pot dry first, you drastically
reduce the danger of cracking. If the outer edge of the pot dries and the
center of the bottom is wet, then as the bottom dries it is like a drum h=
ead
getting tighter and tighter. This is due to the outer edge not giving any=
=2E
eventually the stress in the center of the bottom gets so great that it
pulls itself apart, literally along the path of least resistance.
Tommy
----- Original Message -----
From: "iandol"
To:
Sent: Sunday, April 15, 2001 2:20 AM
Subject: "S" Crack musings
Dear Mudlark,
An interesting post. Got to agree, all of those things you suggest will
alleviate the pesky problem. I like to invert my pots as soon as their ri=
ms
will support their weight without folding. Lets the free water drain
downward from the base
But not one of those solutions tells us why the clay cracks. Cracks resul=
t
=66rom stress relief and have to have a point from which they propagate t=
o
dissipate energy. That is the mystery which must be explained.
Best regards,
Ivor
_________________________________________________________________________=
___
__
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.
vince pitelka on tue 17 apr 01
Ivor wrote:
"It is my opinion that it is impossible to compress clay. The story about
aligning the clay crystals is untrue unless several tons per square inch of
pressure is applied. Under those conditions the plastic fraction is squeezed
past the non plastic fraction to give a slick surface. This occurs during
extrusion and is one cause of laminations in pugged clay."
Ivor:
It is sometimes possible to understand these phenomenon based on extensive
close experience without scientifically gathered evidence. You can argue
semantics, but the reality is that when the clay in the bottom of the pot is
compressed, there is far less liklihood of S-cracks. So what explains that?
Do not tell us that it is not compression unless you can give us another
sure-fire explanation.
And do not summarily tell us that the "story" about aligning clay platelets
is untrue unless you can similarly offer an alternate explanation of all the
phenomena which clearly fit the model of platelet allignment. In the mean
time, in my experience everything you do in manipulating or forming clay
creates some sort of directional grain structure. The platelets are the
only particles which have a distinct linear shape, and therefore seem to be
the logical culprit to explain grain structure and the resulting linear
contraction during shrinking. So many aspects of the behavior of clay are
easily explained in terms of platelet allignment and the resulting grain
structure. As I said, if you can come up with proven scientific
explanations for those phenomenon which summarily disproves the existence of
platelet allignment in studio ceramics, I will defer to your expertise.
Otherwise, my own practical hands-on experience offers me plenty of proof
that compression of clay is a very real phenomenon, and that any means of
working the clay results in allignment of platelets and resulting grain
structure and possible problems with "clay memory."
All of this is offered with the greatest respect for your considerable
technical knowledge and your generous contributions to Clayart, but you do
tend to occasionally develop theories which do not seem to conform to the
actual behavior of the clay. Remember that the actual behavior of the clay
is where it all begins. It is then our challenge to understand and explain
that behavior.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
vince pitelka on tue 17 apr 01
> 3. I never try to get away with using clay straight from the pug mill
without wedging it.
Bill -
For about the last 20 years I have used clay straight from the deairing
pugmill without wedging, and with that clay I do not believe I have ever had
an S-crack in that time. I always wheel-wedge (cone) the clay, and I always
compress the bottom after opening the lump, but I do these things quickly
and automatically, so it is no extra effort at all. I think that using clay
straight from a properly-working deairing pugmill is just fine as long as
you cone the clay to allign the platelets into spiral currents, and as long
as you compress the clay in the bottom of the vessel so that it will be
equal in compression to the clay in the walls. Some people do not believe
in these phenomena, but they are going to have to come up with hard
scientific evidence to convince me.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
Bill Amsterlaw on tue 17 apr 01
I was getting s-cracks in 50% of forms thrown off the hump with my =
stoneware body. I made changes that have all but eliminated the =
problem:
1. Like Tommy Humphries, I observed that s-cracks occurred more often =
when there was delayed drying at the bottom center. For example, pots =
with thin bottoms were less likely to s-crack than those with thick =
bottoms. I changed the way I throw and trim so as to form bottoms that =
gradually get thinner toward the center to speed the drying at the =
center.
2. I found that a small addition of grog or sand greatly reduced the =
tendency of my stoneware body to s-crack.
3. I never try to get away with using clay straight from the pug mill =
without wedging it.
Bill Amsterlaw
Keene, NY, USA
http://amsterlaw.com
wamster@amsterlaw.com
Tommy Humphries wrote:
<< Anytime you let the bottom center of the pot dry first, you =
drastically
reduce the danger of cracking. If the outer edge of the pot dries and =
the
center of the bottom is wet, then as the bottom dries it is like a drum =
head
getting tighter and tighter. >>
Mel Jacobson wrote:
<always been in high talc, or super fine bodies. it never happens
to me with my normal stoneware clay. i have always used an
open body with some sand and grog. (fine grog) i hump throw
often, cone my clay a bit, but the body is made to be open, and
not s crack. i do not have to compress my fingers bloody.>>
vince pitelka on wed 18 apr 01
> From physics we know that at human scale pressures (0-300 psi),
liquids
> and solids are essentially incompressible. People are confusing the ter=
m
> "compress" in this context. A better term in pottery might be "smoothin=
g"
> since what we do is drive the large material down into the surface thro=
ugh
> concentrated application of pressure.
So this has come down to semantics? I have certainly argued semantics on
this list before, but in this case I think we are going a little overboar=
d.
Another post said that pressing the clay removes water, but that water is
uncompressible. Yes, but it is not the water that is being compressed. =
The
clay is being compressed and by doing so we are removing water from the
clay, thereby reducing shrinkage and the chances of cracks. Seems to me
that "compress" is a perfectly appropriate term in this case. So water
cannot be compressed, and perhaps pure clay cannot be compressed within t=
he
range of human pressure, but we are "compressing" wet clay, and that is
perfectly plausible.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
> I tend to agree with Ivor that clay is not compressible, but I am
> building a hydraulic bulk modulus tester to see if there is any
appreciable
> compressibility due to the presence of microscopically entrained air
bubbles
> in moist clay.
> Conceptually, a measured sample of known volume will be placed in a
> closed chamber filled with hydraulic oil. The sample will be subjected =
to
a
> measured and increasing amount of pressure and the volume will be
observed.
> If the volume becomes less at relatively low pressures, it will confirm=
my
> notion that most mixed clay contains a lot of air and it is this air th=
at
> accounts for some of the problems we associate with clay "memory".
> Results to follow.
> Regards,
> Michael Wendt, wendtpot@lewiston.com
> 2729 Clearwater Avenue
> Lewiston, Idaho 83501
> call 1-800-554-3724
>
>
> > Ivor wrote:
> > "It is my opinion that it is impossible to compress clay. The story
about
> > aligning the clay crystals is untrue unless several tons per square i=
nch
> of
> > pressure is applied. Under those conditions the plastic fraction is
> squeezed
> > past the non plastic fraction to give a slick surface. This occurs
during
> > extrusion and is one cause of laminations in pugged clay."
>
> Isn't burnishing a very visual example of the possibility of compressin=
g
> clay with
> very little pressure? The slightest amount of pressure with just ones
finger
> across
> the leather hard surface brings the slick surface you mention. Granted
it's
> only on
> the surface but seems to be proof of the possibility of aligning the
> platelets and
> with a little more pressure, enough to compress the thickness of the cl=
ay
> bottom of
> pots.
>
> Elizabeth
>
>
_________________________________________________________________________=
___
__
> 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.
>
Cindy Strnad on wed 18 apr 01
I agree that when we repeatedly smooth the bottom of a pot, we're not
'compressing' anything. I'm not sure why this works. Maybe it squeezes ou=
t
moisture, as Ivor said. Maybe it simply ensures a well-shaped bottom.
Anyway, I do know that when I don't do it, I get s-cracks. So, whatever i=
t
is I'm doing when I say I'm compressing the bottom, I think I'll keep on
doing it. Sure, I'd like to know why, but whether I understand it or not,
I'm glad it works.
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
earthenv@gwtc.net
http://www.earthenvesselssd.com
Logan Oplinger on wed 18 apr 01
Jessica,
You will probably get similar recommendations from others here, but anyway...
If you knead your clay using the spiral or lotus method, form the cone of kneaded clay into a ball before placing it on the wheel head, or flatten a small ball of clay into a slab on the wheel head and place the cone of clay on top of that.
Good luck,
Logan Oplinger
------------------------------------------------------
Get the Latest News at CNN Interactive: http://CNN.com
elizabet on wed 18 apr 01
> Ivor wrote:
> "It is my opinion that it is impossible to compress clay. The story about
> aligning the clay crystals is untrue unless several tons per square inch of
> pressure is applied. Under those conditions the plastic fraction is squeezed
> past the non plastic fraction to give a slick surface. This occurs during
> extrusion and is one cause of laminations in pugged clay."
Isn't burnishing a very visual example of the possibility of compressing clay with
very little pressure? The slightest amount of pressure with just ones finger across
the leather hard surface brings the slick surface you mention. Granted it's only on
the surface but seems to be proof of the possibility of aligning the platelets and
with a little more pressure, enough to compress the thickness of the clay bottom of
pots.
Elizabeth
>
DEBBYGrant@AOL.COM on wed 18 apr 01
I use my clay straight from the pug mill also and never get S cracks.
However, I never put the cut side down on the wheel head. Rather,
I compress the cylinder of clay into a ball and put the side of the now
round cylinder on the wheel head. I had a student who complained
that she was getting S-cracks after purchasing a pug mill and when
I told her to never put the cut side down the S-cracks stopped. I do
cone my clay as well. For me that is just part of the centering process.
Debby Grant in NH
Howard Scoggins on wed 18 apr 01
Elizabeth--In my opinion "we" are misusing the word 'compress'.
Dewatering may be closer to the mark. The void space between clay
platelets is occupied by water. Dewatered clay is more dense than "wet"
clay. Clay increases its density as water, which can be expelled but not
compressed, is pressured out. Burnishing dewaters the surface and
smoothes dimples where water is wicking out to air. A strong ground
glass will show you what is happening.
Howard Scoggins
elizabet wrote:
>
> > Ivor wrote:
> > "It is my opinion that it is impossible to compress clay. The story about
> > aligning the clay crystals is untrue unless several tons per square inch of
> > pressure is applied. Under those conditions the plastic fraction is squeezed
> > past the non plastic fraction to give a slick surface. This occurs during
> > extrusion and is one cause of laminations in pugged clay."
>
> Isn't burnishing a very visual example of the possibility of compressing clay with
> very little pressure? The slightest amount of pressure with just ones finger across
> the leather hard surface brings the slick surface you mention. Granted it's only on
> the surface but seems to be proof of the possibility of aligning the platelets and
> with a little more pressure, enough to compress the thickness of the clay bottom of
> pots.
>
> Elizabeth
>
> >
>
> ______________________________________________________________________________
> 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.
Michael Wendt on wed 18 apr 01
In response to Elizabeth's reply to Ivor:
From physics we know that at human scale pressures (0-300 psi), liquids
and solids are essentially incompressible. People are confusing the term
"compress" in this context. A better term in pottery might be "smoothing"
since what we do is drive the large material down into the surface through
concentrated application of pressure.
I tend to agree with Ivor that clay is not compressible, but I am
building a hydraulic bulk modulus tester to see if there is any appreciable
compressibility due to the presence of microscopically entrained air bubbles
in moist clay.
Conceptually, a measured sample of known volume will be placed in a
closed chamber filled with hydraulic oil. The sample will be subjected to a
measured and increasing amount of pressure and the volume will be observed.
If the volume becomes less at relatively low pressures, it will confirm my
notion that most mixed clay contains a lot of air and it is this air that
accounts for some of the problems we associate with clay "memory".
Results to follow.
Regards,
Michael Wendt, wendtpot@lewiston.com
2729 Clearwater Avenue
Lewiston, Idaho 83501
call 1-800-554-3724
> Ivor wrote:
> "It is my opinion that it is impossible to compress clay. The story about
> aligning the clay crystals is untrue unless several tons per square inch
of
> pressure is applied. Under those conditions the plastic fraction is
squeezed
> past the non plastic fraction to give a slick surface. This occurs during
> extrusion and is one cause of laminations in pugged clay."
Isn't burnishing a very visual example of the possibility of compressing
clay with
very little pressure? The slightest amount of pressure with just ones finger
across
the leather hard surface brings the slick surface you mention. Granted it's
only on
the surface but seems to be proof of the possibility of aligning the
platelets and
with a little more pressure, enough to compress the thickness of the clay
bottom of
pots.
Elizabeth
Tommy Humphries on wed 18 apr 01
I also do not do anything special to the clay after it comes from the
pugmill, other than occasionally rolling a face on one end to prevent
trapped air. From the moment the clay hits the wheel, to the time it is cut
from the wheel the clay is moving. There are currents in a ball of clay as
it is shaped and worked on the wheel, If one compresses the clay one minute
then the next it is being loosened, then compressed again in another place.
The only place on a pot that this is not true is in the bottom, after the
pot is spread out and the bottom thinned then it is not touched again till
trimming time. It is true that the clay is compressed when opening and
thinning, it can't help but be. I do however find it hard to believe that
with all the different shapes and sizes of particles in a typical clay body
(of which clay may be a scarce thing!) that the platelets of clay will go
into alignment. I am not stating that it is impossible, but like Vince,
until I see scientific proof positive of this alignment through the body...
This is not to say that there is not some clay alignment that is visible...
the outer surface of a pot, where the coarser particles have been forced
down below t he surface is surely aligned, if not then the pot would not be
burnishable, with out the application of terra sig...(which by the way is
composed of 90+% platelets, and is very much aligned when dry and
burnished!)
Tommy Humphries
----- Original Message -----
From: "vince pitelka"
To:
Sent: Tuesday, April 17, 2001 10:44 PM
Subject: Re: "S" Crack musings
> > 3. I never try to get away with using clay straight from the pug mill
> without wedging it.
>
> Bill -
> For about the last 20 years I have used clay straight from the deairing
> pugmill without wedging, and with that clay I do not believe I have ever
had
> an S-crack in that time. I always wheel-wedge (cone) the clay, and I
always
> compress the bottom after opening the lump, but I do these things quickly
> and automatically, so it is no extra effort at all. I think that using
clay
> straight from a properly-working deairing pugmill is just fine as long as
> you cone the clay to allign the platelets into spiral currents, and as
long
> as you compress the clay in the bottom of the vessel so that it will be
> equal in compression to the clay in the walls. Some people do not believe
> in these phenomena, but they are going to have to come up with hard
> scientific evidence to convince me.
> Best wishes -
> - Vince
>
> Vince Pitelka
> Appalachian Center for Crafts
> Tennessee Technological University
> 1560 Craft Center Drive, Smithville TN 37166
> Home - vpitelka@dtccom.net
> 615/597-5376
> Work - wpitelka@tntech.edu
> 615/597-6801 ext. 111, fax 615/597-6803
> http://www.craftcenter.tntech.edu/
>
>
____________________________________________________________________________
__
> 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.
>
mudlark on thu 19 apr 01
I have not thought about this S - crack problem for a very long time. W=
=3D
asn't an
issue for me. But, it seems to be an ongoing dialog with Clayart. When I =
=3D
offered
my input on the matter it seemed to be too simple and then =3D85=3D85ters=
e. I=3D
was told
that I was not in the spirit of furthering knowledge of the Clayart grou=
=3D
p.
I've taught many people to throw and it, (S-cracks) always just went away=
=3D
as their
skills progressed. Why is that?
You can over-think this thing but ... will it make your work more satisfy=
=3D
ing?
better? Will you better make your statement?
Still not a mystery.
If you look at the tip of your skis you will never see what is coming. Wh=
=3D
at is
coming is the challenge that makes it fun.
Thank you Dr. Science.
Later
Tommy Humphries wrote:
> I also do not do anything special to the clay after it comes from the
> pugmill, other than occasionally rolling a face on one end to prevent
> trapped air. From the moment the clay hits the wheel, to the time it i=
=3D
s cut
> from the wheel the clay is moving. There are currents in a ball of clay=
=3D
as
> it is shaped and worked on the wheel, If one compresses the clay one mi=
=3D
nute
> then the next it is being loosened, then compressed again in another pl=
=3D
ace.
>
> The only place on a pot that this is not true is in the bottom, after t=
=3D
he
> pot is spread out and the bottom thinned then it is not touched again t=
=3D
ill
> trimming time. It is true that the clay is compressed when opening and
> thinning, it can't help but be. I do however find it hard to believe th=
=3D
at
> with all the different shapes and sizes of particles in a typical clay =
=3D
body
> (of which clay may be a scarce thing!) that the platelets of clay will =
=3D
go
> into alignment. I am not stating that it is impossible, but like Vince=
=3D
,
> until I see scientific proof positive of this alignment through the bod=
=3D
y...
>
> This is not to say that there is not some clay alignment that is visibl=
=3D
e...
> the outer surface of a pot, where the coarser particles have been force=
=3D
d
> down below t he surface is surely aligned, if not then the pot would no=
=3D
t be
> burnishable, with out the application of terra sig...(which by the way =
=3D
is
> composed of 90+% platelets, and is very much aligned when dry and
> burnished!)
>
> Tommy Humphries
>
> ----- Original Message -----
> From: "vince pitelka"
> To:
> Sent: Tuesday, April 17, 2001 10:44 PM
> Subject: Re: "S" Crack musings
>
> > > 3. I never try to get away with using clay straight from the pug mi=
=3D
ll
> > without wedging it.
> >
> > Bill -
> > For about the last 20 years I have used clay straight from the deairi=
=3D
ng
> > pugmill without wedging, and with that clay I do not believe I have e=
=3D
ver
> had
> > an S-crack in that time. I always wheel-wedge (cone) the clay, and I
> always
> > compress the bottom after opening the lump, but I do these things qui=
=3D
ckly
> > and automatically, so it is no extra effort at all. I think that usi=
=3D
ng
> clay
> > straight from a properly-working deairing pugmill is just fine as lon=
=3D
g as
> > you cone the clay to allign the platelets into spiral currents, and a=
=3D
s
> long
> > as you compress the clay in the bottom of the vessel so that it will =
=3D
be
> > equal in compression to the clay in the walls. Some people do not be=
=3D
lieve
> > in these phenomena, but they are going to have to come up with hard
> > scientific evidence to convince me.
> > Best wishes -
> > - Vince
> >
> > Vince Pitelka
> > Appalachian Center for Crafts
> > Tennessee Technological University
> > 1560 Craft Center Drive, Smithville TN 37166
> > Home - vpitelka@dtccom.net
> > 615/597-5376
> > Work - wpitelka@tntech.edu
> > 615/597-6801 ext. 111, fax 615/597-6803
> > http://www.craftcenter.tntech.edu/
> >
> >
> _______________________________________________________________________=
=3D
_____
> __
> > 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.
> >
>
> _______________________________________________________________________=
=3D
_______
> 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@pcl=
=3D
ink.com.
--
Clyde Tullis
Mudlark Pottery
320 G Street
Salida, CO 81201
719-539-1299
mudlark@chaffee.net
http://www.mudlarkpottery.com
iandol on thu 19 apr 01
Dear Tommy Humphries,
Thank you for joining in to this discussion.
Your posting prompted a thought. Am I right in thinking that when two =
surfaces are moving, in the same way that a ball of clay moves as we =
open and stretch it, that the inside would be moving slowly and the =
outside is moving fast. If this is so, does the difference in speed =
cause turbulence in the stuff in between. If this is so we may be mixing =
the clay up into swirls.
As I said, just a thought.
Best regards,
Ivor Lewis.
Tommy Humphries on fri 20 apr 01
That was just the kind of motion I had in mind. If you stretch the truth
just a bit, clay is like a liquid.
Place a bowl of slip in the center of your wheel head and turn the wheel at
a slow speed. This slip is your ball of clay...place your handin the bowl
and observe the motion of the slip. There will be eddys around your hand of
course, but notice the currents produced by the friction of the bowl on the
outer edge. I can't help but think that much the same thing happens in a
lump of clay, only on a much smaller scale.
Ok, now for a good question, the currents and eddys in the slip show up due
to ??? Is the lighter and darker areas in the slip due to some alignment of
the clay particles, where if the particles are laying flat to the light, it
is reflecting more light? The darker areas where the platelets of clay are
on edge, being non reflective???
Just playing in the slip in the wheel and woolgathering...
Tommy
----- Original Message -----
From: "iandol"
To:
Sent: Thursday, April 19, 2001 2:35 AM
Subject: "S" Crack musings
Dear Tommy Humphries,
Thank you for joining in to this discussion.
Your posting prompted a thought. Am I right in thinking that when two
surfaces are moving, in the same way that a ball of clay moves as we open
and stretch it, that the inside would be moving slowly and the outside is
moving fast. If this is so, does the difference in speed cause turbulence in
the stuff in between. If this is so we may be mixing the clay up into
swirls.
As I said, just a thought.
Best regards,
Ivor Lewis.
____________________________________________________________________________
__
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.
vince pitelka on fri 20 apr 01
Ivor said in response to Tommy Humphries:
"Your posting prompted a thought. Am I right in thinking that when two
surfaces are moving, in the same way that a ball of clay moves as we open
and stretch it, that the inside would be moving slowly and the outside is
moving fast. If this is so, does the difference in speed cause turbulence in
the stuff in between. If this is so we may be mixing the clay up into
swirls."
Ivor -
This is what I have been trying to say, and I am convinced that it is
platelet allignment. The amount of movement is in proportion to the
friction we apply to the outside of the spinning mass of clay. So it
certainly twists the mass, and thus we get spiral currents of platelates
within the clay. If you have ever seen very serious dunting from an
over-vitrified body, the cracks are often in perfect spirals, along the
grain, where the strength is less. Across the grain the strength is
greater. And in drying and firing the shrinkage is greater across the grain
than along the grain due to platelet allignment, because across the grain
there are more water spaces in drying and air spaces in firing. But now I
am getting off the subject.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
Tom Wirt/Betsy Price on sat 21 apr 01
I'm going to continue to climb out on my limb. Marek's observation of
pulling a handle versus swinging a rope of cal back and forth till it
cracks, added this....perhaps what we're dealing with hear has
similarity to attaching a handle. Clay has a shear strength. When
you exceed that either mechanically (swinging the handle) or
electrically (deflocculation) the clay breaks easily.. When coning on
the wheel, you exceed the shear strength at the center. (Does leverage
play any part in this?). This gives an area where the clay is no
longer bonded to itself. The act of "compressing" the center is
analogous to attaching a handle to a pot where the slip and the
wiggling back and forth, rebond the clay...make the particles move
together mechanically and rebond electrically. Use of vinegar as a
joining fluid instead of slip actually improves the joint because of
flocculating the particles at the joint.
Any takers on this one???
Tom Wirt: Re: "S" Crack musings
> I'm in Vince's camp on this one, too. It is platelet movement and
> alignment that causes, stops, s-cracks. Sometime take a pot where
the
> crack has started at leather hard. (If you're unlucky enough to be
> using a clay that is that prone to cracking). You'll find what
looks
> like what I call a twisted pillow.
>
> If your clay is this prone to s-cracking, unless you're getting some
> special effect from that blend, switch clays. There is NO reason to
> put up with this fault. It can easily be blended out of most any
> body.
>
> Tom Wirt
>
Tom Wirt/Betsy Price on sat 21 apr 01
I'm in Vince's camp on this one, too. It is platelet movement and
alignment that causes, stops, s-cracks. Sometime take a pot where the
crack has started at leather hard. (If you're unlucky enough to be
using a clay that is that prone to cracking). You'll find what looks
like what I call a twisted pillow.
If your clay is this prone to s-cracking, unless you're getting some
special effect from that blend, switch clays. There is NO reason to
put up with this fault. It can easily be blended out of most any
body.
Tom Wirt
Subject: Re: "S" Crack musings
> Ivor said in response to Tommy Humphries:
> "Your posting prompted a thought. Am I right in thinking that when
two
> surfaces are moving, in the same way that a ball of clay moves as we
open
> and stretch it, that the inside would be moving slowly and the
outside is
> moving fast. If this is so, does the difference in speed cause
turbulence in
> the stuff in between. If this is so we may be mixing the clay up
into
> swirls."
> Ivor -
> This is what I have been trying to say, and I am convinced that it
is
> platelet allignment.
iandol on mon 23 apr 01
Dear Tom,
I agree with you that leverage plays a big part in what happens as clay =
is coned and I know severe shear can take place. I know also from =
observation that the velocity differential leads to an "S" conformation =
at the centre of rotation. But I think there is more to it than that. =
Both tensile and compressive stress are involved. I have just been =
reading a 1999 paper on the strength of certain plastic materials which =
might be germaine to resolving this problem. It would appear that when =
stress is applied slowly ductile deformation happens but if the stress =
is applied rapidly, brittle fracture occurs. However, this is impossible =
in a duplex clay/water system as described in our literature.
It could be that highly aggresive centering and coning on wheels =
revolving at high speed will impose levered stresses into clay =
sufficient to rupture the fabric.
Release of pressure can induce cavitation. After clay has moved beyond =
the palm of the hand, air may be released into the clay. This would =
happen at the point at which the greatest degree of stress occurs, which =
is in the centre. Released air bubbles, invisible to the eye because =
they are so small, could segregate along planes of weakness and act as =
defects from which cracks would initiate. Imposing pressure on the clay =
may cause the bubbles to redissolve back into the water it came from, =
but that would need to be tested and observed. I doubt if they do. I =
think, and here my observations support my assertion, that applying =
downward pressure radially inwards causes these defects to be =
redistributed to the outer periphery of the thrown clay when it is =
opened. This is where there is less specific stress imposed by drying. =
The fact that vacuum pugged clay is known to be less prone to this fatal =
flaw may support that view, that being De-Aired, there is nothing from =
which bubbles, and hence defects, can form.
But all this is conjecture. Perhaps we will never know.
Best regards,
Ivor.
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