bob johnson on sat 20 feb 99
I am thrilled to learn the reason that s-cracks form
when I hump throw.
This is the first time that I have been able to understand
the physics of this problem.
I would like to know more about using inside throwing
ribs, and I will be very careful with my drying technique.
Good clay, along with good technique. A very good answer
to a problem.
Thank you Ron Roy.
Roberta Johnson
Chicago area
______________________________________________________
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bob johnson on wed 10 mar 99
I would like to report that the problem of s-cracks
is no longer a problem of mine.
I am using and inside rib to compress my clay and
I have switched to a more open body, and without
question, I am drying my pots more slowly.
A big Chicago Pizza to Clayart teachers.
Roberta Johnson
Chicago
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Get Your Private, Free Email at http://www.hotmail.com
carrie or peter jacobson on thu 15 apr 99
Last summer, I had lots of trouble with S-cracks in the bottoms of my large
porcelain bowls. Someone on this list, David Hendley, I think, suggested
that instead of putting spiral-wedged clay on the wheel fat side down and
pointy side up (i.e., the natural way), shape it into a ball and put it on
the wheel sideways. The once-pointy side faces west, the once-fat side
faces east, and the flat, long side faces up (and down).
I have not had one crack since I started using this technique.
I wanted to thank the potter who made this suggestion, and to reinforce it
to others on the list here. It works!
Carrie
Carrie Jacobson
Pawcatuck, CT
mailto:jacobson@brainiac.com
mel jacobson on fri 18 feb 00
ron is confident that i will post this note.
he and i have discussed this at great length.
he is sure that most s cracks are the result of twisted
pressure, bad drying.
i agree, but saying that, i have tested many clay bodies
and have found that high talc, mid range, slippery, baby pooh
clay bodies almost s crack themselves. you can compress them,
dry slow, throw with great skill...and s s s s nice ss nice.
if you are going to insist on throwing tight bodies, i feel it
will serve you well in the long run to add a small amount of sand
or very fine grog, or both.
in kyoto, when throwing very tight mugs, we would set them
on a little disc of dry plaster...sucked out some of the water and
started them drying together...(as ron pointed out)
that would be the reason to set them on newspaper..it will suck the
first drops of water from the base right away.
we have already heard from several potters that say...`hell, never
happens to me`...of course not, all of the conditions are perfect.
professional.
a very important thing to remember about hump throwing....it is
the slowest of all techniques if you do it correctly. it is not about
speed. it is about precision. careful throwing, measure, get right.
then slow drying, inverting...and careful well planned turning.
and only turning when the pots are ready, not you.
that brings up another point. when working as a full time potter,
like david hendley, john baymore.....others.
they work their pots when the pots are perfect...just right.
never wait a day. part time potters come home from work, rush
to the basement, closet, back room or whatever..and want to trim
and turn pots...if they are too wet...well, do it anyway...`oh, god
wally, they are almost as hard as bricks..gonna trim them anyway.`
hell, you might as well use sandpaper to trim them...and they
always fall off, so, you use about 5 pounds of wet
clay to hold them on...big mess.
frustration.
i have always said...`trim the pots when they are ready, not you.`
there was a potter in kyoto that could trim big flower containers
like we would peel an apple in one peel...used a big round trimming tool
set a speed, start at the lip and one great big long perfect snake
coiled on the floor beneath him...amazing.
but, remember, all of his conditions were perfect...wetness, clay body,
tools...all perfect. no guess work.
and, guessing is what gets us all in trouble.
ron does not guess very much...does real research, and
we all get the benefit of that. speaking of treasures.
mel/mn
why is abbreviation such a long word, and what's another word for synonym?
http://www.pclink.com/melpots (website)
from minnetonka, minnesota, u.s.a.
Chris Schafale on sat 19 feb 00
> we have already heard from several potters that say...`hell, never
> happens to me`...
Does anyone else find it puzzling and completely unhelpful when
folks respond to a plea for help by saying, "I've never had that
problem."? What exactly is the message here? "Don't know
what's wrong with you" --? "You must be unusually incompetent" --
? "You shouldn't be having this problem" --? "Too bad you're not as
smart as me" --?
Now, I don't mind a bit when someone says, "I've never had that
problem, and I think the reason is (fill in the blank -- clay body,
technique, mantra, clean living, whatever). At least this gives the
hapless supplicant something different they can try. But to just
write in to say, "Nope, never happens to me" is, I find, both useless
and annoying.
Yes, I have a delete key, and use it frequently, but when I ask a
question, it's usually because I've been struggling with that problem
for months or years, and it's demoralizing to be told that the
problem shouldn't exist. Well, it does exist, and being told
otherwise doesn't make it go away!
There, I feel better now. Thank you for your kind attention. And if
you don't have a problem with people saying they've never had a
problem, well, just keep it to yourself, won't you?
Chris
Light One Candle Pottery
Fuquay-Varina, NC
candle@intrex.net
http://www.angelfire.com/nc2/candle (work in progress)
Rick Mahaffey on mon 21 feb 00
Way back in the early seventies we had a potter from Mashiko stay at
grad school for a year as a special student. He demonstrated many
Japanese pottery techniques. We asked him right away how to prevent
s-cracks when throwing off the hump. He showed us his technique and a
couple of days later he was dismayed that most of his pots had s-cracks
in them. He decided that the clay was prone to s-cracking so he
modified his technique. It took him about three rounds of modified
throwing and waiting to find a technique that worked. He changed clay
bodies and found one that was much more friendly toward his original
technique. Maybe sometimes it is the clay.
Rick Mahaffey Who still gets s-cracks in about 20% of the stuff I make
off the hump (something about not getting to trim it when it is ready).
Ron Roy on fri 25 feb 00
Or maybe it was the potter again. Frequently potters - particularly those
who are trained in a traditional way - are taught to deal with the
traditional clays of the area. This can mean their techniques are from
trial and error over many generations. This way does not always give an
understanding of what is happening but how to avoid it.
I believe a better understanding of what clay is and how it works does
allow a better chance of dealing with problems. I also think I have a
better chance of dealing with these issues because I believe there are
reasons for what is happening.
Changing clays will simply substitute other problems for the one you are
trying to deal with - eventually you need to understand - in one area or
another - and preferably in several.
RR
>----------------------------Original message----------------------------
>Way back in the early seventies we had a potter from Mashiko stay at
>grad school for a year as a special student. He demonstrated many
>Japanese pottery techniques. We asked him right away how to prevent
>s-cracks when throwing off the hump. He showed us his technique and a
>couple of days later he was dismayed that most of his pots had s-cracks
>in them. He decided that the clay was prone to s-cracking so he
>modified his technique. It took him about three rounds of modified
>throwing and waiting to find a technique that worked. He changed clay
>bodies and found one that was much more friendly toward his original
>technique. Maybe sometimes it is the clay.
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
H. L. Rogers on thu 1 jun 00
I began having "s" cracks in bottoms of my pots about the time I
seriously decided to become a potter - went out and bought my own
wheel, kiln, and pug mill. I began turning out a lot of pots in the
evenings after work - also began having many, many pots with cracks
(everything from mugs to casseroles). I asked several potters, read
many books, and none of the remedies worked for me. One day I was
trying to analyze the problem, looking at the way I throw from
beginning to end. As I was taking clay from the pug mill, I realized
that the clay had been "pugged" in a definite direction, just like it
would be if it was wedged by hand - except I had not been wedging by
hand because of "cts". I remembered something one of my teachers had
said about the clay needed to be going in the same direction on the
wheel as it had been wedged - BINGO!! That had been the root of my
problem all along - no more cracks, and it has been years now!
I hope this helps. I haven't noticed anyone else addressing this
particular problem, so I thought I would put my 2cents in.
Joyce In Blairsville, GA - land of mountains, lakes, and streams!
mel jacobson on fri 2 jun 00
this is an old story here on clayart.
it comes up often.
the archives are full of some very good posts
from ron roy on work he has done/ very good stuff.
i also have run many experiments on claybody effects
on s-cracks and have found that certain very tight,
fine bodies will crack almost regardless of what you
do in the throwing. a bit of fine sand or grog is one
of the easiest fixes. we had several local bodies that
had a great deal of talc in them/ sorta cone 6 white that
really were hard to deal with.
if your clay `always s-cracks`/ well, change the body a bit.
or find a new clay.
s-cracks will drive a potter crazy.
the suggestions for help have been right on, but compression
and wedging are not always an easy cure.
often potters use a bad clay for years, get married to it, and don't
have clue that there are great clays that match you much better.
the greatest search that a potter has is clay, never glaze.
mel
looking forward to using vince's clay at the center.
great stuff. all hand made.
FROM MINNETONKA, MINNESOTA, USA
http://www.pclink.com/melpots (website)
Ron Roy on mon 5 jun 00
OK - I will say it - been wanting to for a long time now - I can cure
s-cracks in any clay - Just a matter of technique and drying.
Mel is right - there are clays that will suit your particular style better
than others - that is true but there is a down side to changing clays -
every clay has a different coefficient of expansion - so your glazes may
need some adjustments so make sure you do the testing to find out -
particularly the tests for shivering.
Many potters around the world have to work with bad clay - they find out
ways of making it work - matter of understanding why and not doing that.
Anyone can work with good clay.
RR
>if your clay `always s-cracks`/ well, change the body a bit.
>or find a new clay.
>s-cracks will drive a potter crazy.
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
ARTSHP@AOL.COM on tue 28 nov 00
I think I know what the answer is going to be, but is there anything to be
done with almost bone-dry student handbuilt slab pots that have S-cracks in
the bottom?
Susan in Seattle
Martin Howard on tue 28 nov 00
Fill with spooze. Possibly twice.
Fire, then glaze thickly inside.
It should cover it up.
Does with quite a lot done at my childrens' parties.
Martin Howard
Webb's Cottage Pottery
Woolpits Road, Great Saling
BRAINTREE, Essex CM7 5DZ
England
martin@webbscottage.co.uk
Rod, Marian, and Holly Morris on tue 28 nov 00
Can't help you now, but there is a GREAT article in Pottery Making
Illustrated (F'00) by Jeff Zamek that should end the problem for the future.
> I think I know what the answer is going to be, but is there anything to be
> done with almost bone-dry student handbuilt slab pots that have S-cracks
in
> the bottom?
> Susan in Seattle
>
Cindy Strnad on tue 28 nov 00
Susan,
This is a really good opportunity for your students to learn from their
mistakes. We all have. However, if you really want to save the pots, you can
try a couple of things, which may or may not work.
Fill the cracks in the bone-dry ware with a mixture of paperclay made with
vinegar or Lana's magic water. Make the paper clay very stiff, and add
powdered sugar. The powdered sugar will magically soften the clay. It will
also make it smell really nasty after a week or so. You may need to repeat
this after the bisque firing, and possibly after the glaze firing. In that
case, you'll have to daub glaze over the repair work and then re-fire.
A lot of bother, and you aren't going to get a perfect pot, whatever you do,
but maybe worth it for children's pots. Children's pots merit more effort on
our part, as they're not as replaceable as our pots.
Second, you can mix up a potion of half kiln cement, half powdered clay with
vinegar or magic water and use this in the same way you used the former
concoction.
The advantage to these two patching compounds is that they don't shrink much
themselves. The problem is that the clay does shrink, and will pull away
from the patch. Your biggest problem is the shrinkage (and resultant
cracking) during the glaze firing, as this is the most difficult to
disguise.
Hope this helps. BTW, the first mixture is great for re-attaching stuff that
has fallen off, so long as the stuff in question won't be under stress--best
not to use it on handles, of course.
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
earthenv@gwtc.net
http://www.earthenvesselssd.com
Craig Dunn Clark on tue 28 nov 00
Probably not. Though if you would like a more positive answer, try
taking a little of the clay slip and mixing it with vinegar. Then brush a
bit of the slurry into the cracks. You'll need to do this several times,
until the crack "disappears". It doesn't work well, but you may have some
luck.
As far as the S-cracks are concerned they are most likely the result of
rapid drying and there being to much moisture in the bottom of the pots when
they were built. The same thing happens when a "lake" is left in the bottom
of a pot thrown on the wheel.LOL
-----Original Message-----
From: ARTSHP@AOL.COM
To: CLAYART@LSV.CERAMICS.ORG
Date: Tuesday, November 28, 2000 5:48 AM
Subject: S-Cracks
>I think I know what the answer is going to be, but is there anything to be
>done with almost bone-dry student handbuilt slab pots that have S-cracks in
>the bottom?
>Susan in Seattle
>
>___________________________________________________________________________
___
>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.
>
Karen Sullivan on tue 28 nov 00
Aardvark Ceramic Supply in Santa Ana, CA makes a repair
material they call tenax...you can use it to fill in cracks
on greenware or bisqueware and it holds. It will even hold
a horizontal attachment to a piece.
Think about reinforcing compression concepts when making, but you know
that.
Aardvark can be reached at 714- 541-4157
bamboo karen
on 11/27/00 9:40 PM, ARTSHP@AOL.COM at ARTSHP@AOL.COM wrote:
> I think I know what the answer is going to be, but is there anything to be
> done with almost bone-dry student handbuilt slab pots that have S-cracks in
> the bottom?
> Susan in Seattle
>
> ______________________________________________________________________________
> 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.
>
Chris Schafale on tue 28 nov 00
Paper clay to the rescue!!!! I am completely smitten with this as a
mender, after filling in some 1/8" wide gaps that weren't supposed
to be there. Soak toilet paper in water, whiz in blender, add dry
clay (I used trimming scraps) until you get a thick slip, then slather
on. Smooth carefully while it's wet, because it can be kind of
fragile if you try to sand after bisque firing. Utterly wonderful, and
works on bisque as well as bone-dry or leather hard pots. Yes, I
know, everyone else already knows about this, but sometimes it
takes me awhile -- I never jump on bandwagons until they've
already gone by....
Chris
> I think I know what the answer is going to be, but is there anything to be
> done with almost bone-dry student handbuilt slab pots that have S-cracks in
> the bottom?
> Susan in Seattle
>
> ______________________________________________________________________________
> 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.
>
Frank Gaydos on tue 28 nov 00
Susan,
Believe it or not they can be saved.
This is the method I use: wet the crack with a brush about five or six
times. You want to cross hatch it with a needle tool so you make your own
slurry about half the thickness of the slab.
Meanwhile find some bone dry clay and smash it into a powder with a spoon or
rolling pin. Add a few drops of water to this powder and mix. You want to
create a mix that is as dry as possible but able to hold it's form. The
wetter you make it the more it will shrink later and therefore will recrack.
Use this mix to fill in the crack and burnish the crack with a smooth side
of a spoon or stone. The idea is you want to force the clay into the space.
Leave a little extra thickness that you can sand smooth after it is dried.
You may need to repeat this procedure if it recracks, but, it should not be
as bad a crack.
If it cracks in the bisque firing you can also force clay into the crack and
glaze right over it. Usually cannot tell where it was. Sometimes we cover
with a slip just to make sure.
Hope that helps.
Frank Gaydos
----- Original Message -----
Subject: S-Cracks
> I think I know what the answer is going to be, but is there anything to be
> done with almost bone-dry student handbuilt slab pots that have S-cracks
in
> the bottom?
> Susan in Seattle
>
>
____________________________________________________________________________
__
>
Nanci Bishof on wed 29 nov 00
>From years of experience salvaging middle school clayworks: If the cracks
don't go all the way through the bottom, you can scrape a bone dry scrap of
the same clay and work that into the S-crack. Then fire as usual.
nanci
Dr.Tom Roess on wed 29 nov 00
Susan, I've had good luck fixing cracks with either paper clay or a
product
called Marx Mender which can be used to repair greenware or bisque. Most=
=
ceramic supply houses carry it or a similar product. It's not as good for=
=
filling in gaps as paper clay - I'm going to try mixing it
with paper clay to see if I can get the best qualities of both. (Just
thought
of that as I was writing this to you. Thanks!
Lou in Snowmass
Craig Dunn Clark on wed 29 nov 00
Chris, I for one didn't know about it. Thanx for the suggestion. Maybe I
won't be as quick to scrap cracked pieces as I've been.
-----Original Message-----
From: Chris Schafale
To: CLAYART@LSV.CERAMICS.ORG
Date: Tuesday, November 28, 2000 8:23 PM
Subject: Re: S-Cracks
>Paper clay to the rescue!!!! I am completely smitten with this as a
>mender, after filling in some 1/8" wide gaps that weren't supposed
>to be there. Soak toilet paper in water, whiz in blender, add dry
>clay (I used trimming scraps) until you get a thick slip, then slather
>on. Smooth carefully while it's wet, because it can be kind of
>fragile if you try to sand after bisque firing. Utterly wonderful, and
>works on bisque as well as bone-dry or leather hard pots. Yes, I
>know, everyone else already knows about this, but sometimes it
>takes me awhile -- I never jump on bandwagons until they've
>already gone by....
>
>Chris
>
>> I think I know what the answer is going to be, but is there anything to
be
>> done with almost bone-dry student handbuilt slab pots that have S-cracks
in
>> the bottom?
>> Susan in Seattle
>>
>>
____________________________________________________________________________
__
>> 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.
>>
>
>___________________________________________________________________________
___
>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.
>
David Woof on thu 19 apr 01
has anyone mentioned sloppy throwing practice..... using too much water and not sponging out adequately when finished and/or leaving pots stuck to a nonabsorbent bat while waiting for them to "pop off" on their own. can't think of a more stressing thing to do to a pot. enough has been said about "compressing", just keep doing it, it's a cure for some. each pot and your reputation deserves that much careful attention to foundation detail anyway, and it becomes automatic after the first fifty or so.
David Woof
Earth and Fire Studios
525 Fiesta St.
Clarkdale,Az.86324
woofpots@hotmail.com
Always a little over the edge; reverently takeing an irreverent look at everything.
Get your FREE download of MSN Explorer at http://explorer.msn.com
mel jacobson on mon 28 apr 03
an old story here on clayart. now making the rounds
for the 10th time.
ivor has done some very good work with this subject.
read his pmi piece.
i have written at great length about the subject, as
this is what we did in japan, all the time...every day, by
the thousands. same as lee is doing now.
we never had s-cracks.
and, i do not have them now.
michael is on the track. do not torque the pots.
it is common. keep the throwing smooth. water.
the clay body is important. those high talc, no grit
bodies will crack all the time. never fails.
slow drying is important...and the bottoms cannot be
too thick.
inconsistent. not good for any pots.
most japanese potters use a string lasso to remove
the pots. it is imprint to score the base, right where you
want it cut. use the thumb nail. just enough for a perfect
ring foot. the theory is:
don't leave too much at the bottom...they crack, and you
have to take an extra five minutes to get the thickness correct
and flat,
before you make your ring foot. five minutes by 80 pots...that
is 400 minutes to your day. and potters do that all the time.
mess, mess, mess around. waste time.
remember, the japanese potter almost always uses the rib on the
inside of the pot. it measures, compresses and shapes, all in one.
that is the part people do not know about.
mel
From:
Minnetonka, Minnesota, U.S.A.
web site: my.pclink.com/~melpots
or try: http://www.pclink.com/melpots
new/ http://www.TICK-ATTACK.COM
iandol on tue 29 apr 03
Dear mel jacobson,=20
Thanks for the accolade. I have posted a bit more information today,just =
to reinforce what was said before.
I'd like to get it into people's minds, as did Dave Finkelnberg, that =
clay is not a particularly easy material to compress, in the physical =
sense of decreasing the volume. I like to think in terms of =
"Consolidation", making it more solid, bringing it together, helping =
microfine shear tears to weld, disrupting flow patterns which are =
potentially weak planes where cracks will form.
So teachers, when you are demonstrating how to use the wheel, please =
tell your students that you are "Consolidating the Clay" and why you are =
putting pressure on it as you sweep finger or thumb or rib across the =
base. And never talk about clay crystals sliding across each other =
lubricated by a film of water unless you can provide a demonstration to =
prove such events as a fact.
Best regards,
Ivor Lewis. Redhill, South Australia
Vince Pitelka on wed 30 apr 03
So teachers, when you are demonstrating how to use the wheel, please tell
your students that you are "Consolidating the Clay" and why you are putting
pressure on it as you sweep finger or thumb or rib across the base. And
never talk about clay crystals sliding across each other lubricated by a
film of water unless you can provide a demonstration to prove such events as
a fact.
Thanks for all your technical expertise Ivor. I always appreciate it. But
I shall continue to refer to compressing the clay, because that is the term
in common usage and there is no reason to change it. And I shall continue
to refer to crystals of clay sliding across each other lubricated by a film
of water until you can provide a demonstration to prove such events are not
fact.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Office - wpitelka@tntech.edu
615/597-6801 x111, FAX 615/597-6803
http://iweb.tntech.edu/wpitelka/
Vince Pitelka on thu 1 may 03
Yesterday when I responded to Ivor's message I forgot to identify the part
of his message that was included, so I am resending this to clarify.
Ivor wrote:
"So teachers, when you are demonstrating how to use the wheel, please tell
your students that you are "Consolidating the Clay" and why you are putting
pressure on it as you sweep finger or thumb or rib across the base. And
never talk about clay crystals sliding across each other lubricated by a
film of water unless you can provide a demonstration to prove such events as
a fact."
Thanks for all your technical expertise Ivor. I always appreciate it. But
I shall continue to refer to compressing the clay, because that is the term
in common usage and there is no reason to change it. And I shall continue
to refer to crystals of clay sliding across each other lubricated by a film
of water until you can provide a demonstration to prove such events are not
fact.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Office - wpitelka@tntech.edu
615/597-6801 x111, FAX 615/597-6803
http://iweb.tntech.edu/wpitelka/
John Kudlacek on thu 1 jul 04
Chris,
Right on! I had a similar problem years ago and found that if I put the
clay on the wheel "side ways" after wedging the cracks disappeared.Sort of
like taking a cinamon roll and placing it on its side.
Other things which can help prevent bottom cracks are "wire" cuting to
free the pot from the bat right after throwing, not letting water collect
and sit on the bottom (inside) of the pot and placing pots on paper to dry
allowing the pot to move in the process of shrinking instead of sticking
to a non absorbent surface. Oh yes, compressing the bottom of the pot with
a rib in the early phases of throwing is also very important.
John in Topeka
Chris Trabka on thu 1 jul 04
I have had some problems with s-cracks on the bottom of pots in the past.
I initially burnished the bottom of the pot (using a metal rib). Had a very
nice side effect - the bottom of the pot would be very smooth to the touch
even after a glaze firing. Some posts on clay-art lead me there. I haven't
had any s-cracks in pots with burnished bottoms.
However when making 3-foot tall vases, burnishing the bottom of the
piece was not an option. I noted that some had s-cracks and some didn't.
Thought about all the steps in the process and considered what could
be causing the problem. I noted that after wedging, I generally placed
the "cone" on the wheel either pointing straight up or pointing straight
down (thought it would help me center). Considered the slice of clay out
of a pug mill that had been left to dry and noted a similar s-crack. When
I started the next set of tall vases I made sure that the "cone" on the
wheel was pointing to the side not up or down. Haven't had a problem since.
Chris
Louis Katz on sun 16 oct 05
More data for the mix.
When I throw little 3 inch bowls, i throw a medium grained stoneware
(basic recipe below). I throw off the hump, 2-3 pots per minute. If I
am giving a talk while I throw this slows down to one every 40 seconds.
I use little water, none is left in the bowls.
Most of the time 2/3 of the pots s crack on the bottom. Frankly i could
care less, and when I do care I use a more groggy clay.
Compression; what happens after not reading email for a week and
opening it.
Louis
On Oct 16, 2005, at 1:49 PM, Vince Pitelka wrote:
> Steve Graber wrote:
> "The "compression of the clay" used to *fix* the S crack zone in other
> industries is called surface hardening. it crushes the surface region
> &
> gets smoother (burnishing is a surface hardening). but the foundation
> of
> the material is not affected. ~ the portion further inside from the
> surface
> affected by that *compression*. this *fix* advise means that if a
> compression of the area (surface hardening) *fixes* an S crack it
> still must
> be there under that surface hardened spot."
>
> Steve -
> Wow, that is some impressive extrapolation, considering the weight of
> evidence to the contrary. You would do well to listen to some of the
> most
> experienced potters on this list who have been trying to tell you about
> compression and "S"-cracks.
>
> I agree with most of what Joe Herbert wrote, but my experience tells
> me that
> compression of the bottom is often tied to the formation of
> "S"-cracks. I
> think what Joe is referring to is that experienced potters tend to
> throw
> quickly, and the bottom simply never has a chance to absorb excess
> moisture
> that could cause "S"-cracks. Also, as Joe points out, the choice of
> claybodies has a lot to do with tendency towards S-cracks. Very
> fine-grain
> claybodies without grog or sand have higher drying shrinkage (more
> particles, therefore more water layers) and thus more problems with
> "S"-cracks. My intro to throwing students often start out using our
> "studio
> stoneware" claybody because it is so smooth and buttery, but after
> having
> some trouble with "S"-cracks they sometimes switch over to another of
> our
> stonware bodies that contains 10% fine sand.
>
> I do not mean to deny or demean your clay experience in any way, but
> you
> have come to some odd conclusions. The idea that compression only
> affects
> only the surface like "surface hardening" is about as far-fetched as I
> have
> heard, and is completely contrary to the nature of clay. How can
> someone
> who has run a few tons of clay through his hands come to such
> conclusions?
> The drag of your finger on the clay moves the clay around through its
> full
> thickness, re-aligning the platelets. That is precisely why
> compression
> does such a good job of reducing the chance of "S"-cracks.
>
> Your references to surface hardening on burnished pottery and on metal
> castings both make sense in regards to the unyielding nature of the
> subsurface material, but have nothing at all to do with the behavior of
> plastic clay. In the posts on this subject, you have been given all
> the
> information you need to understand what is going on with water
> absorption,
> comression, and "S"-cracks. If you choose to maintain your own odd
> ideas
> about "S"-cracks despite all evidence to the contrary, that is your
> choice.
>
> At this point I wonder why I am still trying to explain this to you,
> other
> than the hope that other readers will not be misled.
> - Vince
>
> Vince Pitelka
> Appalachian Center for Craft, Tennessee Technological University
> Smithville TN 37166, 615/597-6801 x111
> vpitelka@dtccom.net, wpitelka@tntech.edu
> http://iweb.tntech.edu/wpitelka/
> http://www.tntech.edu/craftcenter/
>
> _______________________________________________________________________
> _______
> 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.
>
>
Louis Katz
http://www.tamucc.edu/~lkatz
steve graber on sun 16 oct 05
wow vince, i guess for you anything contrary to "conventional wisdom" just can't pass for consideration from you. i read several comments here just recently that sound very much like the claybody might have the biggest contribution to the charateristics of S cracking. you still want to push the surface hardening approach of a rework-in-process rather then concider other sources.
it sounds more now to me from other's comments this past day that a "short" claybody is more at the root od S cracks then any anecdotal fix such as *comprsssion*. you even alude to this yourself below. so what if it isn't a complete centering? it's still rooted in some method other then "compress" at a stage of throwing. that was my original comment. the root cause of S cracking has never been shown. people shouldn't be told "compress like this" to stop S cracks. they should be told "we think it's a result of a non-plastic claybody" or some such comment.
for me it's hardly an been an extrapolation - but observation. from other points of view outside the raw only clay exposure.
stay alert - see if you see similar events. don't be so close minded.
sorry to tip your mind some.
see ya
steve
Vince Pitelka wrote:Steve Graber wrote:
"The "compression of the clay" used to *fix* the S crack zone in other
industries is called surface hardening. it crushes the surface region &
gets smoother (burnishing is a surface hardening). but the foundation of
the material is not affected. ~ the portion further inside from the surface
affected by that *compression*. this *fix* advise means that if a
compression of the area (surface hardening) *fixes* an S crack it still must
be there under that surface hardened spot."
Steve -
Wow, that is some impressive extrapolation, considering the weight of
evidence to the contrary. You would do well to listen to some of the most
experienced potters on this list who have been trying to tell you about
compression and "S"-cracks.
I agree with most of what Joe Herbert wrote, but my experience tells me that
compression of the bottom is often tied to the formation of "S"-cracks. I
think what Joe is referring to is that experienced potters tend to throw
quickly, and the bottom simply never has a chance to absorb excess moisture
that could cause "S"-cracks. Also, as Joe points out, the choice of
claybodies has a lot to do with tendency towards S-cracks. Very fine-grain
claybodies without grog or sand have higher drying shrinkage (more
particles, therefore more water layers) and thus more problems with
"S"-cracks. My intro to throwing students often start out using our "studio
stoneware" claybody because it is so smooth and buttery, but after having
some trouble with "S"-cracks they sometimes switch over to another of our
stonware bodies that contains 10% fine sand.
I do not mean to deny or demean your clay experience in any way, but you
have come to some odd conclusions. The idea that compression only affects
only the surface like "surface hardening" is about as far-fetched as I have
heard, and is completely contrary to the nature of clay. How can someone
who has run a few tons of clay through his hands come to such conclusions?
The drag of your finger on the clay moves the clay around through its full
thickness, re-aligning the platelets. That is precisely why compression
does such a good job of reducing the chance of "S"-cracks.
Your references to surface hardening on burnished pottery and on metal
castings both make sense in regards to the unyielding nature of the
subsurface material, but have nothing at all to do with the behavior of
plastic clay. In the posts on this subject, you have been given all the
information you need to understand what is going on with water absorption,
comression, and "S"-cracks. If you choose to maintain your own odd ideas
about "S"-cracks despite all evidence to the contrary, that is your choice.
At this point I wonder why I am still trying to explain this to you, other
than the hope that other readers will not be misled.
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
______________________________________________________________________________
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.
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Vince Pitelka on sun 16 oct 05
Steve Graber wrote:
"The "compression of the clay" used to *fix* the S crack zone in other
industries is called surface hardening. it crushes the surface region &
gets smoother (burnishing is a surface hardening). but the foundation of
the material is not affected. ~ the portion further inside from the surface
affected by that *compression*. this *fix* advise means that if a
compression of the area (surface hardening) *fixes* an S crack it still must
be there under that surface hardened spot."
Steve -
Wow, that is some impressive extrapolation, considering the weight of
evidence to the contrary. You would do well to listen to some of the most
experienced potters on this list who have been trying to tell you about
compression and "S"-cracks.
I agree with most of what Joe Herbert wrote, but my experience tells me that
compression of the bottom is often tied to the formation of "S"-cracks. I
think what Joe is referring to is that experienced potters tend to throw
quickly, and the bottom simply never has a chance to absorb excess moisture
that could cause "S"-cracks. Also, as Joe points out, the choice of
claybodies has a lot to do with tendency towards S-cracks. Very fine-grain
claybodies without grog or sand have higher drying shrinkage (more
particles, therefore more water layers) and thus more problems with
"S"-cracks. My intro to throwing students often start out using our "studio
stoneware" claybody because it is so smooth and buttery, but after having
some trouble with "S"-cracks they sometimes switch over to another of our
stonware bodies that contains 10% fine sand.
I do not mean to deny or demean your clay experience in any way, but you
have come to some odd conclusions. The idea that compression only affects
only the surface like "surface hardening" is about as far-fetched as I have
heard, and is completely contrary to the nature of clay. How can someone
who has run a few tons of clay through his hands come to such conclusions?
The drag of your finger on the clay moves the clay around through its full
thickness, re-aligning the platelets. That is precisely why compression
does such a good job of reducing the chance of "S"-cracks.
Your references to surface hardening on burnished pottery and on metal
castings both make sense in regards to the unyielding nature of the
subsurface material, but have nothing at all to do with the behavior of
plastic clay. In the posts on this subject, you have been given all the
information you need to understand what is going on with water absorption,
comression, and "S"-cracks. If you choose to maintain your own odd ideas
about "S"-cracks despite all evidence to the contrary, that is your choice.
At this point I wonder why I am still trying to explain this to you, other
than the hope that other readers will not be misled.
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Craig Martell on tue 18 oct 05
Steve was saying:
>my discussion in all this started from the often stated fix of "compression".
>my issue is with people teaching only of the immediate repair of
>"compression" without regard for further causes.
Hi:
I think that your main problem with the S crack issue is that you are
assuming that compression is a fix or repair as stated above. My
disagreement here is that if compression is used, there is nothing to
repair so your use of fix and repair with regard to S cracks and
compression are incorrect. Sure, there are clay bodies that are prone to S
cracking no matter what sort of forming methods are used but the first
thing we do is throw the pots in a way that we know will prevent S
cracking. If this flaw still occurs, we are forced to look deeper.
My claybodies don't have the S crack problem. I formulate and mix them
myself so if there are any problems it's my fault and I'm the one who has
to fix them. I did have a stoneware body that I used decades ago that S
cracked with total abandon. Turned out to be excessive raw shrinkage and
poor particle size. I worked that out and the cracks stopped. So there
are stages to this problem. Primary and secondary causes of S
cracks. Improper formulation and mixing of claybodies and, or, poor
working technique.
regards, Craig Martell Hopewell, Oregon
Ivor and Olive Lewis on wed 19 oct 05
Dear Louis Katz,
Interesting to speculate why adding grog tends to prevent "S" cracks. I =
say this because it adds nothing to the physical strength of a plastic =
clay. In fact, if you add an excessive amount it can reduce the strength =
of a clay body.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
John Hesselberth on thu 20 oct 05
Hi Ivor,
My speculation would be that it introduces multiple discontinuities and
places for stress to accumulate thereby keeping any single point from
becoming a point of high stress buildup and exceeding the breaking
point of the clay structure. Essentially the same reason why rounded
corners are better at resisting cracks than square corners, but in a
different system. Stress concentration is something you want to avoid
when working with clay.
Regards,
John
On Oct 19, 2005, at 3:48 AM, Ivor and Olive Lewis wrote:
> Interesting to speculate why adding grog tends to prevent "S" cracks.
> I say this because it adds nothing to the physical strength of a
> plastic clay. In fact, if you add an excessive amount it can reduce
> the strength of a clay body
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
Vince Pitelka on thu 20 oct 05
Ivor Lewis wrote:
"Interesting to speculate why adding grog tends to prevent "S" cracks. I say
this because it adds nothing to the physical strength of a plastic clay. In
fact, if you add an excessive amount it can reduce the strength of a clay
body."
Hey Ivor, we're waiting for you to speculate. All I know is that it makes a
big difference. Some of my students get seduced early-on by fine-grain,
gritless stoneware or porcelain claybodies, but then discover the greater
tendency towards cracking, especially "S"-cracks. We have a Mackenzie
stoneware formula that contains 10% fine sand, and it rarely "S"-cracks. I
think John is right about the "multiple discontinuities" and their tendency
to disrupt fracture planes, but the added sand or grog also just reduces
drying shrinkage. I also suspect that a claybody containing sand or grog
moves around internally in a different manner than a very fine-grain
claybody. It may be that "compression" is simply more effective on a
claybody containing sand or grog.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Hank Murrow on thu 20 oct 05
Dear Group;
I've been staying out of this thread, but Vince's remarks served to
spur me into action(well, waiting for my firing to cool added to the
urgency). My teacher, Bob James, suggested his "Room Full of
Basketballs Theory of Clay Formulation". This postulated that if you
had a certain # of basketball-size particles, then there would be room
for tennisball-size particles in between, then Marbles, then BBs, then
etc. The idea is that all particles would receive wetting and surface
area would be mazimized while the larger particles would prevent
extreme shrinkage. Thus, we looked at the particle size range and
percentages of the many clays and other materials available to us and
composed our clay bodies with that information in a rule of thumb way.
it seemed to work, because there were many bodies in use, and we seldom
had S-cracks.
Cheers, Hank, still in Eugene
On Oct 20, 2005, at 6:03 PM, Vince Pitelka wrote:
> Ivor Lewis wrote:
> "Interesting to speculate why adding grog tends to prevent "S" cracks.
> I say
> this because it adds nothing to the physical strength of a plastic
> clay. In
> fact, if you add an excessive amount it can reduce the strength of a
> clay
> body."
>
> Hey Ivor, we're waiting for you to speculate. All I know is that it
> makes a
> big difference. Some of my students get seduced early-on by
> fine-grain,
> gritless stoneware or porcelain claybodies, but then discover the
> greater
> tendency towards cracking, especially "S"-cracks. We have a Mackenzie
> stoneware formula that contains 10% fine sand, and it rarely
> "S"-cracks. I
> think John is right about the "multiple discontinuities" and their
> tendency
> to disrupt fracture planes, but the added sand or grog also just
> reduces
> drying shrinkage. I also suspect that a claybody containing sand or
> grog
> moves around internally in a different manner than a very fine-grain
> claybody. It may be that "compression" is simply more effective on a
> claybody containing sand or grog.
www.murrow.biz/hank
Tom Sawyer on thu 20 oct 05
It seems to me that the answer to S-cracking when throwing off the hump may
be related to the lesser tendency to S-crack when throwing off the wheel
head or non-plaster bats. It seems logical that "compression" [if I may use
that word] to align clay particles would be more effective when applied
against a solid surface rather than a softer clay surface. This is true even
when using bodies with grog. Could grog add drag that allows the particle to
line up more easily?
Tom Sawyer
Ivor and Olive Lewis on fri 21 oct 05
Dear Vince,=20
Nice to "rap" with you again.
Yes. I go along with John's idea of multiple discontinuities and in a =
way, it is similar to some metallic alloys that gain strength and =
freedom from fracture by having a duplex or two phase structure. In =
those examples, a fracture in one phase of the material has its passage =
interrupted by the second phase. But in the case of a well grogged clay =
I speculate it is a shrinkage void between the large particles and the =
Kaolinite/Water complex in which the fracture developing that prevents =
the fractures form enlarging.
If I can get hold of a CCD for the microscope I may be able to make =
images of clay body structure that might reveal what is actually =
happening.
As I say, I can speculate but that does not give us the facts.
Best regards,
Ivor.
Heather Pedersen on sat 22 oct 05
Many years ago, I took a class on material properties. One of the things
we covered was inhibiting crack propigation. It turns out that if you
have some mid-sized particles in your base material, not too big, not too
small, cracks have a difficult time finding a path, and will stop. That's
probably why grog tends to help with the s-crack problem.
-= Heather Pedersen
-= Lone Tree Ceramics
On Wed, 19 Oct 2005 17:18:00 +0930, Ivor and Olive Lewis
wrote:
>Dear Louis Katz,
>
>Interesting to speculate why adding grog tends to prevent "S" cracks. I
say this because it adds nothing to the physical strength of a plastic
clay. In fact, if you add an excessive amount it can reduce the strength
of a clay body.
>
>Best regards,
>
>Ivor Lewis.
>Redhill,
>S. Australia.
>
Ivor and Olive Lewis on sun 23 oct 05
Dear Heather Pedersen,
Thanks for your contribution.
In the case of Grog, clay shrinks away from it as drying proceeds. This =
creates a void across which a fracture will not propagate. The energy =
seems to dissipate as an echo. The interesting questions is "Which =
medium is the crack travelling through in a drying plastic clay body ? " =
Kingery et Al, "Introduction to Ceramics" have a chapter on Defects =
which covers cause and propagation.
Best regards
Ivor
James and Sherron Bowen on sun 23 oct 05
Look in thearchives for a solution to this problem posted by Mudlark
Karen Nakakihara on wed 1 nov 06
Hello,
One of the assignments in my ceramics class is to make 8 bakers, 8" wide
and 2" high. I have thrown 27 to date and only have 1 to show for it -
all the rest had S-cracks (if not in the greenware, it cracks in the
bisque). I was told to compress the floor more and so I have - over and over
and over and still it cracks. I used a sponge, my fingers, a rib, and a
squared off scraper.
Does anyone have any tips they can give me?
Frustrated in Alpine,
Karen
Marcia Selsor on thu 2 nov 06
Are you getting the center too thin when you cut under it? ? Are you
leaving a heavy corner around the edge? Make sure all is even. Try
putting a thumb tack through the walls, bottom and bottom corner. OR
measure with your pin tool..
Are you drying slow? Turn your pieces over as they dry.
You might try putting a little coil the size of the foot in an "s"
shape on the bottom. Look at old porcelain platters in antique shops.
They have these.
Your piece of 8" shouldn't warrant this, but it shouldn't hurt.
Marcia Selsor
http://marciaselsor.com
On Nov 1, 2006, at 9:51 PM, Karen Nakakihara wrote:
> Hello,
>
> One of the assignments in my ceramics class is to make 8 bakers, 8"
> wide
> and 2" high. I have thrown 27 to date and only have 1 to show for
> it -
> all the rest had S-cracks (if not in the greenware, it cracks in the
> bisque). I was told to compress the floor more and so I have -
> over and over
> and over and still it cracks. I used a sponge, my fingers, a rib,
> and a
> squared off scraper.
>
> Does anyone have any tips they can give me?
>
> Frustrated in Alpine,
> Karen
>
> ______________________________________________________________________
> ________
> 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.
>
Timothy Joko-Veltman on thu 2 nov 06
On 11/2/06, Karen Nakakihara wrote:
> Hello,
>
> One of the assignments in my ceramics class is to make 8 bakers, 8" wide
> and 2" high. I have thrown 27 to date and only have 1 to show for it -
> all the rest had S-cracks (if not in the greenware, it cracks in the
> bisque). I was told to compress the floor more and so I have - over and over
> and over and still it cracks. I used a sponge, my fingers, a rib, and a
> squared off scraper.
>
> Does anyone have any tips they can give me?
I have seen the suggestion to center the clay, remove it from the
bat/wheelhead, flip it over, and recenter as a means of compressing
the clay even further before opening. I personally have never needed
this, but it may do the trick for you.
Cheers,
Tim
Dave Finkelnburg on thu 2 nov 06
Karen,
I'm sorry to hear of the many failures of your
work. That is very discouraging for anyone. :-(
One of the great misconceptions perpetrated by
pottery instructors is that clay is "compressible."
Certainly, increasing the density of clay in the foot
of ware is helpful to preventing cracks when the clay
dries. And, it is possible that one can open clay so
fast that one creates small (micro) tears (rips, not
the crying kind) in the clay that can weaken the clay.
Burnishing the clay, which the so-called
"compressing" does, can reduce the weakness caused by
those tears and that does prevent cracking of the
foot.
The real cause of S-cracks, though, is how thrown
pieces dry. Vince Pitelka has described this very
succinctly on list within the last two weeks.
Briefly, the rim dries faster than the foot. That's
what causes the S-crack.
Some ways to prevent S-cracks include covering the
rim with a material that slows evaporation
there--plastic or paper or cloth are used by many
potters. On very wide pieces I drape strips of the
lightest plastic I can find on the rim--it literally
sticks to the wet clay without causing distortion or
damage.
Another solution is to put the ware on an
absorbent material so the foot is losing moisture at a
similar rate to the rim. Not possible in a very dry
atmosphere, like one gets with winter heating or
summer desert, though. Simly trying this and laying
sheets of newspaper over the top may be enough to
cause even drying, though the drying will be slower.
Another solution is to invert the ware onto its
rim, which slows evaporation from the rim and speeds
drying of the foot. For wide, shallow, ware, though,
this can lead to slumping of the center if it's
unsupported, so this may be least effective in your
particular case. It does work well once the ware
reaches stiff leather hard and will no longer slump.
In all cases you need to watch the ware closely
and "manage" it during the drying process. S-cracks
in bisque, by the way, start in drying. The cracks
are just so small you haven't noticed them in the
greenware. If you did, you wouldn't waste time firing
them.
I hope this helps.
Dave Finkelnburg
--- Karen Nakakihara wrote:
> One of the assignments in my ceramics class is to
> make 8 bakers, 8" wide
> and 2" high. I have thrown 27 to date and only have
> 1 to show for it -
> all the rest had S-cracks (if not in the greenware,
> it cracks in the
> bisque). I was told to compress the floor more and
> so I have
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William & Susan Schran User on thu 2 nov 06
On 11/1/06 10:51 PM, "Karen Nakakihara" wrote:
> I was told to compress the floor more and so I have - over and over
> and over and still it cracks. I used a sponge, my fingers, a rib, and a
> squared off scraper.
>
> Does anyone have any tips they can give me?
What clay are you using?
Some finer grained clays are more susceptible to the type of cracking your
describe. Try changing to a clay that has a variety of particle sizes and
contains some grog.
Break one of the forms in half and examine the cross section of the bottom
in comparison to the walls, especially where the bottom transitions to the
wall. Too rapid a change in thickness will also contribute to cracking.
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
Bruce Girrell on thu 2 nov 06
Karen Nakakihara wrote:
> One of the assignments in my ceramics class is to make 8 bakers, 8" wide
> and 2" high. I have thrown 27 to date and only have 1 to show for it -
> all the rest had S-cracks
How fast are you drying them? If you try to dry them too fast, you will
create s-cracking. To be more accurate, I should say, "If you allow them to
dry unevenly, you will create s-cracking." When pieces are dried quickly,
they generally dry unevenly, resulting in cracks.
How fast is too fast? Again, it's a matter of uniformity, but I would allow
probably at least a week for an 8", flat-bottomed piece to dry. We can nit
pick on that if you want.
Other possible crack initiators:
1) Allowing the bottom to stay wet while throwing. Allowing the bottom to
remain wet changes the clay composition and water content relative to the
remainder of the piece.
2) Insufficient or improper clay preparation prior to throwing. The clay
must be made homogeneous prior to throwing. Pugging doesn't count. In fact
pug mills can exacerbate the problem. Wedge your clay prior to use or at the
very least place the clay on the wheelhead so that the axis of the pug is
perpendicular to the axis of the wheel and then cone up and down until you
feel a consistent texture.
Good luck with your project
Bruce Girrell
Snail Scott on thu 2 nov 06
At 07:51 PM 11/1/2006 -0800, you wrote:
...make 8 bakers, 8" wide
>and 2" high...had S-cracks...I was told to compress the floor more and so
I have...
Could be a drying issue. Can you
flip them over when they become
leather-hard, to dry upside-down?
If not, try covering the rims
and leaving the bottom/middle
exposed, to let the drying process
even out. If they are sitting on
non-absorbent surfaces to dry, try
transferring them to an absorbent
board or bat, or to a fresh one
if they're already on an absorbent
surface.
-Snail
Donald G. Goldsobel on thu 2 nov 06
In my experience, S cracks are the result of uneven drying in the bottom of
the pot which causes tension which evidences itself in the crack. If you let
water accumulate in the bottom as you throw the result is more water in the
clay at the bottom of the pot than the rest of the body=S cracks. Constantly
remove the bottom water as you throw, dry the piece up side down as soon as
the piece has enough integrity to bear its own weight. Slow the drying by
covering with newspaper then plastic film. The newspaper keeps the
condensation from returning to the pot. As you are more experienced you will
use less water and have fewer problems. Pay attention to the nature of clay
and how it changes as you change its form. Adding moisture rearranges things
and as the clay dries there is a different rearrangement of the particles.
Unevenness in the moisture in the various parts of the pot between leather
hard and bone dry is where the chief problems occur.
Be patient. Take your time and don't rush the drying process.
Good luck
Donald
Ann Brink on thu 2 nov 06
Hi Karen,
When I was using a local earthenware, I had to be careful about pots shaped
like casseroles, i.e. a flat bottom, and a quick transition to a vertical
side. I had much better results when I made that corner a rounded one, a
smooth continuous curve, and, as other posts have said, even thickness.
Also, drying upside down helps (as soon as the rim has firmed), as someone
else mentioned.
Ann Brink in Lompoc CA
(mostly about pottery)
----- Original Message -----
From: "Karen Nakakihara"
> Hello,
>
> One of the assignments in my ceramics class is to make 8 bakers, 8" wide
> and 2" high. I have thrown 27 to date and only have 1 to show for it -
> all the rest had S-cracks (if not in the greenware, it cracks in the
> bisque). I was told to compress the floor more and so I have - over and
> over
> and over and still it cracks. I used a sponge, my fingers, a rib, and a
> squared off scraper.
>
> Does anyone have any tips they can give me?
>
> Frustrated in Alpine,
> Karen
>
>
Veena Raghavan on thu 2 nov 06
Hi Karen,
How frusrating. You have all my sympathy.
Are you sponging out the water when you throw, so that water does not
accumulate at the bottom? Water can make the bottom soggy, so it is better to sponge
it out frequently.
Are you cutting the pots off the bat or wheel before you let them dry to
leatherhard for trimming? Often, leaving the pot on the wheel to dry out before
cutting off can cause problems.
When you do cut off the wheel or bat, if you take a pin tool and make a
groove for the wire, and if you slant the pin tool when making this groove, it
helps to cut off the pot leaving very little clay on the bat, so the bottom is not
thinned out.
Hope some of this helps and good luck.
Veena
In a message dated 11/2/2006 11:04:30 AM Eastern Standard Time,
wschran@COX.NET writes:
> On 11/1/06 10:51 PM, "Karen Nakakihara" wrote:
>
> >I was told to compress the floor more and so I have - over and over
> >and over and still it cracks. I used a sponge, my fingers, a rib, and a
> >squared off scraper.
> >
VeenaRaghavan@cs.com
Michael Wendt on thu 2 nov 06
Karen,
I know I harp on wedging and everyone
groans so let me share a story:
In the March-April issue of Pottery
Making Illustrated is printed an article
I wrote on how to stack and slam
wire wedge.
A potter who makes mostly tile called
me after that and said she had read
the article but thought that the wedging
would be impractical in her setting
because she does hundreds at a time.
I suggested she do a trial run.
She emailed me a few months later
with this note:
>"Hi Michael,
I wanted to thank you again for insisting that it was
worth the effort to
wire wedge all the clay for my tiles. My back is sore,
but all 160 tiles
came out flat except one! I was having about 20% loss,
so this is a big
deal. What a pleasure it is to open the kiln and not
have to throw out all
this work.
I've added some other things into the process. With
fresh clay, I add 1 tsp
of calcined EPK to 1/4 bag of clay when I am wedging.
Linda Arbuckle
suggested this, and perhaps it contributes to killing
the plasticity, but
since I'm wedging anyway, I might as well add it. I am
also finding that
the edge trimming clay (which can be on the dry side),
can be wedged back to
a decent consistency if I squirt some water between
each wedge. The clay
remains fairly hard, but is uniform and makes nice
tiles. It is a lot more
work to wedge when the clay is dry, but there is no
waste clay (since I
don't have a pugmill).
The other thing I have noticed is how different the
wetness of the clay is
from bag to bag. If the clay is just mush, I roll it
out and let it set up
for a few hours before I press and I'm sure that helps,
too. But with the
wire wedging, I can mix wet and dry clay and it still
comes out ok.
Janet "
In general, flat items have a much
lower loss rate if you wire wedge
them as illustrated in the article.
Best of Luck,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, Idaho 83501
USA
wendtpot@lewiston.com
www.wendtpottery.com
Vantuil Varges on thu 2 nov 06
Sometimes I will use plastic shower caps while drying plates and wide
pieces. They keep the wall moist but allow the bottom to dry. Put a
shower cap on the pot and flip it upside down letting the bottom dry.
Vantuil
On Nov 2, 2006, at 11:03 AM, Snail Scott wrote:
> At 07:51 PM 11/1/2006 -0800, you wrote:
> ...make 8 bakers, 8" wide
>> and 2" high...had S-cracks...I was told to compress the floor more
>> and so
> I have...
>
>
> Could be a drying issue. Can you
> flip them over when they become
> leather-hard, to dry upside-down?
> If not, try covering the rims
> and leaving the bottom/middle
> exposed, to let the drying process
> even out. If they are sitting on
> non-absorbent surfaces to dry, try
> transferring them to an absorbent
> board or bat, or to a fresh one
> if they're already on an absorbent
> surface.
>
> -Snail
>
> ______________________________________________________________________
> ________
> 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.
claybair on thu 2 nov 06
Hey Karen,
A few questions...
What claybody are you using?
Do you leave water setting in the bottom?
How thick is the bottom in relation to the walls?
Do you wire them off the bat?
How are you drying them?
What kind of surface do you dry them on?
Do you trim them and if so is it leather hard, drier or softer?
Gayle Bair
Bainbridge Island, WA
Tucson, AZ
http://claybair.com
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG]On Behalf Of Karen
Nakakihara
Sent: Wednesday, November 01, 2006 7:52 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: S-cracks
Hello,
One of the assignments in my ceramics class is to make 8 bakers, 8" wide
and 2" high. I have thrown 27 to date and only have 1 to show for it -
all the rest had S-cracks (if not in the greenware, it cracks in the
bisque). I was told to compress the floor more and so I have - over and
over
and over and still it cracks. I used a sponge, my fingers, a rib, and a
squared off scraper.
Does anyone have any tips they can give me?
Frustrated in Alpine,
Karen
____________________________________________________________________________
__
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.
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Lynn Goodman Porcelain Pottery on thu 2 nov 06
On Nov 1, 2006, at 10:51 PM, Karen Nakakihara wrote:
> Hello,
>
> One of the assignments in my ceramics class is to make 8 bakers, 8"
> wide
> and 2" high. I have thrown 27 to date and only have 1 to show for
> it -
> all the rest had S-cracks (if not in the greenware, it cracks in the
> bisque). I was told to compress the floor more and so I have -
> over and over
> and over and still it cracks. I used a sponge, my fingers, a rib,
> and a
> squared off scraper.
>
> Does anyone have any tips they can give me?
>
> Frustrated in Alpine,
> Karen
Hi Karen,
Do you have access to plaster bats? Since I switched over, I can
count the number of s-cracks I have gotten on one hand. The reason
they help is, the bottom of the pot dries at a similar rate to the
wall and rim, reducing twisting in the floor to a minimum. As a
bonus, the pot will pop off the bat when ready to trim--much easier,
and less warpage to boot.
Lynn
Lynn Goodman
Fine Porcelain Pottery
Cell 347-526-9805
www.lynngoodmanporcelain.com
Jennifer Boyer on fri 3 nov 06
To add to this, as someone who uses a fine grained "touchy" clay, I
love hardy backer board (cement) shelves as ware boards. When I throw
off the hump I put the pots on those boards and they act like a
plaster bat in terms of drying out the bottoms and eliminating S-
cracks. Some clay bodies are just prone to S-cracking. You can be the
most experienced thrower out there and still those bottoms crack. We
all know how hard it is to match a clay body with the glazes you want
to use and it's nice to be able to keep using a clay body by
switching ware boards! By the way I also switched from plastic to
plaster bats for all my wide forms. I bought one of those bat making
systems at NCECA and they are great. The system has little plastic
bat pin hole thingies that work great....
Take Care
Jennifer, firing Big Bertha on a nice clear day, just the kind she
likes....
On Nov 2, 2006, at 7:28 PM, Lynn Goodman Porcelain Pottery wrote:
> On Nov 1, 2006, at 10:51 PM, Karen Nakakihara wrote:
>
>> Hello,
>>
>> One of the assignments in my ceramics class is to make 8 bakers, 8"
>> wide
>> and 2" high. I have thrown 27 to date and only have 1 to show for
>> it -
>> all the rest had S-cracks (if not in the greenware, it cracks in the
>> bisque). I was told to compress the floor more and so I have -
>> over and over
>> and over and still it cracks. I used a sponge, my fingers, a rib,
>> and a
>> squared off scraper.
>>
>> Does anyone have any tips they can give me?
>>
>> Frustrated in Alpine,
>> Karen
>
> Hi Karen,
>
> Do you have access to plaster bats? Since I switched over, I can
> count the number of s-cracks I have gotten on one hand. The reason
> they help is, the bottom of the pot dries at a similar rate to the
> wall and rim, reducing twisting in the floor to a minimum. As a
> bonus, the pot will pop off the bat when ready to trim--much easier,
> and less warpage to boot.
>
> Lynn
>
>
> Lynn Goodman
> Fine Porcelain Pottery
> Cell 347-526-9805
> www.lynngoodmanporcelain.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.
>
Jennifer Boyer
http://www.vtpots.com
jennifer@vtpots.com
Montpelier. Vermont
William & Susan Schran User on fri 3 nov 06
>
>> On Nov 1, 2006, at 10:51 PM, Karen Nakakihara wrote:
>>
>>> Hello,
>>>
>>> One of the assignments in my ceramics class is to make 8 bakers, 8"
>>> wide
>>> and 2" high. I have thrown 27 to date and only have 1 to show for
>>> it -
>>> all the rest had S-cracks (if not in the greenware, it cracks in the
>>> bisque).
In my original response I asked about the clay body, but one thing I failed
to ask was if other students in the class were encountering the same issues
of cracking. If Karen is the only individual suffering the "S" cracking
issue, I would suggest consulting with her colleagues and observe what they
do differently to avoid the problem.
I would also like to know what the instructor's advice has been to deal with
the cracking problem.
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
Bonnie Staffel on fri 3 nov 06
Karen, you may be barking up the wrong tree by doing all the things you
mentioned. You are getting S cracks because the walls are drying faster
than the bottom. Either place your pot on a piece of drywall to hasten =
the
drying of the bottom, or cover the wall with plastic for a while to =
allow
the bottom to dry before the walls. Most cracks occur because of uneven
drying or firing. When loading in the kiln, it might be a good idea to =
lift
the pot from the kiln shelf by small shards so that the heat can get =
under
the pot as well as over the surface. Or put the pot upside down after =
it
gets to be leather hard in order to dry evenly.. =20
Your teacher should be able to guide you in this problem. It is just =
common
sense that the bottom is staying wet too long while the sides are =
getting
dry. The bottom has no place to go but to crack. =20
Wish you success.
Bonnie Staffel
http://webpages.charter.net/bstaffel/
http://vasefinder.com/bstaffelgallery1.html
DVD Throwing with Coils and Slabs
DVD Beginning Processes
Charter Member Potters Council
Jaymes, The Pottery Pimp on fri 10 nov 06
I'm sure you've gotten your answer by now--but the only cure and sure fix
to "S" crackes either during drying, bisque fire or actual glaze fire is
slow down the drying.
I had the world's most horrible "S" crackes--once I began really slowing
down my drying times (greenware) I've not had an "S" crack in years now.
I actually cover all my thrown or slab pieces for sometime to slow the
drying process for as long as a week, then slowly open up a corner of the
plastic covering day by day until the pieces is evenly dried. Keeping
it "evenly" moist for the drying phase will prevent those "S" cracks from
happening!
Good luck with your studies--and better luck with your art!
Jaymes,
aka, The Pottery Pimp
http://www.werhandmaid.com
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