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cracking problems with double walled vessels....anyone with

updated wed 10 may 06

 

Judi Buchanan on thu 4 may 06

experience?

Craig, I make a couple of dozen double walled vessels every year and the
only cracking I get is the two walls separating where they are joined if I
don't wrap the outer wall while it dries. Of course I carve the outer wall
until it is just filigree before the bisque and that would take away the
stress and air pressure problems. Are you making a hole in the outer wall
when you are refining the inside contour? That would cause stress also.

Judi Buchanan, Flutter-by Pottery, Heidelberg, Mississippi




Hello all, I have recently thrown several "double walled" vessels. I
have admired them for years and decided to give the form a whirl. Well,
the throwing part went well enough, but I have now cracked two of the
in separate bisque firings. They are about 16 inch diameter, double
walled bowl shapes with nice fleshy lips.

Ann Brink on thu 4 may 06

experience?

Hi Craig,

I've only made a few, and early on one cracked the way you describe, but in
the glaze firing. It's inner bowl was suspended, and the outer wall was
where the weight rested. I figured I should have put grog under the
feet/edge, to allow movement as the piece shrunk.

However, there isn't much size alteration in the bisque firing, unless
there is with your clay. Depending on how they are constructed...can the
heat access the inner part as readily as the outer part?. I'm sure you too
have been picturing what may happen to the pieces, and when. It it's
cracking on cooldown, it would be because the inner part is still hotter and
remains a bit larger, not allowing the outer edge to shrink a bit.
(disclaimer- just conjecturing here)

Let us know what you try next and if it works!

Ann Brink in Lompoc CA.....going to be on Local Origination TV- the taping
is this afternoon-am bringing various pots and will be interviewed. Hope my
tongue doesn't get ahead of my brain.




----- Original Message -----
From: "Craig Clark"
To:
Sent: Thursday, May 04, 2006 8:05 AM
Subject: cracking problems with double walled vessels....anyone with
experience?


> Hello all, I have recently thrown several "double walled" vessels. I
> have admired them for years and decided to give the form a whirl. Well,
> the throwing part went well enough, but I have now cracked two of the
> in separate bisque firings. They are about 16 inch diameter, double
> walled bowl shapes with nice fleshy lips. One stands about 12 inches
> tall and the other was a shallower form that was a bit wider. I put a
> slip texture on the outside of the first one and thougth that perhaps
> the cracking was from the application of the slip a resultant uneven
> drying between the outer wall and the inner wall. But the second one did
> not have any slip application and it also ended up with an outer wall
crack.
> The clay body is one that I have used extensively and have not
> experienced any type of problem with. It is a real toothy mix from
> Laguna called WSO. The cracks appear only on the outer wall. Nothing on
> the bottom or the inside. One of the cracks goes partially into the
> lip.The bisque was what I call nice and slow. I start off with the kiln
> on low for an over night and then begin bringing it up the next day
> after it has reached red heat. Once the bisque has gotten to a nice
> red/orange glow I rock and roll to an 04.
> Does anyone on the list have any experience with successfully firing
> double walled vessels? Do I need to fire even slower as if I were firing
> a large sculptural form? Could the cracking be occuring on the way down,
> rather than on the way up? I let the kiln cool naturally and I have not
> arranged any type of slow cooling brick network around the perimeter of
> the piece as I would for a large platter form. Nor am I using Vince's
> infamous starburst array under the piece. Do I need to do this?
> Thank you for any and all help or suggestions
> Craig Dunn Clark
> 619 East 11 1/2 St
> Houston, Texas 77008
> (713)861-2083
> mudman@hal-pc.org
>
>
____________________________________________________________________________
__
> 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.

Linda Ferzoco on thu 4 may 06

experience?

Is Doug Gray a member of Clayart? He did an amazing
display of double-walled vessels at Mendocino in 2005.
Those who attended might recall his firing
techniques.

Linda Ferzoco
> From: "Craig Clark"
>
>
> > Hello all, I have recently thrown several
> "double walled" vessels. I
> > have admired them for years and decided to give
> the form a whirl. Well,
> > the throwing part went well enough, but I have now
> cracked two of the
> > in separate bisque firings.

Carl Finch on thu 4 may 06

experience?

At 08:05 AM 5/4/2006, Craig Clark wrote:

> Hello all, I have recently thrown several "double walled" vessels.

> Does anyone on the list have any experience with
> successfully firing double walled vessels?

I don't, but perhaps Carol Lebreton can help. Her work was featured on
yesterday's "Oregon Artbeat" TV program. She makes "singing bowls,"
double-walled bowls with some sort of beads within the space between to
make 'em sing when spun about.

http://www.carollebreton.com/

--Carl
in Medford, Oregon

Ron Roy on fri 5 may 06

experience?

Hi Craig,

If the crack is not open then it happened on the way down - outside clay
cools faster than the inside clay - during the quartz inversion - tries to
contract but - because the inside clay has not reached the right
temperature to go through the quartz inversion - the part of the pot that
does go through the contraction - cannot contract.

If getting it off the shelf did not work you need to slow the cooling -
make sure no spys are open - lid fits properly - and vent system is off.

If that does not work then - if you have a controller - set it to turn the
kiln back on about 700C (to cool at 50C per hour) and then off again at
500C.

Pyrometers are not that accurate so you may want to see what yours says the
temp is with a low fire cone in a dish - say cone 022 which is supposed to
go down around 600C and Hamer says that is dull red in an electric kiln -
if you know you pyrometer is going to give good info to your controller
then you only have to slow down between 600 and 550C. Just remember - kiln
bottoms usually cool faster - maybe firing those pots at the top will work
because they will cool slower there.

You may also find that other bodies - which don't have as much free silica
will not be a problem.

So I managed to get through that whole thing without saying - the quartz
inversion happens at 573C - if you could see the pot as it cracks the crack
would be open - until the rest of the quartz in the pot cooled enough to go
though the inversion.

RR

> Hello all, I have recently thrown several "double walled" vessels. I
>have admired them for years and decided to give the form a whirl. Well,
>the throwing part went well enough, but I have now cracked two of the
>in separate bisque firings. They are about 16 inch diameter, double
>walled bowl shapes with nice fleshy lips. One stands about 12 inches
>tall and the other was a shallower form that was a bit wider. I put a
>slip texture on the outside of the first one and thougth that perhaps
>the cracking was from the application of the slip a resultant uneven
>drying between the outer wall and the inner wall. But the second one did
>not have any slip application and it also ended up with an outer wall crack.
> The clay body is one that I have used extensively and have not
>experienced any type of problem with. It is a real toothy mix from
>Laguna called WSO. The cracks appear only on the outer wall. Nothing on
>the bottom or the inside. One of the cracks goes partially into the
>lip.The bisque was what I call nice and slow. I start off with the kiln
>on low for an over night and then begin bringing it up the next day
>after it has reached red heat. Once the bisque has gotten to a nice
>red/orange glow I rock and roll to an 04.
> Does anyone on the list have any experience with successfully firing
>double walled vessels? Do I need to fire even slower as if I were firing
>a large sculptural form? Could the cracking be occuring on the way down,
>rather than on the way up? I let the kiln cool naturally and I have not
>arranged any type of slow cooling brick network around the perimeter of
>the piece as I would for a large platter form. Nor am I using Vince's
>infamous starburst array under the piece. Do I need to do this?
>Thank you for any and all help or suggestions
>Craig Dunn Clark

Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0

Craig Clark on mon 8 may 06

experience?

First off, thank you to everyone who responded with suggestions. I
shall be putting them to use in the next several days and will report
back to the board with hopefully more positive results.
Specifically to Ron, I will try and fire down one of the pieces
latter on this week. There will be an additional problem though if this
turns out to be the only way to get these pots to not crack. I am
planning on raku firing them. I know of at least one potter who does
this sucessfully, his name is James Watkins and he co-published a book
about "primitive" firing techniques. His forms a wonderful. Have not
been able to contact him yet.
I use the WSO body because I like the tooth, it's thermal shock
resistance, and the fact that it is a nice white body, which gives me
the bright white crackle I'm after without any difficulties. I just use
a clear crackle. No muss , no fuss. Nice brilliant white crackle every
time. It burns a really nice deep black without having to flame it too
much. This keeps the white and yellow crackle much "brighter" than they
would otherwise be on a darker body that takes more time to blacken. Can
you suggest another body, with less silica, that would give similar results?
Thank you again for any and all help
Craig Dunn Clark
619 East 11 1/2 St
Houston, Texas 77008
(713)861-2083
mudman@hal-pc.org


Ron Roy wrote:

>Hi Craig,
>
>If the crack is not open then it happened on the way down - outside clay
>cools faster than the inside clay - during the quartz inversion - tries to
>contract but - because the inside clay has not reached the right
>temperature to go through the quartz inversion - the part of the pot that
>does go through the contraction - cannot contract.
>
>If getting it off the shelf did not work you need to slow the cooling -
>make sure no spys are open - lid fits properly - and vent system is off.
>
>If that does not work then - if you have a controller - set it to turn the
>kiln back on about 700C (to cool at 50C per hour) and then off again at
>500C.
>
>Pyrometers are not that accurate so you may want to see what yours says the
>temp is with a low fire cone in a dish - say cone 022 which is supposed to
>go down around 600C and Hamer says that is dull red in an electric kiln -
>if you know you pyrometer is going to give good info to your controller
>then you only have to slow down between 600 and 550C. Just remember - kiln
>bottoms usually cool faster - maybe firing those pots at the top will work
>because they will cool slower there.
>
>You may also find that other bodies - which don't have as much free silica
>will not be a problem.
>
>So I managed to get through that whole thing without saying - the quartz
>inversion happens at 573C - if you could see the pot as it cracks the crack
>would be open - until the rest of the quartz in the pot cooled enough to go
>though the inversion.
>
>RR
>
>
>
>> Hello all, I have recently thrown several "double walled" vessels. I
>>have admired them for years and decided to give the form a whirl. Well,
>>the throwing part went well enough, but I have now cracked two of the
>>in separate bisque firings. They are about 16 inch diameter, double
>>walled bowl shapes with nice fleshy lips. One stands about 12 inches
>>tall and the other was a shallower form that was a bit wider. I put a
>>slip texture on the outside of the first one and thougth that perhaps
>>the cracking was from the application of the slip a resultant uneven
>>drying between the outer wall and the inner wall. But the second one did
>>not have any slip application and it also ended up with an outer wall crack.
>> The clay body is one that I have used extensively and have not
>>experienced any type of problem with. It is a real toothy mix from
>>Laguna called WSO. The cracks appear only on the outer wall. Nothing on
>>the bottom or the inside. One of the cracks goes partially into the
>>lip.The bisque was what I call nice and slow. I start off with the kiln
>>on low for an over night and then begin bringing it up the next day
>>after it has reached red heat. Once the bisque has gotten to a nice
>>red/orange glow I rock and roll to an 04.
>> Does anyone on the list have any experience with successfully firing
>>double walled vessels? Do I need to fire even slower as if I were firing
>>a large sculptural form? Could the cracking be occuring on the way down,
>>rather than on the way up? I let the kiln cool naturally and I have not
>>arranged any type of slow cooling brick network around the perimeter of
>>the piece as I would for a large platter form. Nor am I using Vince's
>>infamous starburst array under the piece. Do I need to do this?
>>Thank you for any and all help or suggestions
>>Craig Dunn Clark
>>
>>
>
>Ron Roy
>RR#4
>15084 Little Lake Road
>Brighton, Ontario
>Canada
>K0K 1H0
>
>______________________________________________________________________________
>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.
>
>
>

Taylor Hendrix on tue 9 may 06

experience?

Craig,

I've seen James Watkins throw his double walled cauldrons. I believe
he adds a little somin' somin' to his claybody but I don't have his
book so I could be mistaken. Anyway, I can't remember if he takes
care to clear off any throwing slip before he throws the inner wall to
meet the outer. He immedietally put an air hole in his forms fresh
from the wheel.

Fingers crossed, he's coming here later in the month to pit fire in my
back yard. Nee ner nee ner.

If you need his email, contact me OL.

Tay in Roc TX