Paul B on wed 3 aug 05
I recently had a lot of pots crack in strange places during an electric
bisque, perhaps the most bizarre were a number of covered casseroles that
cracked vertically from the rim downward, lids fired on them as usual.
Nearly everything was done exactly as it had been hundreds of times before
and i have never had casseroles crack like that. I did make a relatively
minor change to the clay body i use by adding more kaolin but i find it
hard to believe that was the cause.
What i think caused it was that we had about 2 weeks of extremely humid
weather, and even though the pots had plenty of time to dry and seemed to
be dry before going into the bisque (which was the usual 10-12 hours)it
seems they were not as dry as i thought, and having the lids on the entire
time probably did not help.
But my question, which would help me really figure this out, is could a
crack like that really be associated with moisture still in the pot? I
always thought that caused explosions rather than cracks. What do you think?
thanks,
Paul
dannon rhudy on wed 3 aug 05
> But my question, which would help me really figure this out, is could a
> crack like that really be associated with moisture still in the pot? I
> always thought that caused explosions rather than cracks. What do you
think?
> thanks,
> Paul
Moisture in the pots can cause all kinds of trouble and wierd
damage. The short answer to your question is "yes", the
moisture could be at fault. The cracks starting at the lip
and going down are a little suspicious, but I don't know
your clay. The additional small amount of kaolin shouldn't
affect the body substantially. If it was wet enough to trap
moisture inside the casseroles, and the lids were a tight
fit and/or stuck because of wetness - they could and would
crack at the weakest point. How thin are the lips? Hmm?
No matter how long your pots have been drying, it is well
to let them sit in the kiln for perhaps 3/4 of an hour, kiln
on low, lid propped open a bit. Humid climates are tricky
to deal with re: greenware. Unless you have substantially
altered a known clay body, or your technique has changed
a lot - then suspect the humidity. Least common denominator.
regards
Dannon Rhudy
marianne kuiper milks on thu 4 aug 05
Hello Paul,
I am the -10c (below..near nowhere) expert here, but
thought I'd tell you soemthing that happened this
summer.
I was at Alfred for a month, with about 70,80
ceramists of all levels, 39 kilns and a crate-full of
assistants and MFAs who ran the kilns.
The number of strange cracks was unbelievable and it
happened to almost everyone. one woman had a "T"
shaped crack in the middle of the outer rim. Etc. Some
pieces fell apart, but most had these weird cracks. I
believe all (certainly most) pieces were bisqued in
gas kilns, glazing (fine) in everything available.
At first people blamed the assts...not letting the
kiln candle long enough, roughness, etc. Then it was
the clay (3 kinds Laguna as well as home-made and
brought clay. Not the problem either. It was our
method of constructing (many professionals there)
no...humidity, no...firing too soon, no.. etc. The
humidity and heat were extremely high there (June
27-July 22), so your thought may not be so strange.
And we also had the lid-good,pot bad situations. I
wish mine had cracked more and theirs less, because
theiur work was so good, but mine was actually fine: 2
cracks out of 61 pieces.
This is just my -10C/10cents' worth. hope you find an
answer.
Marianne
Marianne Kuiper Milks, 22 revelstone Dr. , Honesdale,
PA 18431 travelartnmusic@yahoo.com
--- Paul B wrote:
> I recently had a lot of pots crack in strange places
> during an electric
> bisque, perhaps the most bizarre were a number of
> covered casseroles that
> cracked vertically from the rim downward, lids fired
> on them as usual.
> Nearly everything was done exactly as it had been
> hundreds of times before
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Snail Scott on thu 4 aug 05
At 10:28 PM 8/3/2005 -0400, you wrote:
>...a number of covered casseroles that
>cracked vertically from the rim downward, lids fired on them as
usual....could a
>crack like that really be associated with moisture still in the pot?
Generally moisture causes steam, which causes
outward breakage in a spall-type pattern. (I'm
coming to dislike the term 'explosion', as it
seems to give people a 'deus ex machina' vision
of what's happening, rather that keeping in mind
that it's the pressure of expanding steam within
the pores of the clay that's responsible.)
Moisture rarely causes straight-line cracks,
except when the stress of a steam explosion
(OK, I give up; I'll call it that) is carried
beyond the area of dampness.
If these were tightly sealed containers, I'd
not be surprised that internal pressure would
cause such a crack, but it seems very unlikely
to me that greenware casseroles, even with
well-fitted lids, could contain pressure that
effectively. Or perhaps you do make awesome
bad-ass lids? Maybe next time, put some wadding
around the rims, to create narrow air-gaps.
So, in summary, I don't know. Dampness dosn't
seem like the likely cause, but it's possible.
-Snail
Michael Wendt on thu 4 aug 05
Paul,
Since you fired electric, there is a possibility that a relay stuck on
longer than it should have and caused a faster heating cycle in part of the
kiln. This has happened to me more than once and required the replacement of
the relay.
I'd check to see if any are stuck in the closed position.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, Idaho 83501
USA
wendtpot@lewiston.com
www.wendtpottery.com
Jim Willett on fri 5 aug 05
Great answer, we do the similar in electric fire, use a lot of bisque
spacers, we make our own, roll out a slab and cut into 1 x 1 inch squares.
Fire once and then use as spacers. Between ware and shelves, ware and ware,
where ever. That and a long soak at or near 180 before ramping up, seems to
work wonders.
Jim
Out of the Fire Studio
Edmonton, Alberta, Canada ( we have no mad cows...no really....they are all
in Montana...trust me...)
http://www.outofthefirestudio.com
http://potblog.outofthefirestudio.com
..............
Bisquing plates is tricky, if you stack them tightly the heat doesn't
penetrate to the center of the stack. When stacking plates in the bisque,
try setting each one on three small shards, separating them and allowing the
kiln gasses to flow between. It evens out the stress.
Best,
Paul Herman
......................
Heather Pedersen on fri 5 aug 05
Hi Paul,
The only times I have gotten cracks like that myself was with rice bowls
that were a bit too dry when I trimmed them, and I believe the pressure
from sitting on their rims during the trimming caused the vertical cracks
in the rims. The clay I was using at the time (orion stout I think it
was? It's been a few years now) seemed very prone to that sort of thing.
Too short perhaps?
-= Heather Pedersen
On Wed, 3 Aug 2005 22:28:10 -0400, Paul B wrote:
>I recently had a lot of pots crack in strange places during an electric
>bisque, perhaps the most bizarre were a number of covered casseroles that
>cracked vertically from the rim downward, lids fired on them as usual.
>Nearly everything was done exactly as it had been hundreds of times before
>and i have never had casseroles crack like that. I did make a relatively
>minor change to the clay body i use by adding more kaolin but i find it
>hard to believe that was the cause.
>thanks,
>Paul
Carole Fox on fri 5 aug 05
Paul - you may want to check out the description in Hamer & Hamer of
readsorption, which occurs when a pot that has thoroughly dried takes up
water vapor from humid air. Readsorption causes fine vertical cracks from
the rim down. I used to have this problem occasionally until I installed
a dehumidifier in my studio, which is in a fairly damp basement. It
generally happened when pieces were not drying in the studio, so I took
them outside to dry, then returned them to the humid studio environment
before firing.
You may also want to read Hamer's general descriptions of cracks,
particularly their Figure A, which shows multiple examples of vertical
cracks going down from the rim along with explanations.
Carole Fox
Dayton, OH
Paul B on fri 5 aug 05
Thanks everyone for the replies.
I thought to add a little more info, in case it helps someone else who may
have this problem or whatever.
Another thing that was very strange about this one was that in one layer of
the kiln (which by the way is a manual electric kiln) there were small,
wide rim and thin bowls nested 3-4 inside each other, the way i always
bisque them. They almost never crack, aside from a rare s-crack on the
bottom. About 5 stacks of them, and in each case the top bowl only had a
crack all the way through the bottom. The ones below were all fine, and
they all had footed bottoms which, for me, seems to decrease the chance of
base cracks to almost zero. Was this the moisture from all the other pots
below each stack getting absorbed in the base of the top bowl or something?
These bowls are very thin and almost never crack.
The other thing that was strange is that i had several plates in the kiln,
in some cases there would be two nested together with no problems. But on
the bottom of the kiln there were three stacks of dinner size plates, and
then nested inside each of them were one smaller plate. In all three cases,
the smaller plate was fine but the dinner plates developed a 90 degree-type
crack all the way through.
Also, this is the clay body:
50 EPK
15 OLD HICKORY BALL CLAY
25 HAWTHORNE
10 G200
10 MULLITE GROG
It is not much different than what i normally use and i don't get many
cracks unless unusual drying conditions cause greenware cracks. In fact i
just made about 20 large platters with this body (a very soft mix) and
dried them very slow, and so far not a single greenware crack which is
actually really unusual.
Anyway, i just thought to post a few more specifics here just in case
anyone has any other thoughts. Sometimes things just don't work out, it
could have just been a freak thing. Actually, the last two weeks or so have
been absolutely filled with breakdowns of everything from cars to phone
lines and i have spent most of the time helping people fix things.
Yesterday, for example, a 95 degree day, the entire water system in the
house i live and work in shut down and it me and some friends a full TWENTY
FOUR HOURS to fix it. After doing everything we possible could think of
including replacing the water pump to no avail, i finally realized at 4:00
a.m. this morning that there could only be one thing left so instead of
going to sleep i drove to Lowes at 80 mph at around 6:00 a.m. to get a
bunch of pipes and stuff and finally fixed it by around 9:00 am, exactly 24
hours from the time it first broke down.
Now its time to make pots again.
thanks again and sorry for getting off topic with my little story here,
Paul
Paul Herman on fri 5 aug 05
Hi Paul,
Bisquing plates is tricky, if you stack them tightly the heat doesn't
penetrate to the center of the stack. When stacking plates in the
bisque, try setting each one on three small shards, separating them and
allowing the kiln gasses to flow between. It evens out the stress.
Best,
Paul Herman
Great Basin Pottery
Doyle, California US
http://www.greatbasinpottery.com/
----------
>From: Paul B
> In all three cases,
> the smaller plate was fine but the dinner plates developed a 90 degree-type
> crack all the way through.
Ron Roy on sat 6 aug 05
Hi Paul,
First thing is to determine if the cracks happened on the way up or on the
way down.
If the crackes are open - then probably on the way up - if closed then they
are bisque dunting.
RR
>I recently had a lot of pots crack in strange places during an electric
>bisque, perhaps the most bizarre were a number of covered casseroles that
>cracked vertically from the rim downward, lids fired on them as usual.
>Nearly everything was done exactly as it had been hundreds of times before
>and i have never had casseroles crack like that. I did make a relatively
>minor change to the clay body i use by adding more kaolin but i find it
>hard to believe that was the cause.
>What i think caused it was that we had about 2 weeks of extremely humid
>weather, and even though the pots had plenty of time to dry and seemed to
>be dry before going into the bisque (which was the usual 10-12 hours)it
>seems they were not as dry as i thought, and having the lids on the entire
>time probably did not help.
>But my question, which would help me really figure this out, is could a
>crack like that really be associated with moisture still in the pot? I
>always thought that caused explosions rather than cracks. What do you think?
>thanks,
>Paul
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
William & Susan Schran User on sat 6 aug 05
On 8/5/05 1:32 PM, "Heather Pedersen" wrote:
> The only times I have gotten cracks like that myself was with rice bowls
> that were a bit too dry when I trimmed them, and I believe the pressure
> from sitting on their rims during the trimming caused the vertical cracks
> in the rims.
Been thinking about this issue of vertical cracking. Have not experienced it
myself, but have seen it happen occasionally with my student's work. Never
could put my finger on the cause, but I don't think humidity would cause
this issue.
It is some type of stress and may happen more in some clays. It seems this
happens most often in forms that are bellied out, so it may be the clay
becoming thinner, in relation to the rest of the wall, in the widest area
and/or lack of compression in this same area.
--
William "Bill" Schran
Fredericksburg, Virginia
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