Dale Cochoy on tue 29 jan 02
Well, my question a few days ago about using antifreeze in slip w/or wo
paperclay seemed to spark some good answers. At least for me.
Lets try another.
I make lots of slab-built pots, some using 25-30 lbs of clay. VEERRYY time
consuming. Sometimes I have feet pop off during firing ( both bisque or high
fire) , but not often, but, very often I have small cracks at foot/wall
junctions where joined. These usually are not bad enough to destroy pot,
but makes them "unsellable". Many of the pots have feet as integral parts of
the walls, that is, the feet are cut into wall slabs so not attached. These
are happening when I high fire ( mostly) and pot is shrinking. Here's my
query. Do you think this is because of the weight of the pot on the
smallish joined feet junctions as the pot shinks/moves.?? I've thought
lately of bisque firing these heavy pots upside down to help with strain of
the feet during bique at least ( harder to do this in final fire of course).
Can I use something on shelves perhaps to help with this, especially during
final firing ( they are coated with kiln wash, but some have rougher spots
since old and repaired "sticks" a few times, but, overall are not bad). Any
EXPERIENCED thoughts on this.
Dale Cochoy
Snail Scott on tue 29 jan 02
At 06:34 PM 1/29/02 -0500, you wrote:
>...Do you think this is because of the weight of the pot on the
>smallish joined feet junctions as the pot shinks/moves.??
I wouldn't be surprised. A heavy object on small
supports can add up to tremendous PSI on those
tiny points of contact. Larger areas of contact
can be eased through shrinkage by putting sand
or grog on the shelf to act as 'ball bearings'.
This doesn't work well on small points, though.
I don't do many slab-built vessels, but I often
do figural/animal work where the full weight is
supported on just the toes, for instance. Although
I selsom get cracking at the support points, the
legs can 'splay' out as they remain in place and the
piece above them shrinks, sometimes causing cracks
at the 'armpits'. I solve this by firing on slabs
of the same claybody, which helps tie the separate
points of support together and distribute the load
(and I put the grog 'bearings' under it, instead).
My work seldom has a level top, but I did do a few
large slab bowls (20" diameter) with little weird
feet. Firing them on the rim worked sort of OK,
though they were prone to warping due to the
friction with the shelf. Lighter-weight forms might
fare better, though.
-Snail
Dave Murphy on tue 29 jan 02
----- Original Message -----
From: "Dale Cochoy"
To:
Sent: January 29, 2002 6:34 PM
Subject: Slab feet firing cracks
> Do you think this is because of the weight of the pot on the
> smallish joined feet junctions as the pot shinks/moves.?
>
> Hi Dale:
When I have a large or even wide form to fire I use placing sand which is
just sand box sand. You can either put it in your wax if you use latex; if
you use parafin you can dip the just waxed pot in a bit of sand put on the
table top; or you can just put a bit on the kiln shelf itself and the sand
will act as tiny ballberrings when the clay shrinks. I also use it in the
bisque if the pots are in the range that you talk about. Hope this helps.
Barbara Murphy
Waterloo County Pottery
Waterloo Ontario Canada
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>
Terrance Lazaroff on tue 29 jan 02
Dale;
This problem could be caused any where along the production of your large
slab built piece. It is due to the clay shrinkage. I suggest the
following;
When you complete your clay piece place it on a smooth surfaced bat that has
two layers of dry cleaning plastic film laid over the surface of the bat.
Failing this use two or three layers of newspaper. This will ease the
stress caused by the first stages of shrinkage, during the first drying
stages of your project.
At the same time you make your object roll our a slab that will serve as a
saucer when you bisque and when you fire. Place it under your project when
both items are leather hard or even greenware stage. This slab will follow
the object into the kiln. It will be what takes the stress of shrinkage
during the firing. You will probably see that this slab will crack in the
fining but that is to be expected.
Try this and see if it makes a difference.
Let me know your results.
Terrance
----- Original Message -----
From: "Dale Cochoy"
To:
Sent: Tuesday, January 29, 2002 6:34 PM
Subject: Slab feet firing cracks
> Well, my question a few days ago about using antifreeze in slip w/or wo
> paperclay seemed to spark some good answers. At least for me.
> Lets try another.
> I make lots of slab-built pots, some using 25-30 lbs of clay. VEERRYY time
> consuming. Sometimes I have feet pop off during firing ( both bisque or
high
> fire) , but not often, but, very often I have small cracks at foot/wall
> junctions where joined. These usually are not bad enough to destroy pot,
> but makes them "unsellable". Many of the pots have feet as integral parts
of
> the walls, that is, the feet are cut into wall slabs so not attached.
These
> are happening when I high fire ( mostly) and pot is shrinking. Here's my
> query. Do you think this is because of the weight of the pot on the
> smallish joined feet junctions as the pot shinks/moves.?? I've thought
> lately of bisque firing these heavy pots upside down to help with strain
of
> the feet during bique at least ( harder to do this in final fire of
course).
> Can I use something on shelves perhaps to help with this, especially
during
> final firing ( they are coated with kiln wash, but some have rougher spots
> since old and repaired "sticks" a few times, but, overall are not bad).
Any
> EXPERIENCED thoughts on this.
> Dale Cochoy
>
>
____________________________________________________________________________
__
> 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.
Cat Jarosz on tue 29 jan 02
HI Dale I am sure your gonna get a zillion same replys to this querry...
but I have been in your shoes and can say emphatically roll out one more
slab for that piece to sit on during drying stage, bisque stage and high
fire stage.. I think they call em donuts ?? I am not great on wordage
just grunt and point for the most part... gee I need to get out more ;o) it
has solved all foot problems completely... Cat Jarosz
Maid O'Mud on wed 30 jan 02
try sprinkling silica sand about 1/8" deep on the shelf. I fired ^10 ox for
years, and that little tip made a huge difference.
Sam - Maid O'Mud Pottery
Melbourne, Ontario CANADA
"First, the clay told me what to do.
Then, I told the clay what to do.
Now, we co-operate."
sam 1994
http://www.ody.ca/~scuttell/
----- Original Message -----
From: "Dale Cochoy"
To:
Sent: Tuesday, January 29, 2002 6:34 PM
Subject: Slab feet firing cracks
> Well, my question a few days ago about using antifreeze in slip w/or wo
> paperclay seemed to spark some good answers. At least for me.
> Lets try another.
> I make lots of slab-built pots, some using 25-30 lbs of clay. VEERRYY time
> consuming. Sometimes I have feet pop off during firing ( both bisque or
high
> fire) , but not often, but, very often I have small cracks at foot/wall
> junctions where joined. These usually are not bad enough to destroy pot,
> but makes them "unsellable". Many of the pots have feet as integral parts
of
> the walls, that is, the feet are cut into wall slabs so not attached.
These
> are happening when I high fire ( mostly) and pot is shrinking. Here's my
> query. Do you think this is because of the weight of the pot on the
> smallish joined feet junctions as the pot shinks/moves.?? I've thought
> lately of bisque firing these heavy pots upside down to help with strain
of
> the feet during bique at least ( harder to do this in final fire of
course).
> Can I use something on shelves perhaps to help with this, especially
during
> final firing ( they are coated with kiln wash, but some have rougher spots
> since old and repaired "sticks" a few times, but, overall are not bad).
Any
> EXPERIENCED thoughts on this.
> Dale Cochoy
>
>
____________________________________________________________________________
__
> 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.
>
Dale Cochoy on wed 30 jan 02
Thanks folks,
I got 5 great answers, all involving sand under feet and the slab under
feet. I'm going to try the slab idea..
I usually make my items on formica covered boards ( like old kitchen top
sink cut-outs) or (usually) on old fiberglass hospital food trays. Both are
"fairly" smooth. I "Flip" them often to dry evenly, and cover with trash
bags to dry sslllooowwww. I open them a while each day. More problems in
winter with dry heat than during summer.
Most often my feet cracks appear in final fire, but sometimes in drying
and bisque ( possibly ther, just not noticable), and sometimes I can get
them repaired with the paperclay/vinegar solution..
I'm definitely going to try the slab idea,
Thanks,
Dale
----- Original Message -----
From: "Snail Scott"
To:
Sent: Tuesday, January 29, 2002 8:19 PM
Subject: Re: Slab feet firing cracks
> At 06:34 PM 1/29/02 -0500, you wrote:
> >...Do you think this is because of the weight of the pot on the
> >smallish joined feet junctions as the pot shinks/moves.??
>
> I wouldn't be surprised. A heavy object on small
> supports can add up to tremendous PSI on those
> tiny points of contact. Larger areas of contact
> can be eased through shrinkage by putting sand
> or grog on the shelf to act as 'ball bearings'.
> This doesn't work well on small points, though.
>
> I don't do many slab-built vessels, but I often
> do figural/animal work where the full weight is
> supported on just the toes, for instance. Although
> I selsom get cracking at the support points, the
> legs can 'splay' out as they remain in place and the
> piece above them shrinks, sometimes causing cracks
> at the 'armpits'. I solve this by firing on slabs
> of the same claybody, which helps tie the separate
> points of support together and distribute the load
> (and I put the grog 'bearings' under it, instead).
>
> My work seldom has a level top, but I did do a few
> large slab bowls (20" diameter) with little weird
> feet. Firing them on the rim worked sort of OK,
> though they were prone to warping due to the
> friction with the shelf. Lighter-weight forms might
> fare better, though.
>
> -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.
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