Kate Johnson on fri 20 aug 04
I'm curious--I have 3 wonderful pieces done by two different potters that
just fascinate me. I don't see HOW they did them without trapping air
bubbles and then having problems with exploding in the kiln.
Two are made up of coils, obviously set into a mold from the inside--that
is, the sides and bottom are all made up of many sizes of spirals set side
by side until they make up a vessel. They're quite beautiful, and
functional for holding dry things, if not wet. The third is a large jug,
made much the same way, but with blobs and balls and ropes of clay, pressed
into a form, then with the top of the jug thrown and attached later (I'm
assuming). It's very organic and textural, wildly interesting.
I would be scared to DEATH of air pockets, however...it's quite thick and
rather heavy. I'd love to try this technique but...any ideas on how to
avoid the dread exploding pots problem? Is it really just a matter of
letting them get very, _very_ dry before firing?
Regards,
Kate Johnson
Graphics/Fine Arts
http://www.cathyjohnson.info/
http://www.epsi.net/graphic/
Candace Young/Norman Czuchra on fri 20 aug 04
Kate,
I know that every so often a pot gets thrown around an air bubble and fires
without exploding. I do check every single piece with a moisture meter
before loading the kiln if I'm not sure they are dry. I also do a very
slow temperature climb and no longer have explosions. However, I know
potters who put very damp pots in their kilns wihout problems so perhaps
you'll get some more information on how it works.
Candace
>avoid the dread exploding pots problem? Is it really just a matter of
>letting them get very, _very_ dry before firing?
>
>
>Regards,
>Kate Johnson
Candace Young Mailto:candace@bayriverpottery.com
Norm Czuchra Mailto:norm@bayriverpottery.com
(252) 745-4749
107 S. Water Street
PO Box 394
Bayboro, NC 28515
http://bayriverpottery.com
Earl Brunner on fri 20 aug 04
The idea that air bubbles cause pots to explode is overrated. It maybe that
occasionally the rapid expansion of air might cause a pot to explode.
However, if reasonable care is taken to avoid sudden temperature changes in
the early stages of the firing, there shouldn't be a problem. You can
actually fire hollow forms with very large air pockets and no openings
without blowing them up.
Earl Brunner
Las Vegas, NV
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Kate Johnson
Sent: Friday, August 20, 2004 7:49 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: air bubble query
I'm curious--I have 3 wonderful pieces done by two different potters that
just fascinate me. I don't see HOW they did them without trapping air
bubbles and then having problems with exploding in the kiln.
Bonnie Staffel on sat 21 aug 04
One of my favorite methods of making forms from clay is to use the press
mold. I have NEVER had to be concerned with air bubbles. Could be because
the wad or coil is firmly pressed against the wall of the mold. I have more
trouble making sure there are no openings between the applied clay pieces so
that if the pot is functional, it won't leak. When I was recovering from
CTS and able to handle clay, I made a few of the most interesting (to me)
teapots. I am mainly a classic potter and these press mold teapots took me
into another realm of creativity. I use the press mold to teach beginners,
as it frees up the student to make coils, balls of clay, slabs and texture
to get the "feel" of clay and besides to make an acceptable form. In the
beginning, I made these "molds" to just make pressed mold bowls. Then when
in Denmark, felt something was needed to give the bowl something more. So I
designed tops that would set each press molded pot apart from the prior one.
Put two press molded clay forms together, more objects of interest. I had
as much fun in designing the molds as I did then making the forms in them.
I do not believe in putting a new student on the wheel before he/she has had
handbuilding experience. Learning the texture and feel of the clay is so
important to get it to respond to wheel work. I have also worked with
students who had no prior experience in actually using their hands to make
something. To me, that is a big drawback. Most often I would hear that the
"ladies" didn't like the feel of clay under their long fingernails. That
was clue #1 of what was important to them. Those of us who were dedicated
to learning about clay at the art museum, often kidded about the Thursday
night ladies who came for their evening out. Out of those first classes
came a nucleus of potters who formed the Toledo Potters Guild. Harvey
Littleton helped them form the rules for the Guild having come from the Ann
Arbor Guild success. I am happy to say the TPG is going strong with some
new blood and with their own building. There are a few Clayarters who
belong to the guild.
Getting back to the press molds, with practice one can get rather light
weight pieces which can be fired in many different ways. I usually pit fire
mine or just wash with oxides and put a clear glaze over. One has to be
sure to press the little pieces together so they are one complete mass,
otherwise the parts may separate and cause problems you don't want. I
highly recommend this method as a way to get your hands dirty but come out
with a very unusual pot. One can also use different colored clays.
Warm regards,
Bonnie Staffel
http://webpages.charter.net/bstaffel
http://www.vasefinder.com/
Potters Council member
Kathy Forer on sun 22 aug 04
On Aug 22, 2004, at 1:19 PM, Earl Brunner wrote:
> At the art center (where we fire a LOT of thicker, beginner pots) I
> program
> the bisque to preheat at 190 degrees for 6-8 hours.
Would it be acceptable to do a low-low preheat for 6-8 hours, as you
describe, and then turn off the kiln for overnight and start over again
in the AM for another long 18-24 hour slow bisque, just not so slow in
the beginning? The point being to dry out the clay thoroughly, then get
some sleep before the 'real' firing.
At school, we used to fire the kilns for 2-3 days, and I'm not looking
forward to doing that, though the work may, at times, require it.
Kathy Forer
www.kforer.com
Ron Roy on sun 22 aug 04
Hi Kate,
Air bubbles are not the cause of clay blowing up during a firing - just
make sure all the water is out of the clay before you go over 100C.
I expect potters thought the air bubbles were the problem because - when
the clay is forced apart by super heated steam - it gives way at the
weakest point - so if there is a bubble - you can sometimes see it - and
blame it.
A good way to "see" if there is moisture left in the clay is to hold a
mirror - or shiny metal at the top spy hole (with the bottom spy open) if
there is moisture coming out as indicated by condensation on the cooler
surface of the mirror - then wait some more.
RR
>I'm curious--I have 3 wonderful pieces done by two different potters that
>just fascinate me. I don't see HOW they did them without trapping air
>bubbles and then having problems with exploding in the kiln.
>
>Two are made up of coils, obviously set into a mold from the inside--that
>is, the sides and bottom are all made up of many sizes of spirals set side
>by side until they make up a vessel. They're quite beautiful, and
>functional for holding dry things, if not wet. The third is a large jug,
>made much the same way, but with blobs and balls and ropes of clay, pressed
>into a form, then with the top of the jug thrown and attached later (I'm
>assuming). It's very organic and textural, wildly interesting.
>
>I would be scared to DEATH of air pockets, however...it's quite thick and
>rather heavy. I'd love to try this technique but...any ideas on how to
>avoid the dread exploding pots problem? Is it really just a matter of
>letting them get very, _very_ dry before firing?
>
>
>Regards,
>Kate Johnson
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Snail Scott on sun 22 aug 04
At 09:48 AM 8/20/2004 -0500, you wrote:
>I would be scared to DEATH of air pockets, however...
Air pockets don't blow up pots. It's moisture
(in the form of steam) that blows up pots.
Air pockets can create weak spots that the
expanding steam will exploit, but they are
not the cause of the explosion by themselves.
Fairly large air pockets, such as might be
created by an overlying attached slab, can
cause expanding air to be trapped. This air
can expand enough that the attached clay may
be popped off by the pressure inside the
pocket. This is NOT an explosion, though.
How a big a trapped pocket must be to cause
trouble depends on several factors. One is
the speed of firing. With a sufficiently
slow firing, the expanding air will find its
way out through the porosity of most clays
without causing harm. The porosity of the
clay matters, too, since air can escape from
a coarse clay far more readily than from a
'tight', fine-grained body. With the right
clay and a slow firing, closed air pockets
of substantial volume (I've seen liter/
quart sized spaces) may be fired with
reliable success. The thickness of the clay
walls is also a factor. Naturally, both air
and moisture will need extra time to escape
from very thick clay, but I have seen foot-
thick work fired without damage. It's usually
possible to achive the same appearance using
hollow methods, but truly solid clay can
sometimes have a presence and effect which
justifies the extra care required.
The thing that kills clay is MOISTURE. The
heat expansion of dry air, as discussed above,
is a slow, gradual thing. Steam expansion,
unfortunately, is many times greater and it
occurrs in a very short span of time. So,
to fire work which is likely to have air
bubbles, dryness and slow firing (through
the lower temperature ranges) are key. If
it's dry enough, and slow enough, you can
fire damn near anything successfully.
Air doesn't kill clay, water kills clay!
-Snail Scott
Kate Johnson on sun 22 aug 04
Thanks, Earl...I thought I remembered someone saying that on this list a
couple of years ago. I think the idea was the possibility of moisture
expansion trapped in the bubble...I've seen some pretty spectacular
explosions in the kiln at school, in the kids' work, not sure why.
> The idea that air bubbles cause pots to explode is overrated. It maybe
that
> occasionally the rapid expansion of air might cause a pot to explode.
> However, if reasonable care is taken to avoid sudden temperature changes
in
> the early stages of the firing, there shouldn't be a problem. You can
> actually fire hollow forms with very large air pockets and no openings
> without blowing them up.
Excellent, and thank you, all who answered my query. I know my buddy who
runs the kiln does set it to warm up quite slowly, so must be doing
something right--I haven't had a problem for a couple of years, when the
legs blew off 3 of my pinch-pot pipkins.
Regards,
Kate Johnson
Graphics/Fine Arts
http://www.cathyjohnson.info/
http://www.epsi.net/graphic/
Earl Brunner on sun 22 aug 04
At the art center (where we fire a LOT of thicker, beginner pots) I program
the bisque to preheat at 190 degrees for 6-8 hours. I have only lost maybe
three pieces in the last two years. And yes they were very thick. If you
can get the moisture and/or air out slow enough, they won't blow. The
spectacular explosions you refer to are almost certainly steam related. Very
powerful stuff.
Earl Brunner
Las Vegas, NV
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Kate Johnson
Sent: Sunday, August 22, 2004 6:32 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: air bubble query
Thanks, Earl...I thought I remembered someone saying that on this list a
couple of years ago. I think the idea was the possibility of moisture
expansion trapped in the bubble...I've seen some pretty spectacular
explosions in the kiln at school, in the kids' work, not sure why.
Earl Brunner on mon 23 aug 04
Sure, that would be better than NOT presoaking. The Skutt controller gives
a readout at the end of the firing for how long it has fired, and WITH the
6-8 hour preheat (and you wouldn't need that long with your own work
probably if it was dry and not too thick) I think the Program says that that
the total firing time is still less than 20 hours, I think in the
neighborhood of 16 to 18.
Earl Brunner
Las Vegas, NV
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Kathy Forer
Sent: Sunday, August 22, 2004 6:30 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: air bubble query
On Aug 22, 2004, at 1:19 PM, Earl Brunner wrote:
> At the art center (where we fire a LOT of thicker, beginner pots) I
> program
> the bisque to preheat at 190 degrees for 6-8 hours.
Would it be acceptable to do a low-low preheat for 6-8 hours, as you
describe, and then turn off the kiln for overnight and start over again
in the AM for another long 18-24 hour slow bisque, just not so slow in
the beginning? The point being to dry out the clay thoroughly, then get
some sleep before the 'real' firing.
At school, we used to fire the kilns for 2-3 days, and I'm not looking
forward to doing that, though the work may, at times, require it.
Kathy Forer
www.kforer.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
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Snail Scott on mon 23 aug 04
At 09:30 PM 8/22/2004 -0400, you wrote:
>Would it be acceptable to do a low-low preheat for 6-8 hours, as you
>describe, and then turn off the kiln for overnight and start over again
>in the AM for another long 18-24 hour slow bisque...
Sure. It won't absorb much moisture (if any)
before the 'real' firing the next day. (By
the way, unless this is some seriously thick
work, a 24-hour bisque is probably overkill.)
-Snail Scott
Kathy Forer on mon 23 aug 04
On Aug 23, 2004, at 4:16 PM, Snail Scott wrote:
> (By
> the way, unless this is some seriously thick
> work, a 24-hour bisque is probably overkill.)
I'm sure I may come to be more experimental and thrifty with my own
kiln, but all my sculptures so far have been fired on a schedule very
similar to what John Jensen described: On May 12, 2004, at 7:47 PM,
John Jensen wrote:
> First section of the firing: Slowly (50 degrees/hr)go to around 200
> degrees F and hold that for ten hours, then go up to 220 degrees and
> hold that for 8 hours...the work should be dry by this point. Then go
> up at 25 degrees per hour till you reach 500 F. Next go up by 40
> degree
> per hour till you reach 900 50 degrees till you get to 1200, then 75
> per hour till you get to 1940 (cone 04) up from there you are on your
> own.
> Just a suggestion.
>
> The whole thing takes about two and a half days. Check out the
> sculpture on my web page to see some of the things I didn't blow up.
>
> John Jensen, Mudbug Pottery
> mudbug@toadhouse.com , http://www.toadhouse.com
As clayart has impressed on me the importance of technique, and having
a show has taught the drawbacks to moving and displaying heavy pieces,
I'm a little less cavalier about making and firing 4" thick pieces, but
they may not be evenly 1.000", or 72 pixels thick, so I'm asking a kiln
for room for comfort. Maybe a day and a half.
I'm excited about the prospect of firing. A visitor to my studio, with
nary a pot in sight, noticed the kiln and asked "are you a potter?" I'm
getting there.....
Kathy
kforer.com
Ivor and Olive Lewis on tue 24 aug 04
Dear Friends,
Snail Scott tells you "Air doesn't kill clay, water kills clay!"
Please believe her.
Mel tells us to heed the Physics and the Chemistry of our Art and
Craft.
I can explain why.
Water does not need space other than its own volume to cause trouble.
There is no need for an air pocket. Just something like two
millilitres, or two grams, of water distributed through solid clay
will instantly change to about 2 litres of steam at its temperature
rises from 99=BA C to 101=BA C. I have had this happen when I have left
excess thickness in the base of a pot. In some instances the clay has
been totally shattered, fragmented into ceramic shrapnel. An unseen
patch of leather hard clay an inch in diameter and a quarter of an
inch thick in the base of a pot will cause this to happen.
Air only expands by 1/273 of its volume as its temperature increases
by one degree Celsius. Dry porous clay can cope with that happening in
cavities which are small.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
----- Original Message -----
From: "Snail Scott"
To:
Sent: Monday, 23 August 2004 1:09
Subject: Re: air bubble query
> At 09:48 AM 8/20/2004 -0500, you wrote:
> >I would be scared to DEATH of air pockets, however...
>
>
> Air pockets don't blow up pots. It's moisture
> (in the form of steam) that blows up pots.
> Air pockets can create weak spots that the
> expanding steam will exploit, but they are
> not the cause of the explosion by themselves.
>
> Fairly large air pockets, such as might be
> created by an overlying attached slab, can
> cause expanding air to be trapped. This air
> can expand enough that the attached clay may
> be popped off by the pressure inside the
> pocket. This is NOT an explosion, though.
>
> How a big a trapped pocket must be to cause
> trouble depends on several factors. One is
> the speed of firing. With a sufficiently
> slow firing, the expanding air will find its
> way out through the porosity of most clays
> without causing harm. The porosity of the
> clay matters, too, since air can escape from
> a coarse clay far more readily than from a
> 'tight', fine-grained body. With the right
> clay and a slow firing, closed air pockets
> of substantial volume (I've seen liter/
> quart sized spaces) may be fired with
> reliable success. The thickness of the clay
> walls is also a factor. Naturally, both air
> and moisture will need extra time to escape
> from very thick clay, but I have seen foot-
> thick work fired without damage. It's usually
> possible to achive the same appearance using
> hollow methods, but truly solid clay can
> sometimes have a presence and effect which
> justifies the extra care required.
>
> The thing that kills clay is MOISTURE. The
> heat expansion of dry air, as discussed above,
> is a slow, gradual thing. Steam expansion,
> unfortunately, is many times greater and it
> occurrs in a very short span of time. So,
> to fire work which is likely to have air
> bubbles, dryness and slow firing (through
> the lower temperature ranges) are key. If
> it's dry enough, and slow enough, you can
> fire damn near anything successfully.
>
> Air doesn't kill clay, water kills clay!
>
> -Snail Scott
>
>
______________________________________________________________________
________
> 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.
Snail Scott on tue 24 aug 04
>On Aug 23, 2004, at 4:16 PM, Snail Scott wrote:
...unless this is some seriously thick
>> work, a 24-hour bisque is probably overkill.)
At 10:22 PM 8/23/2004 -0400, Kathy F. wrote:
>they may not be evenly 1.000", or 72 pixels thick, so I'm asking a kiln
>for room for comfort. Maybe a day and a half.
At 1" or more, I'd call that 'thick', and a
24 hour bisque is not unreasonable, especially
when so much of that time is allocated to the
lowest temperature range.
-Snail Scott
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