Tony Ferguson on wed 27 aug 03
Cindi,
You are too worried about this.
I've taken 2 inch walled pieces made the day before and fast dried them in
the kiln. The trick is your clay body and monitoring the moisture content
as it is burning off--putting your kiln on low and closing your lid with
your peeps open should be safe and sufficient for completely drying your
kiln load in 2 to 6 hours time.
NOW, that said, it all depends on your clay body what you can get away with.
If it is a stoneware type with grog in it, your are pretty safe. I have
also dried porcelain sculpture this way so really anything should work--but
you don't know until you try.
6 months--yikes. Don't be worried about moisture during firing. Who told
you that? Perhaps I need to be educated, but I thought moisture escaped
during a firing. You can feel it with your hands, a mirror, or glasses.
When it is gone, proceed brining your kiln up. If you are really worried
about it, put it on low over night and start bringing it up the next day.
If you sculpture is structurally sound, you can stack it and fire it as it
will stand. I like to fire anything I can that is in pieces together for
potential deformation--and this has everything to do with the claybody and
how it was dried--if tension was set up on one side of a work because the
other dried faster, then you may have some problems--benefit of firing them
together as they keep each other in check. I don't believe you have
anything to worry about.
Trapped steam? Who told you that? Educate me. The only danger with steam
is firing too fast so the steam can't get out according to the porosity of
the clay body. If you want to single fire it (sorry, I don't have any cone
6 single fire glazes--ask Linda Blossom), you fire it as you would a bisque
and keep going. The thicker or tricky'ier the work, go slower till you hit
red heat (well, a bit pass red heat because the inner core of the work may
not be fully red yet). Once you've hit red heat, you're worries are pretty
much gone.
Send me a picture if you can take a dig image and I will make any further
recommendations. Otherwise, fire away.
Thank you.
Tony Ferguson
On Lake Superior, where the sky meets the Lake
Stoneware, Porcelain, Raku and more
by Coleman, Ferguson, Winchester...
www.aquariusartgallery.com
218-727-6339
315 N. Lake Ave
Apt 312
Duluth, MN 55806
----- Original Message -----
From: "Cindi Anderson"
To:
Sent: Wednesday, August 27, 2003 6:43 PM
Subject: Suggestions for firing large closes sculpture
> I made a sculpture 25" tall, about 1 foot diameter and 1" thick. It is
> hollow but closes on the top. It is sectioned into 3 pieces. I was going
> to fire it with the pieces stacked, but I realize that isn't a good idea
> because moisture could become trapped inside the piece. (It has been
drying
> 6 months but I am concerned about the moisture that is created during
> firing.) But I am afraid that if I take the top piece off and fire it
> separately, it will deform. Any ideas?
>
> My one thought is that maybe I could separate the top two sections with a
> series of clay wads, leaving space between the wads for steam to escape.
I
> am a little concerned they won't be able to take the weight, but I could
> squeeze the wads out the sides helping keep it in place (ah, maybe that
> won't work because the side wings would probably crack off as they shrink
> and dry...)
>
> Any brilliant ideas out there? I FINALLY got it through my skull not to
> make things that won't fit in my kiln, but I guess I have to think about
how
> I will fire it in advance. I should have made the cuts straight, instead
I
> made them wavy so they would "lock" together, not thinking about how it
> might deform in firing.
>
> I was going to fire this once up to Cone 6 (it has no glaze). I suppose I
> could bisque it in pieces and hopefully it wouldn't deform at the low
> temperature, and then I could put it together and fire it to Cone 6. Is
> there any danger of trapped steam during a glaze firing if it has already
> been bisqued?
>
> Thanks
> Cindi
> Fremont, CA
>
>
____________________________________________________________________________
__
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
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>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
>
Carolyn Bronowski on wed 27 aug 03
John Toki (he does large scale sculptures in the Bay Area--owns Leslies on
San Pablo, Berkeley) says to dry sooooo slowly, then to fire sooooo slowly.
He told us at a workshop at University of the Pacific that he often fires 4
to 5 days using a gas kiln. I have a large piece that I made at the
workshop using IMCO Sculpture 412 clay--I plan to fire it sometime next
spring. Carolyn
____________________________________________________________________________
__
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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Cindi Anderson on wed 27 aug 03
I made a sculpture 25" tall, about 1 foot diameter and 1" thick. It is
hollow but closes on the top. It is sectioned into 3 pieces. I was going
to fire it with the pieces stacked, but I realize that isn't a good idea
because moisture could become trapped inside the piece. (It has been drying
6 months but I am concerned about the moisture that is created during
firing.) But I am afraid that if I take the top piece off and fire it
separately, it will deform. Any ideas?
My one thought is that maybe I could separate the top two sections with a
series of clay wads, leaving space between the wads for steam to escape. I
am a little concerned they won't be able to take the weight, but I could
squeeze the wads out the sides helping keep it in place (ah, maybe that
won't work because the side wings would probably crack off as they shrink
and dry...)
Any brilliant ideas out there? I FINALLY got it through my skull not to
make things that won't fit in my kiln, but I guess I have to think about how
I will fire it in advance. I should have made the cuts straight, instead I
made them wavy so they would "lock" together, not thinking about how it
might deform in firing.
I was going to fire this once up to Cone 6 (it has no glaze). I suppose I
could bisque it in pieces and hopefully it wouldn't deform at the low
temperature, and then I could put it together and fire it to Cone 6. Is
there any danger of trapped steam during a glaze firing if it has already
been bisqued?
Thanks
Cindi
Fremont, CA
Tony Olsen on thu 28 aug 03
Cindi,
How about putting 2 half shelves butted together on the bottom of the =
kiln and putting the sculpture on them? The joint between the shelves =
would let any steam out.
Tony Olsen, Galveston TX
neslot@houston.rr.com
http://tonyolsen.com/up/
Malcolm Schosha on thu 28 aug 03
Cindi,
In most climates six months to dry is way more than necessary.
(However, I can remember one winter in Sesto Firentino, Italy when it
was so damp that vases I had thrown were still too wet to trim after
three weeks.) With sculpture with walls 1" thick the biger problem is
chemically combined water, wich does not burn off until about 800f.
Fire very slow up to that temp.
You might try making the walls a little thiner. The Greeks frequently
threw large knobs on lids without making any hole for steam to
escape. But they threw the knob very thin, and fired slow.
Good luck.
Malcolm
..............................................
--- In clayart@yahoogroups.com, Cindi Anderson wrote:
> I made a sculpture 25" tall, about 1 foot diameter and 1" thick.
It is
> hollow but closes on the top. It is sectioned into 3 pieces. I
was going
> to fire it with the pieces stacked, but I realize that isn't a good
idea
> because moisture could become trapped inside the piece. (It has
been drying
> 6 months but I am concerned about the moisture that is created
during
> firing.) But I am afraid that if I take the top piece off and fire
it
> separately, it will deform. Any ideas?
>
> My one thought is that maybe I could separate the top two sections
with a
> series of clay wads, leaving space between the wads for steam to
escape. I
> am a little concerned they won't be able to take the weight, but I
could
> squeeze the wads out the sides helping keep it in place (ah, maybe
that
> won't work because the side wings would probably crack off as they
shrink
> and dry...)
>
> Any brilliant ideas out there? I FINALLY got it through my skull
not to
> make things that won't fit in my kiln, but I guess I have to think
about how
> I will fire it in advance. I should have made the cuts straight,
instead I
> made them wavy so they would "lock" together, not thinking about
how it
> might deform in firing.
>
> I was going to fire this once up to Cone 6 (it has no glaze). I
suppose I
> could bisque it in pieces and hopefully it wouldn't deform at the
low
> temperature, and then I could put it together and fire it to Cone
6. Is
> there any danger of trapped steam during a glaze firing if it has
already
> been bisqued?
>
> Thanks
> Cindi
> Fremont, CA
>
>
______________________________________________________________________
________
> Send postings to clayart@l...
>
> 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@p...
Snail Scott on thu 28 aug 03
At 04:43 PM 8/27/03 -0700, you wrote:
>I made a sculpture 25" tall, about 1 foot diameter and 1" thick. It is
>hollow but closes on the top. It is sectioned into 3 pieces. I was going
>to fire it with the pieces stacked, but I realize that isn't a good idea
>because moisture could become trapped inside the piece.
But I am afraid that if I take the top piece off and fire it
>separately, it will deform...
Don't worry about trapping moisture in the cavity.=20
It won't happen. No way is the seal between two=20
dry pieces of clay tight enough to prevent the=20
escape of air (or water vapor). It's the water=20
trapped in the clay itself that causes explosions.
The piece was probably as dry as it was going to=20
get within a month after you unwrapped it for the=20
final time (or less, in dry climates). All the=20
time spent since then has just been spent sitting=20
at the ambient humidity level, which is never=20
zero anywhere. That last bit of moisture, and the=20
moisture which is chemically bonded to the clay=20
molecules, can only be driven off by firing. Go=20
slowly at first, holding the temperature at below=20
180=BAF for a few hours before starting the heat=20
rise.
Keeping the sections stacked will reduce firing=20
deformation of the upper sections, but may=20
increase it for the bottom, since the added weight=20
will increase the friction between the clay and=20
the kiln shelf and inhibit free shrinkage.
If fired separately, each section's bottom risks
deformation in the same way, but the risk will be=20
reduced, since the weight of each section is so=20
much less than that of the whole piece.
If you are using a decently groggy stoneware which=20
is not overly pyroplastic, I wouldn't worry too=20
much about deformation in either case. If it were=20
my own work, I'd probably fire it stacked, but with=20
sand or grog on the shelf to help it shrink and=20
move freely.=20
To aid with such issues in the future, there are=20
a few ways to build the work which can help.=20
Build with a taper in the wall thickness. Give it=20
that full 1" at the bottom if it needs it (though=20
that sounds like overkill for a 25" piece), but=20
make the walls thinner and thinner as you go up.=20
The upper sections aren't supporting that much=20
weight, and by making them thinner, you reduce=20
the stress on the lower sections.
I don't know how you build the bottoms of your=20
pieces, but many people simply start with the=20
bottom of the wall and go up. This creates a lot=20
of friction when the entire weight of the piece=20
is resting on the bottom edge. Cracks tend to=20
occur as the piece has difficulty overcoming the=20
friction with the shelf. (Friction is a function=20
of pressure: a small area of support means more=20
weight per square unit of area.)
Another practice which causes cracking is the=20
opposite approach - full slab bottoms. While=20
this option does create a greater bearing area,=20
it also heats and cools much slower than the rest
of the piece, and was also the last part to dry.
It's the drying that usually starts the cracking,=20
and leaves the slab with little strength to=20
withstand friction and also gives a starting=20
point for firing-stress cracks. Also, the middle=20
of the bottom isn't really helping bear the weight,=20
since clay has little tensile strength until=20
after firing. Middles of bottoms generally cause=20
more trouble than they solve.
So, the alternative: partial bottoms, extending=20
just an inch or two (depending on the size of=20
the piece) inside the wall of the object. This=20
gives added surface area to reduce friction, while=20
being close enough to the outside of the piece=20
to allow more even drying and firing. Use sand=20
anyway, though.
Another option is to make a firing slab - a flat=20
piece of clay big enough to hold the sculpture=20
off the shelf. The slab takes all the friction=20
stress, and shrinks at the same rate as the work.
Put the sand under the slab instead. It can follow=20
the piece through bisque and on to high-fire. It=20
is waste afterward, but a few pounds of clay is a=20
small price to pay for preserving a major piece=20
of work intact. (Maybe you could pave a path with=20
them, or paint on them...)
The partial-bottom method works best for pieces that=20
are approximately round, i.e. with a continuous=20
perimeter and a base large enough to be worth leaving=20
the middle out of.=20
The firing-slab method works best for pieces with=20
multiple points of support which are not contiguous=20
with one another. It keeps the relative spacing as=20
the clay shrinks. Without it, the top of the piece=20
shrinks in firing, and the bottom tries, but its=20
'feet' are 'nailed down', and keep their pre-
shrinkage spacing. This shelf friction will tend=20
to result in a splayed form, with cracks if they=20
are necessary to allow for it.=20
All of these factors are of greater concern with=20
large heavy work, and increase with size. Smaller=20
work may not need any of these methods to fire=20
successfully. Note that 'heavy' is an important=20
consideration, though. If your work doesn't really=20
need to be 1" thick all over, consider going=20
thinner. Added thickness can be counterproductive.
You may have done it to make it stronger, but the=20
weight creates added stress, too. Use the strength=20
down low where it's most needed. Up top, thickness=20
becomes more of a liability than an aid.
-Snail
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