Elizabeth Priddy on thu 13 nov 08
One more tip on this.
=A0
I use the ear canal as a convenient place to put holes of about 1/4 inch al=
l the way through to the interior of the head.=A0 I also bore out the nostr=
ils.
=A0
Talk about steam coming out of your ears, but I find that there is no reaso=
n to fire it to bisque upright.=A0 And the flow of steam and lack of interi=
or pressure solve a lot of bust problems.=A0 Freaks the students out, as th=
ey think it is going to 'slump' out of shape, but it will not from my exper=
ience with stacking the bisque kiln in this manner.=A0=20
=A0
just make sure they don't make it taller than it would fit in your kiln lay=
ing down.
=A0
I also make a characteristic notch under the foot of platters over 12 inche=
s across for the same reason.
=A0
Elizabeth Priddy
Beaufort, NC - USA
Workshops and pottery online at: http://www.elizabethpriddy.com
Personal Blog: http://open.salon.com/user_blog.php?uid=3D943
Natural Instincts Conference Information:
http://downtothepottershouse.com/NaturalInstincts.html
Kiln pictures and such: http://www.flickr.com/photos/7973282@N03/
=0A=0A=0A
David Todd on thu 13 nov 08
I am just picking up the peices of a hollowed out bust form I tried to fire
for someone else.
The head is intact with part of the neck but the shoulders forming the base
are in peices.
Any suggestions as to what I did wrong and any way to make use of what I
have left?
I have been drying the form for 10 weeks in my kiln room which gets quite
warm every time I fire.
Examining the shoulders I see some places it is 3/4 inch thick and other
places 2 1/2 inches thick. There are also some small pockets along the
fracture lines.
I have a skutt km-1027-3. I candled it for 4 hours and then put it on slow
fire.
Any suggestions what to do with the head?
Thanks
David Todd
John Rodgers on thu 13 nov 08
Ouch!
David, that has to be a tough loss.
I once fired a big platter that had been drying in a hot room at summer
time temperatures in the deep south for 5 months. . The platter exploded
into a gazillion pieces ( I know - exaggeration) in the kiln when I went
to bisque fired it, even those I took all the usual precautions - drying
in the kiln overnight at 204F - kiln lid cracked, etc, etc. There were
thick and thin places in that platter, similar to what you described,
and I was really puzzled by the reaction.
Lessons:
1) Greenware clay will retain moisture at whatever the atmospheric
humidity is.
2) Entrapped moisture at atmospheric levels cannot necessarily be driven
from the piece except over a very long period of time - several hours,
and perhaps even days, near the boiling point.
3) Wall thicknesses over 1/2 inch lose water very slowly, and sometimes
moisture becomes sealed inside., unable to escape through the pores of
the clay, even when heated. This converts to steam a steam explosion,
destroying the piece.
4) In very thick clay, the organic components and some chemical
components - even when dry, will gasify and the expanding gases will
explode the piece if the gasses cannot find a way to the surface to
relieve the pressure.
Summary: Make walls relatively thin, dry thoroughly first with air then
with heat, then fire very very slowly for large thick pieces.
These are things I learned from the experience.
Regards,
John Rodgers
Chelsea, AL
David Todd wrote:
> I am just picking up the peices of a hollowed out bust form I tried to fire
> for someone else.
> The head is intact with part of the neck but the shoulders forming the base
> are in peices.
> Any suggestions as to what I did wrong and any way to make use of what I
> have left?
> I have been drying the form for 10 weeks in my kiln room which gets quite
> warm every time I fire.
> Examining the shoulders I see some places it is 3/4 inch thick and other
> places 2 1/2 inches thick. There are also some small pockets along the
> fracture lines.
> I have a skutt km-1027-3. I candled it for 4 hours and then put it on slow
> fire.
> Any suggestions what to do with the head?
> Thanks
> David Todd
>
>
>
Dannon Rhudy on thu 13 nov 08
David said:
> I am just picking up the peices of a hollowed out bust form I tried to
fire
> for someone else.
> The head is intact with part of the neck but the shoulders forming the
base
> are in peices.
> Any suggestions as to what I did wrong and any way to make use of what I
> have left?.......
>
Well, finish the head any way you like it,
and mount it on a piece of metal, to lift it up.
I've used all kinds of metals, from stainless to
re-bar, depending on the piece and the effect
I was looking for. If the break is uneven it
will look all the better.
The bust was too un-even in thickness to
fire easily. You'd have to fire a lot slower
for something of such thick/thin to make it
through without cracking along the stress
points where thick changed to thin.
regards
Dannon Rhudy
Weiland, Jeff on thu 13 nov 08
David,
The firing timeline is only part of the issue. No matter how long you
let the work sit, there will still be atmospheric moisture. Slow warming
to evaporate the water is important. More important than that is the
clay body. It needs to be made for sculpting. It should be heavy on the
fireclay and wedge or mix in 5 to 10% medium grog. That will make the
body short and crumbly but should eliminate the cracking problems and
reduce the shrinkage. Another suggestion would be to use a needle tool
and poke holes in the thicker areas to help venting. Be careful not to
create air pockets that where the moisture can collect. Lastly, the more
uniform the thickness, the less problems that occur.
Jeff Weiland
Greenfield-Central High School
810 North Broadway
Greenfield, Indiana 46140
317-462-9211
jweiland@gcsc.k12.in.us
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of David Todd
Sent: Thursday, November 13, 2008 12:26 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Firing a bust Form
I am just picking up the peices of a hollowed out bust form I tried to
fire
for someone else.
The head is intact with part of the neck but the shoulders forming the
base
are in peices.
Any suggestions as to what I did wrong and any way to make use of what I
have left?
I have been drying the form for 10 weeks in my kiln room which gets
quite
warm every time I fire.
Examining the shoulders I see some places it is 3/4 inch thick and other
places 2 1/2 inches thick. There are also some small pockets along the
fracture lines.
I have a skutt km-1027-3. I candled it for 4 hours and then put it on
slow
fire.
Any suggestions what to do with the head?
Thanks
David Todd
William & Susan Schran User on thu 13 nov 08
On 11/13/08 12:26 PM, "David Todd" wrote:
> I have been drying the form for 10 weeks in my kiln room which gets quite
> warm every time I fire.
> Examining the shoulders I see some places it is 3/4 inch thick and other
> places 2 1/2 inches thick. There are also some small pockets along the
> fracture lines.
We have just finished a portrait bust in the sculpture class I teach.
I have everyone hollow out the form such that no areas are more than about
1/2" thick and try to maintain as even a thickness throughout the form. I
also have the students pierce all over the interior with a needle to open
any air pockets that may have been formed when they added layers of clay
during construction.
Air pockets might cause small cracks/separations but do not cause the
explosive shattering - that is due to moisture content. Yes, the bust was
not completely dry! More than likely the areas of explosion were the thicker
regions that still retained some moisture.
Bill
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
Snail Scott on thu 13 nov 08
On Nov 13, 2008, at 11:26 AM, David Todd wrote:
> I am just picking up the peices of a hollowed out bust form I tried to
> fire
> for someone else...I see some places it is 3/4 inch thick and other
> places 2 1/2 inches thick. There are also some small pockets along the
> fracture lines... I candled it for 4 hours and then put it on slow
> fire...
A four-hour candle is what I use for work that's less than
an inch thick. If you know that it's thicker than that, you need
to vastly increase the candle time, AND the time spent in
the below-400F temperature range. (Of course, you may
not have known.
Imagine that your clay has little water molecules living in
the interstices between the particles. (It does.) No matter
how long you let it sit out to dry, some of them will remain
because the air isn't completely dry either. It has humidity.
So, we candle to make the clay drier than the air can. When
we candle, we dry out the air in the kiln, and the water in
the clay will try to equalize, so it leaves the clay to fill the dry
air, where it evaporates in turn, until all the water has left
the clay and has evaporated. If your clay is 1/4" thick, which
is reasonable for thrown functional pottery or slip-cast work,
then the furthest any water molecule can be from freedom
is 1/8" - not too long a trip. If the clay is 1/2" thick - typical
for some sculpture - then the innermost water must travel
twice as far to escape, so you'd better figure on candling
twice as long. If the clay is an inch thick, then the water in
the innermost areas must travel half an inch - four times
further. Now, for this bust, we're talking two inches thick or
more - that's eight times as far for that little water molecule
to hike! And eight times longer to candle than you'd need
for typical thrown or cast work (the assumption that's used
when the kiln manufacturer makes that handy automatic
'slow-firing' computerized schedule, or when your thin-
pottery-throwing friends make helpful firing suggestions.)
So, if you would normally candle for two hours, you'd need
to candle for (let's do the math)...16 hours! And that doesn't
include the extra time you'd need to allow when you start
temperature rise past 200 degrees F. The whole thickness
won't heat up instantly. Clay is a lousy conductor of heat,
so you would need to take it super-slow, so that the inner
layers can heat up at least a little before the outer layers
get too far ahead of them. So, you'd still need to take it super-
crazy-slow; hours and hours just raising the temperature
past 250 F. And so on. It's not impossible to fire very thick
clay, but it takes time. A lot of it.
Of course, that two-hour candling time probably has a
safety margin built in, and an eight-hour candling time
might be fine for even this thick piece, but you get the
idea. THICKNESS MATTERS! The artist of the bust should
have been more responsible in hollowing it out properly,
but often, people trying this for the first time have no prior
experience with firing clay and don't realize how serious
the thickness issue can be. You, as the firer of the work,
may not have known is was over-thick, and the artist not
being a ceramist (I assume) may not have known it
mattered - recipe for disaster!
Sorry about your friend's sculpture. It would have been
a real commitment to fire it as slowly as it would have
required, and unless you knew how thick it was, you
can't really be responsible for the outcome. Next time,
you'll both know. They've got to make it thinner, or
you'll need to bake it slower. There's no reason that a
portrait bust can't be as thin as a (beefy) thrown pot, but
it will take them a bit of effort. If they aren't sure of the
thickness, they can jam a needle tool into it. When it comes
out the other side, they can better judge the thickness, and
go after the inside with a loop tool. If there are parts that
can't be reached, or are tricky (hair, or around eye sockets)
they can stick the needle in and turn it into swiss cheese,
perforated thoroughly on a 1/4" spacing. The holes must
go all the way through to the inner cavity, and if the needle
tool is too short, use a sharpened bit of welding rod or a
turkey-lacing needle. Then, smooth over the perforations
on the outside and restore the outside appearance. The
holes will remain to vent the moisture to the inside, and
then out through the bottom of the neck. Why does this
work? Because now, that little hapless water molecule
doesn't have to travel an inch or more to escape. It only
has to make it to the nearest perforation, 1/8" or so away,
and follow that channel out to freedom. And a more normal
(though still slow) firing cycle.
Better luck next time, to both of you.
-Snail
(...currently teaching a portrait-sculpture class to people
who've never used clay before. Next session will in fact
be 'hollowing it out and fixing it up again' day.)
Vince Pitelka on thu 13 nov 08
David Todd wrote:
"I am just picking up the peices of a hollowed out bust form I tried to fire
for someone else.
The head is intact with part of the neck but the shoulders forming the base
are in peices.
Any suggestions as to what I did wrong and any way to make use of what I
have left?
I have been drying the form for 10 weeks in my kiln room which gets quite
warm every time I fire.
Examining the shoulders I see some places it is 3/4 inch thick and other
places 2 1/2 inches thick. There are also some small pockets along the
fracture lines. I have a skutt km-1027-3. I candled it for 4 hours and then
put it on slow
fire."
David -
You should candle longer with work like that - at least eight to twelve
hours, and always with the lid raised at least three or four inches. We
generally candle thick things for twelve hours with a soft brick beneath the
lid, raising the higher edge of the lid 4.5".
In this case, I think that the problem was mostly due to the small pockets
of air along the fracture lines. I have become something of an expert on
this, because no matter how much I caution my intro students about avoiding
trapped air spaces, some of them get exuberant and add clay so fast that
they inevitably trap some air. All of my intro students do a life-size
coil-built head-and-shoulders bust, and although they are suppose to control
the shape as they are coil-building, some of them inevitably end up adding
clay to the surface after the form is built. We just bisque-fired them last
week. One of them ended up in large pieces on the kiln shelf. Several
others blue off significant chunks. In every case, there were obvious
trapped air spaces along the breaks.
We fire L&Ls, and I have been meaning to research the "slow-bisque" firing
ramp. I think that in our case the breakage is occurring during the
water-smoking period, when the chemical water is being released from the
ware - above 900 degrees Fahrenheit. Steam gathers in closed internal
spaces, and can create enough pressure to blow the clay apart. I am
wondering if the slow-bisque ramp really accommodates this adequately.
If you are closing the lid before you fire, then in your case the breakage
likely occurred very early in the candling period. No matter how dry the
piece seems, there is still atmospheric humidity in the clay, and even
taking the piece up to 200 degrees quickly could create enough steam to
cause the trapped air spaces to pop. Leaving the lid propped up keeps air
circulating around the wares, and they do not heat up nearly as quickly.
Every once in a while someone accidentally bisque-fires thick forms and
closes the lid at the start of the candling period, and things frequently
blow up. So we really try to avoid ever doing that.
Hope this helps.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net; wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka
Vince Pitelka on thu 13 nov 08
Bill Schran wrote:
"Air pockets might cause small cracks/separations but do not cause the
explosive shattering - that is due to moisture content."
Bill -
You are a skilled and experienced artist and teacher, but this needs
clarification. It is the steam expansion that causes explosive shattering,
but the steam gathers in air pockets, and that is where the first breaks
often occur, and they can be big ones. In a form that does not have any
other problems, and little discrepancy in wall thickness, large sections can
pop off, revealing obvious air pockets. I have a slide of a Japanese Jomon
clay mask from around 4000 BC, and one of the raised eyes is missing. You
can clearly see score marks and a large air pocket in the space where the
eye should be. Potters have been making the same mistakes for at least six
millennium.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net; wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka
Elizabeth Priddy on thu 13 nov 08
another tip is to fire it on it's side so that there is no captured air ins=
ide it.=A0 If they made it flush to the ground and ou fired it sitting upri=
ght, there coud have been steam trapped inside it as it heated up.
=A0
If you lay it on it's side instead, or at least set it up on a few kiln pos=
ts to raise it off the floor, it stands a better chance of getting through =
the candle.
Elizabeth Priddy
Beaufort, NC - USA
Workshops and pottery online at: http://www.elizabethpriddy.com
Personal Blog: http://open.salon.com/user_blog.php?uid=3D943
Natural Instincts Conference Information:
http://downtothepottershouse.com/NaturalInstincts.html
Kiln pictures and such: http://www.flickr.com/photos/7973282@N03/
--- On Thu, 11/13/08, Snail Scott wrote:
From: Snail Scott
Subject: Re: Firing a bust Form
To: CLAYART@LSV.CERAMICS.ORG
Date: Thursday, November 13, 2008, 6:05 PM
On Nov 13, 2008, at 11:26 AM, David Todd wrote:
> I am just picking up the peices of a hollowed out bust form I tried to
> fire
> for someone else...=0A=0A=0A
Snail Scott on fri 14 nov 08
On Nov 13, 2008, at 6:50 PM, Elizabeth Priddy wrote:
> another tip is to fire it on it's side so that there is no captured=20
> air inside it.=A0 If they made it flush to the ground and ou fired it=20=
> sitting upright, there coud have been steam trapped inside it as it=20
> heated up.
I don't think even the most flat-bottomed and heavy
work can form an airtight seal with a kiln shelf.
It doesn't take much to vent an enclosed space.
A single needle-tool hole is sufficient for a volume
of two cubic feet, maybe more, if fired at a medium-
slow rate of heat rise. The slight gap between clay
and shelf will be greater in cross-section than one
little hole. Fairly porous clay can even get away with
no hole at all, if the inner cavity is small enough, and
not sealed over with dense slip or fired too fast. Steam
can only do damage if pressure builds up, and if
pressure gets too high, it will open up an escape route
at the weakest point. In this case, it would be the 'joint'
between the work and the shelf. The steam may lift the
work slightly up on whatever side of the piece is lightest,
but only be enough to release the pressure - hardly
enough to jostle it, really. If the piece were entirely
closed, the pressure (if not allowed to escape) would
finally split the piece at the weakest point - a bad
joint, or a thin spot, or a hidden air pocket.
I know that I've theorized in the past that air pockets
may serve as 'collection points' for steam, and cause
breakage that wouldn't occur otherwise, but I don't
believe it's as frequent as I once thought. I now suspect
that although that might be a factor, the simple fact that
air pockets are weak points in the structure is sufficient
to explain why cracks so often intersect air pockets.
In addition, there are plenty of air pockets that we never
know about because the work didn't break at that point,
so we never saw them at all. Air pockets alone do not
cause breakage. I believe it because I have often made
multi-part work which required 'truing up' the contact
surfaces, and in some cases revising the form by removing
inches of its height or length before refitting the joint.
I work with a fairly loose-and-fast coil-building method
and I build thick for outdoor projects, and when cutting
through, I have often encountered air pockets of
considerable size (1/4" and more) in work with no
firing cracks at all. I attribute the survival of this work
to the coarse, porous clay which allows air and steam
to easily escape, but it proves that air pockets, in and of
themselves, are not the cause or breakage.
Firing too fast for the porosity of the green clay to release
the moisture (either as vapor or steam) is the fundamental
problem. If this is dealt with (by firing slower, or using
a coarser clay, or adequate venting), there will be no \
breakage, air pockets or no.
-Snail=
Elizabeth Priddy on fri 14 nov 08
The other advantage of firing it on its side is that the heat can enter fro=
m the opening and heat it more evenly.=A0 It is not just about air pockets.
Elizabeth Priddy
Beaufort, NC - USA
Workshops and pottery online at: http://www.elizabethpriddy.com
Personal Blog: http://open.salon.com/user_blog.php?uid=3D943
Natural Instincts Conference Information:
http://downtothepottershouse.com/NaturalInstincts.html
Kiln pictures and such: http://www.flickr.com/photos/7973282@N03/
--- On Fri, 11/14/08, Snail Scott wrote:
From: Snail Scott
Subject: Re: Firing a bust Form
To: CLAYART@LSV.CERAMICS.ORG
Date: Friday, November 14, 2008, 10:00 AM
On Nov 13, 2008, at 6:50 PM, Elizabeth Priddy wrote:
> another tip is to fire it on it's side so that there is no captured
air inside it.=A0 If they made it flush to the ground and ou fired it sitti=
ng
upright, there coud have been steam trapped inside it as it heated up.
I don't think even the most flat-bottomed and heavy
work can form an airtight seal with a kiln shelf.
It doesn't take much to vent an enclosed space.
=0A=0A=0A
john dellow on fri 14 nov 08
Vince Pitelka wrote:
> David Todd wrote:
> "I am just picking up the peices of a hollowed out bust form I tried to=
fire
> for someone else.
> The head is intact with part of the neck but the shoulders forming the =
base
> are in peices.
> Any suggestions as to what I did wrong and any way to make use of what =
I
> have left?
> I have been drying the form for 10 weeks in my kiln room which gets qui=
te
> warm every time I fire.
> Examining the shoulders I see some places it is 3/4 inch thick and othe=
r
> places 2 1/2 inches thick. There are also some small pockets along the
> fracture lines. I have a skutt km-1027-3. I candled it for 4 hours and =
then
> put it on slow
> fire."
>
> David -
> You should candle longer with work like that - at least eight to twelve
> hours, and always with the lid raised at least three or four inches. W=
e
> generally candle thick things for twelve hours with a soft brick beneat=
h the
> lid, raising the higher edge of the lid 4.5".
>
>
>
> I am
> wondering if the slow-bisque ramp really accommodates this adequately.
> =20
> I think the nature of an electric kiln makes a slow bisque of larger pi=
eces difficult because when the elements ramp on there is a rapird incres=
e of temp on pieces near the elements.
> If this happens near the water soak temp then this quick increse may be=
a problem.
--=20
John Dellow
The potter formelly known as=20
=91The Flower Pot Man=92
www.welcome.to/jkdellow
http://www.flickr.com/photos/sagers/
Ron Roy on fri 21 nov 08
Hi David,
I have fired two kiln loads of busts (28 in each load) for a teacher - life
sized terra cotta - all self potraits by teenaged girls - did not lose one.
In fact I have been firing heavy scupltures for others for years now.
All you have to do - in a fuel kiln or electric is hold at 90C or 200F till
there is no more steam detectable - use a mirror or shiny metal to "see"
any steam at the spy holes - then hold for a few hours more.
For the big loads I just left my gas kiln on pilot for a few days - after
that you can fire at a normal speed.
Let me know if you need more on this - RR
>I am just picking up the peices of a hollowed out bust form I tried to fire
>for someone else.
>The head is intact with part of the neck but the shoulders forming the base
>are in peices.
>Any suggestions as to what I did wrong and any way to make use of what I
>have left?
>I have been drying the form for 10 weeks in my kiln room which gets quite
>warm every time I fire.
>Examining the shoulders I see some places it is 3/4 inch thick and other
>places 2 1/2 inches thick. There are also some small pockets along the
>fracture lines.
>I have a skutt km-1027-3. I candled it for 4 hours and then put it on slow
>fire.
>Any suggestions what to do with the head?
>Thanks
>David Todd
Ron Roy
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Ron Roy on fri 21 nov 08
I think Snail has it exactly right here - worth reading over by everyone -
the better you understand the problem the better your chances of finding
the solutions.
RR
>I don't think even the most flat-bottomed and heavy
>work can form an airtight seal with a kiln shelf.
>It doesn't take much to vent an enclosed space.
>A single needle-tool hole is sufficient for a volume
>of two cubic feet, maybe more, if fired at a medium-
>slow rate of heat rise. The slight gap between clay
>and shelf will be greater in cross-section than one
>little hole. Fairly porous clay can even get away with
>no hole at all, if the inner cavity is small enough, and
>not sealed over with dense slip or fired too fast. Steam
>can only do damage if pressure builds up, and if
>pressure gets too high, it will open up an escape route
>at the weakest point. In this case, it would be the 'joint'
>between the work and the shelf. The steam may lift the
>work slightly up on whatever side of the piece is lightest,
>but only be enough to release the pressure - hardly
>enough to jostle it, really. If the piece were entirely
>closed, the pressure (if not allowed to escape) would
>finally split the piece at the weakest point - a bad
>joint, or a thin spot, or a hidden air pocket.
>
>I know that I've theorized in the past that air pockets
>may serve as 'collection points' for steam, and cause
>breakage that wouldn't occur otherwise, but I don't
>believe it's as frequent as I once thought. I now suspect
>that although that might be a factor, the simple fact that
>air pockets are weak points in the structure is sufficient
>to explain why cracks so often intersect air pockets.
>In addition, there are plenty of air pockets that we never
>know about because the work didn't break at that point,
>so we never saw them at all. Air pockets alone do not
>cause breakage. I believe it because I have often made
>multi-part work which required 'truing up' the contact
>surfaces, and in some cases revising the form by removing
>inches of its height or length before refitting the joint.
>I work with a fairly loose-and-fast coil-building method
>and I build thick for outdoor projects, and when cutting
>through, I have often encountered air pockets of
>considerable size (1/4" and more) in work with no
>firing cracks at all. I attribute the survival of this work
>to the coarse, porous clay which allows air and steam
>to easily escape, but it proves that air pockets, in and of
>themselves, are not the cause or breakage.
>
>Firing too fast for the porosity of the green clay to release
>the moisture (either as vapor or steam) is the fundamental
>problem. If this is dealt with (by firing slower, or using
>a coarser clay, or adequate venting), there will be no \
>breakage, air pockets or no.
>
> -Snail
Ron Roy
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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