Judith Marvin on mon 12 feb 07
A friend has asked me to bisque fire a sculpture and I'm a bit goosey. It
is a horse standing upright. My concern is that shrinkage during firing
might be hard on the legs. Am I on the right track in thinking that a
dusting of grog on the shelf under the feet would help?
Thanks in advance.
Judith Marvin
_________________________________________________________________
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John Jensen on mon 12 feb 07
Is there any reason not to fire the horse on its side? My experience
is that there is no noticeable shrinkage in my bisque firings. Still
horse legs are so fragile, I'd risk them as little as possible.
John Jensen, Homewood Pottery
homewoodpottery.com
johnjensen@homewoodpottery.com
> A friend has asked me to bisque fire a sculpture and I'm a bit
> goosey. It
> is a horse standing upright. My concern is that shrinkage during
> firing
> might be hard on the legs. Am I on the right track in thinking that
> a
> dusting of grog on the shelf under the feet would help?
>=20
> Thanks in advance.
>=20
> Judith Marvin
>=20
Michael Wendt on mon 12 feb 07
Judith,
There is no appreciable shrinkage in most
clay bodies between bone dry and bisque.
Sand on the shelf is o.k. though to level the
piece.
Hi to you and Gary,
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
USA
208-746-3724
http://www.wendtpottery.com
wendtpot@lewiston.com
Judith wrote:
A friend has asked me to bisque fire a sculpture and
I'm a bit goosey. It
is a horse standing upright. My concern is that
shrinkage during firing
might be hard on the legs. Am I on the right track in
thinking that a
dusting of grog on the shelf under the feet would help?
Thanks in advance.
Judith Marvin
timothy knick on mon 12 feb 07
I have never made or fired something like that, but I have two thoughts:
Check the shrinkage percentage from green to bisque. A lot of clays shrink
very little, or not even 1 percent from green to bisque. From green/bisque
to hi fire is where major shrinkage occurs. Second, you might roll straps
to lay flat on the shelf for the legs to stand on. Maybe an "H" shape with
one leg on each of the four ends. Use the same clay as the sculpture is
made of. My theory is that the straps would shrink the same amount as the
sculpture, contracting as the legs pulled inward. I would also put powder
alumina under the straps. Please correct me if I misunderstood the
question.
Tim
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Craig Clark on mon 12 feb 07
One time while I was in school I had to fire several larger pieces
from a sculpture class. They were not horses, but all of the pieces were
quite irregular and there was not any really good way to fire them
without putting undue stress on a fragile place and risking cracking in
the bisque. It was a class of "true artists" and the last thing on their
minds was any consideration of how pieces would actually be placed into
a kiln and fired. I was irritated, pacing back and forth in the kiln
room trying to come up with a solution and I literally stepped on the
answer.
There was a fired cone pat, which I had just stepped on, laying on
the floor near the kiln. I looked at it for a minute and then really
looked at it. With all of the holes that are poked in a cone pat it is
easily fired green. No warps, cracks or breaks. I wondered whether or
not I could use several large shapes of clay to form saddles for the
pieces to lay in and then poke a bazillion holes in them just like a
cone pat.
I ended up doing a dry run with some older wonky pieces that were
laying about and it worked. I then took one of the pieces and tried
something similar. It worked as well. Ended up doing all of the
remaining pieces in the next firing. I was all done in an old 60 Cubic
Foot Alpine updraft that I knew very well. I took it nice and easy on
the pieces. Took the better part of two days. All of the pieces made it
through the firing intact without any warpage or cracking. I probably
could have fire things more quickly, but I never had the need to fire
such oddly shaped large pieces in that kiln again so I can't speak from
experience.
Hope this helps
Craig Dunn Clark
619 East 11 1/2 St
Houston, Texas 77008
(713)861-2083
mudman@hal-pc.org
John Jensen wrote:
> Is there any reason not to fire the horse on its side? My experience
> is that there is no noticeable shrinkage in my bisque firings. Still
> horse legs are so fragile, I'd risk them as little as possible.
>
> John Jensen, Homewood Pottery
> homewoodpottery.com
> johnjensen@homewoodpottery.com
>
>
>> A friend has asked me to bisque fire a sculpture and I'm a bit
>> goosey. It
>> is a horse standing upright. My concern is that shrinkage during
>> firing
>> might be hard on the legs. Am I on the right track in thinking that
>> a
>> dusting of grog on the shelf under the feet would help?
>>
>> Thanks in advance.
>>
>> Judith Marvin
>>
>>
>
> ______________________________________________________________________________
> 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.
>
>
Vince Pitelka on tue 13 feb 07
Snail wrote:
> Grog or sand WILL insulate the slab and
> increase the risk of heating-or cooling-
> related cracking the slab, but I see the
> increased risk as only slight. We're not
> talking about a thick bed of sand here,
> just a sprinkling.
Snail -
It is a well-known fact that a sprinkling of grog or sand will help a
sculpture to move in the kiln as it shrinks, but if it cracks, what have you
accomplished? If it is a strong slab, then it will shrink and move just
fine on wadding, and you eliminate the chance of the cracking.
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Snail Scott on tue 13 feb 07
At 02:41 PM 2/12/2007 -0800, you wrote:
>A friend has asked me to bisque fire a sculpture and I'm a bit goosey. It
>is a horse standing upright. My concern is that shrinkage during firing
>might be hard on the legs. Am I on the right track in thinking that a
>dusting of grog on the shelf under the feet would help?
Yes, it may help. Cetainly it will
do no harm. There is not much
shrinkage from dry to bisque though,
so I wouldn't worry. I'm assuming that
they aren't one and the same here - i.e.
not earthenware being fired to maturity
before a lower glaze firing, or a single-
shot firing without glaze, which would
also (technically) be a bisque. In other
words, this will be a 'normal' studio
potter's style low-fire bisque of a high-
fire clay?
It's the vitrification firing that will
matter the most. The shrinkage of the
clay will determine how big an issue it
really is.
The weight of the piece will be a big
factor when firing things with small,
widely spread points of support. When
possible, I like to do this sort of
thing on a 'firing slab' - a thick
slab of the same clay body, which will
sit on top of the sand-or-grog-covered
kiln shelf. The sand will act like ball
bearings, and the slab will shrink in
the same propertion as the artwork. This
will prevent the legs of the piece from
splaying outward in firing, as friction
keeps them in place on the shelf while
the clay shrinks. The slab and the piece
will all all shrink together.
Naturally, the slab needs to be at the
same state of firing as the artwork. So,
although the slab is not needed for a
low bisque, it'll be needed in a bisque
state when the bisqued art gets refired
higher later on. If a higher firing is
in the plan, go ahead and bisque the slab
now, too. If you want to single-fire
anything this way, naturally the slab
would get single-fired as well.
No need to worry for a low bisque, though.
And you can always bisque a firing slab
later if they need it for high fire.
-Snail
Frank Colson on tue 13 feb 07
Big Tang! I am currently working on a large stylized Tang horse. The legs
alone are nearly two feet high. As I have done in the past, often., the
legs are separated from the extention of the head,body. However, a template
was made, at the cut off point, of each leg, and a bevel is made (1/2"
wide) at the top of the cut off point of each leg, and bottom of the body
extensions to each leg. I also included an internal cavity extention
"gasket" protruding 1" beyond the cut off point.
After one firing all units together, the extensions (internal protrusions)
will fit up into the body leg protrusion. When the units are married, a
small "vee" groove will be apparent around the cut off point. of each leg.
The first action of fitting each leg up into the body may be epoxied. In
finishing the, external "vee" groove is half filled with epoxed, and then
filled again with an auto body filler or Durhams Rock Hard Water Putty, or
Stainable Wood Filler. After set,ting, that area can be touched up to match
the exterior patinsa (which I use usually), or glaze coloring. When
finished, this piece will be more than five feet high standing on four legs,
and total strong!
I developed this approach when I had a one person exhibition of some 12
works, from six to nine feet high each, in the main lobby of Rockefeller
Plaza , NYC, in 1980., and again
with a series of "Armored Horses" (in clay), in 1994, for the Fletcher
Challange Ceramic Award, in Auckland, New Zealand.
Frank Colson
----- Original Message -----
From: "Craig Clark"
To:
Sent: Monday, February 12, 2007 7:22 PM
Subject: Re: bisque sculpture
> One time while I was in school I had to fire several larger pieces
> from a sculpture class. They were not horses, but all of the pieces were
> quite irregular and there was not any really good way to fire them
> without putting undue stress on a fragile place and risking cracking in
> the bisque. It was a class of "true artists" and the last thing on their
> minds was any consideration of how pieces would actually be placed into
> a kiln and fired. I was irritated, pacing back and forth in the kiln
> room trying to come up with a solution and I literally stepped on the
> answer.
> There was a fired cone pat, which I had just stepped on, laying on
> the floor near the kiln. I looked at it for a minute and then really
> looked at it. With all of the holes that are poked in a cone pat it is
> easily fired green. No warps, cracks or breaks. I wondered whether or
> not I could use several large shapes of clay to form saddles for the
> pieces to lay in and then poke a bazillion holes in them just like a
> cone pat.
> I ended up doing a dry run with some older wonky pieces that were
> laying about and it worked. I then took one of the pieces and tried
> something similar. It worked as well. Ended up doing all of the
> remaining pieces in the next firing. I was all done in an old 60 Cubic
> Foot Alpine updraft that I knew very well. I took it nice and easy on
> the pieces. Took the better part of two days. All of the pieces made it
> through the firing intact without any warpage or cracking. I probably
> could have fire things more quickly, but I never had the need to fire
> such oddly shaped large pieces in that kiln again so I can't speak from
> experience.
> Hope this helps
> Craig Dunn Clark
> 619 East 11 1/2 St
> Houston, Texas 77008
> (713)861-2083
> mudman@hal-pc.org
>
> John Jensen wrote:
>> Is there any reason not to fire the horse on its side? My experience
>> is that there is no noticeable shrinkage in my bisque firings. Still
>> horse legs are so fragile, I'd risk them as little as possible.
>>
>> John Jensen, Homewood Pottery
>> homewoodpottery.com
>> johnjensen@homewoodpottery.com
>>
>>
>>> A friend has asked me to bisque fire a sculpture and I'm a bit
>>> goosey. It
>>> is a horse standing upright. My concern is that shrinkage during
>>> firing
>>> might be hard on the legs. Am I on the right track in thinking that
>>> a
>>> dusting of grog on the shelf under the feet would help?
>>>
>>> Thanks in advance.
>>>
>>> Judith Marvin
>>>
>>>
>>
>> ______________________________________________________________________________
>> 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.
>>
>>
>
> ______________________________________________________________________________
> 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.
Vince Pitelka on tue 13 feb 07
>A friend has asked me to bisque fire a sculpture and I'm a bit goosey. It
>is a horse standing upright. My concern is that shrinkage during firing
>might be hard on the legs. Am I on the right track in thinking that a
>dusting of grog on the shelf under the feet would help?
Some of you may have seen Meagan Kieffer's amazazing horse sculptures in
various exhibitions. She makes them in many sizes, and from what I know of
her work there are three ways that she fires them to prevent serious damage.
The choice depends on the design of the particular horse. The ones that are
freestanding on their own legs are sometimes fired on a pedestal that stands
under the horse's belly. Others are fired on a clay slab that shrinks along
with the horse in the firing. The pedestal or slab is always made at the
same time as the horse and from the same clay, and is allowed to shrink
through drying and firing along with the horse. Some of her horses are made
with an integral slab, and those ones do fine, although the horses on slabs
(integral or separate) are always fired on wads to allow heat and atmosphere
to pass beneath the piece. It's possible that placing sand or grog under
the horses legs, as Snail suggests, might do the trick, but it's not nearly
as sure a thing as firing it on a slab made of the same clay. A piece with
a flat bottom like a slab should never be fired on sand or grog, because itt
worsens a problem that is already there - it further insulates the bottom of
the piece from changes in temperature during heating and cooling (and the
associated expansion and contraction), and you run a very high risk of
cracking the piece. Always use wadding or softbrick shims in such a
situation.
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Snail Scott on tue 13 feb 07
At 07:00 PM 2/13/2007 -0600, you wrote:
>...It's possible that placing sand or grog under
>the horses legs, as Snail suggests, might do the trick, but it's not nearly
>as sure a thing as firing it on a slab made of the same clay. A piece with
>a flat bottom like a slab should never be fired on sand or grog, because itt
>worsens a problem that is already there - it further insulates the bottom of
>the piece from changes in temperature during heating and cooling (and the
>associated expansion and contraction), and you run a very high risk of
>cracking the piece...
Actually I didn't suggest grog under the
legs; that was in the original post. I
mainly said that it couldn't hurt.
I generally use slabs myself, when firing
such forms. I disagree with Vince about the
virtues and detriments of grog underneath,
though. As with the 'tool discussion', I'm
not disagreeing with the basic principle,
but with specific possibilities.
Grog or sand WILL insulate the slab and
increase the risk of heating-or cooling-
related cracking the slab, but I see the
increased risk as only slight. We're not
talking about a thick bed of sand here,
just a sprinkling. Since I moved to the
Midwest, I've discovered something they
call 'shuffleboard', a game played in bars
and taverns on elaborate long game tables
with little hand-shoved pucks; the highly
polished 'lane' covered with a regulation
scattering of little table-salt-looking
'beads' to let the pucks slide smoothly
and almost frictionlessly. If this cultural
reference makes sense to you, THAT'S the
amount of sand I mean: a good dusting,
but nowhere more than 1 grain thick, and
not a solid coating by any means. As
insulation, it ain't much.
The only time I'd use more sand is when the
shelf (either due to abuse or irregular
kiln wash) is not very smooth and flat. We
don't always have the shelves we wish we
had, and their condition may not be ours to
fix. If this is the case, a layer of sand
applied more heavily, but scraped back to
the thinnest layer possible with a very flat
tool like a ruler, will level the shelf. I
believe that although the thicker layer of
sand in this case might increase thermal
mass, I see the shelf irregularities as
a greater hazard, if unequal friction causes
localized 'snagging' during shrinkage. And
I also suspect that most irregularities wil
be of the 'pit' variety, not the 'bump'
variety, since bumps can easily be ground
down. If so, the added sand is really filling
little more than the spots of absent shelf,
not adding much additional mass.
By the way, I seldom make enclosed work with
a broad flat bottom slab (integral, not just
a disposable firing slab), because in addition
to the irregular heating and cooling, getting
them to dry on the bottom is a bitch if they're
big.I make a wide lip on the bottom to help
spread the load of the weight above over a wider
area of shelf, but the middle (to the extent
that the shape allows) is empty, like rolled-
out sugar-cookie dough with the cookie already
cut out. Two inches or three (for biggish work)
is as wide a bottom flange as I want. Wider
than that, it won't dry evenly even on drywall,
and cracking is just waiting to happen, even
without firing stress to exacerbate the issue.
-Snail
Ivor and Olive Lewis on wed 14 feb 07
Dear Judith Marvin,=20
You are right to be cautious. If this Horse is constructed by hand it is =
always possible that the clay varies in thickness, that some of the =
joints between parts are suspect, that inequality in thickness could =
impose strains, that the body will be heavy imposing additional stress =
on the legs.
Perhaps searching for information about firing clay sculpture would help =
you to devise good ways of overcoming anticipated problems. Snail will, =
I am sure, will offer good suggestions.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
,
William & Susan Schran User on wed 14 feb 07
On 2/13/07 11:25 PM, "Vince Pitelka" wrote:
> It is a well-known fact that a sprinkling of grog or sand will help a
> sculpture to move in the kiln as it shrinks, but if it cracks, what have you
> accomplished? If it is a strong slab, then it will shrink and move just
> fine on wadding, and you eliminate the chance of the cracking.
Some input please - we use Advancers in our gas fired kiln, firing to ^10.
One of the issues with these shelves are thermal mass and temperature
variations that might lead to cracking/breaking. It's recommended that
hardbricks/soaps not be used as shelf posts as the heat variance where
shelf/brick meet make cause cracking.
When I have a student work that has a large "foot print", say a slab pot
that covers at least half the shelf surface I have some concerns the clay
may act like a brick, holding the heat. Then again, I would expect the clay
wall/bottom, that is thinner than the shelf, to lose heat faster than the
shelf and shouldn't be concerned.
When I spoke with Marshall Brown about the Advancers, I voiced my concerns
about this. I told him I use crumbled pedestals from our crystalline
firings. The pedestals are composed of alumina/kaolin/sawdust, so after
they're fired, it becomes like a soft fire brick. We just crumble the used
ones into a coarse grog size and put this under large flat bottomed forms.
Marshall thought this was a good process.
Every once in a while we do have a large flat form crack, and I assume this
is due to the issues Vince wrote about in his earlier message. My question
is: with wadding, would there be problems with warping of the slab,
especially with the clay becoming protoplastic at our top firing
temperature?
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
Vince Pitelka on wed 14 feb 07
Bill Schran wrote:
> Every once in a while we do have a large flat form crack, and I assume
> this
> is due to the issues Vince wrote about in his earlier message. My question
> is: with wadding, would there be problems with warping of the slab,
> especially with the clay becoming protoplastic at our top firing
> temperature?
Bill -
It all depends on the pattern and quantity of wadding. It's so easy to do,
and you can easily do a network of 30 or 40 wads on the back of a large
piece. Just stick them in place on the bottom of the piece with dots of
white glue, and then set the piece in place right away while the wads are
still damp so that the whole piece settles down slightly. Then you have
absolutely even support under the piece, and less chance of warpage than
with any other means of support.
With some pieces, it works better to use parallel coils of wadding rather
than dots. You don't ever want to use a grid of wadding, because that
creates closed spaces and defeats the purpose of raising the piece up on
wadding.
For round pieces like large platters, a starburst pattern of coils works
best.
When using wadding for this purpose in anything except salt or soda, just
use a 50-50 mix of any clay and flint. Recycle slurry works great mixed
with plenty of flint. With this mix, the clay ventillates so easily, and
will not build up steam pressure even when heated abruptly. I've always
meant to do some tests with a propane torch to see exactly how fast you can
heat up this kind of wadding.
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
William & Susan Schran User on thu 15 feb 07
On 2/14/07 8:11 PM, "Vince Pitelka" wrote:
> When using wadding for this purpose in anything except salt or soda, just
> use a 50-50 mix of any clay and flint. Recycle slurry works great mixed
> with plenty of flint. With this mix, the clay ventillates so easily, and
> will not build up steam pressure even when heated abruptly. I've always
> meant to do some tests with a propane torch to see exactly how fast you can
> heat up this kind of wadding.
Vince - Thanks I'll give it a try, though with one caveat, I think the
wadding for our situation should be 50/50 alumina/kaolin.
There's an issue with Advancers where we get plucking with porcelain, so I
think a mix with silica would stick to the shelves.
What I may do is put together a batch of the pedestal mix, roll out a slab
and cut it up in small pieces to have wads to use more than once, or roll
out several small coils and do the same.
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
Snail Scott on sun 18 feb 07
At 10:25 PM 2/13/2007 -0600, Vince wrote:
>It is a well-known fact that a sprinkling of grog or sand will help a
>sculpture to move in the kiln as it shrinks, but if it cracks, what have you
>accomplished? If it is a strong slab, then it will shrink and move just
>fine on wadding, and you eliminate the chance of the cracking...
I suppose I evolved my current system when
I was making heavy stuff and firing front-
loaders. I loaded the slab and work as a
unit, shoving it straight across from the
work board to the kiln, rolling/sliding it
into place on the pre-sanded shelf. 150 lb.
stuff, some of it. I could never have gotten
the leverage to lift the work cleanly onto
a pre-placed slab, and wadding implies
placing the slab first, or lifting the slab-
and-work together.
Certainly wadding can be a good way to go,
if the geometry of kiln loading allows for
it. I do think that the difference between
a slightly elevated (wadded) slab and a
not-elevated slab (sanded) is still quite
small, especially if there's much wadding.
And I do agree that although the thermal
issues are somewhat diminished by the small
air-gap allowed by wadding, sand still
allows a freer shrinkage of the slab (less
friction) and a more even distribution of
the load on top.
-Snail
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