Snail Scott on mon 30 dec 02
At 01:23 PM 12/30/02 -0500, you wrote:
>They are hollow and the base is open in that there is no bottom, just the
bottom edges of the walls like an upside-down pot. My electric kiln will
accommodate them, but I can't find a shelf big enough to keep the pieces
from spanning at least two shelves.
I do a lot of large hollow sculpture, too. I found
out, early on, that walls that come straight down to
the shelf and stop can be a real problem. The full
weight of the piece is borne on a very small surface
area. And since the bottom edge of the piece is
unlikely to be a perfect match for the surface of the
shelves, the points that touch the shelf will have
tremendous friction, and will tend not to move with
the piece as it shrinks. The weight _per square inch_
(or other unit of choice) can be huge, even if the
total weight of the art doesn't seem like much. (Ever
been stepped on by a spike-heel wearer?) What you need
is the functional equivalent of snowshoes, instead.
The big full kiln shelf is certainly an option, but not
the only one. One I often use is to make a thick sturdy
slab out of the same clay body, which will shrink right
along with it as it fires. It's a one-time use, but if
you've spent a month on the piece, don't compromise its
success by being stingy with the clay for the slab. A
firing slab won't reduce the p.s.i, but you won't have
to deal with friction due to shrinkage (non)differential.
Also, I never make a piece that has walls that simply
rise up from the shelf. I make a lip for the inside of
the bottom edge, to give a wider area of support. Most
often, I just make a slab, then build the piece on it.
Then, when I've finalized the footprint of the piece, I
cut away the excess form the outside, and cut out a hole
from the inside. Full-across bottom slabs tend to crack,
since shelf contact alone prevents even heating, but
by removing the middle, it reduces the stress quite a
lot. (For a round/columnar piece, the bottom would look
like a flat donut, etc...)
And, a thin bed of sand or grog on the shelf can act
like ball-bearings, and help ease the friction between
the shelf and the piece.
I use all three methods simultaneously, sometimes, or
two out of three, depending on the piece.
Since you've already made this piece, it's a bit too
late to modify the bottom, but do make a firing slab
and use sand. And in the future, give plenty of surface
area to your wall-bottoms. It's all about distributing
the load, and reducing friction.
-Snail
Brenda Beeley on mon 30 dec 02
All my sculptural and tile pieces are too big to fit on one shelf. I
haven't had any problem coming from spanning the shelves. If I have huge,
heavy tile pieces then I might sprinkle the shelf with silica or grog
first to let them move if they need to.
And often my pieces are too heavy to lift in myself......they still do
okay. Plus I often single-fire my pieces to cone 6. No cracking as long
as I cool them slowly from 1400-800 F.
Brenda
+++++++++++++++++
ClaySpace on Puget Sound
P.O. Box 1339
Suquamish, WA 98392-1339
(360) 598-3688
email: mtimes@telebyte.net
http://www.clayspaceonpugetsound.com
e wilson farrington on mon 30 dec 02
Dear ClayArters,
I remember all the discussion a while back about techniques for =
firing large platters, etc, that involved different types of clay =
patties, coils, etc. to aid with the movement of the clay during firing. =
I know you're not supposed to fire large items across two kilns shelves; =
so my problem is this: I have large sculptures that measure about 22" =
wide X 31" long at the base. They are hollow and the base is open in =
that there is no bottom, just the bottom edges of the walls like an =
upside-down pot. My electric kiln will accommodate them, but I can't =
find a shelf big enough to keep the pieces from spanning at least two =
shelves. I understand I may be able to order a very large shelf, ($$$$ =
and prone to cracking, not to mention heavy). Unfortunately I did not =
think this far ahead while I was into making the pieces. (I do that a =
lot- those creative juices get going and reality be damned...) So, any =
suggestions? I was thinking about some combination of clay patties, =
coils stuffed between the shelves...hell, I don't know. HELP!!!
Thanks!--Willie
cmhatfield on mon 30 dec 02
Willie
I suggest you mix a high firing clay with plenty of grog and make six golf
ball sized balls. Place the shelves on three balls each and with a large
straight edge or board press them level. Allow to dry and re check they are
still level. If all is well put about six sheets of newspaper on the
shelves, load and fire very slowly.
Good luck
Colin Hatfield
Chipping Sodbury, UK
Hank Murrow on mon 30 dec 02
On Monday, December 30, 2002, at 10:23 AM, e wilson farrington wrote:
> Dear ClayArters,
> I remember all the discussion a while back about techniques for
> firing large platters, etc, that involved different types of clay
> patties, coils, etc. to aid with the movement of the clay during
> firing. I know you're not supposed to fire large items across two
> kilns shelves; so my problem is this: I have large sculptures that
> measure about 22" wide X 31" long at the base. They are hollow and the
> base is open in that there is no bottom, just the bottom edges of the
> walls like an upside-down pot. My electric kiln will accommodate them,
> but I can't find a shelf big enough to keep the pieces from spanning
> at least two shelves. I understand I may be able to order a very large
> shelf, ($$$$ and prone to cracking, not to mention heavy).
> Unfortunately I did not think this far ahead while I was into making
> the pieces. (I do that a lot- those creative juices get going and
> reality be damned...) So, any suggestions? I was thinking about some
> combination of clay patties, coils stuffed between the shelves...hell,
> I don't know. HELP!!!
> Thanks!--Willie
Dear Wilson;
Why don't you use strips of fiber the piece and on top of the existing
shelves? I do this when size constraints require it.
Best, Hank in Eugene (soon to San Diego!)
Dave Finkelnburg on mon 30 dec 02
Dear Willie,
Colin Hatfield's suggestion is excellent!
I do the second step a little differently. After leveling the shelves
as close as possible, I wad under the large piece with refractory clay or
regular clay dipped in alumina. This wadding can be small balls, or
sections of coils, depending on the ware. The idea is to get even support
under the foot of a piece that may not have a perfectly flat foot. If you
set the piece on soft wadding, and allow it to settle into the wadding, it
will be fully supported. Keeping the wadding very refractory is important.
You want it to be able to move with the ware, and not stick to the shelf at
all!
With important work I set the piece in place, draw around it with a
pencil, take it up again and lay out the wadding just inside the pencil
line, then set the ware back on the wadding. Others actually glue the
wadding on with an inexpensive white glue. I have tried that, also, and it
works fine, too.
By the way, whomever the silly bugger is who started the tale that one
can't stack ware across two shelves created one more rule to be broken.
Sure, it would be easier and better to have bigger shelves, but in their
absence I stack across two shelves practically every firing.
Good firing! Reality, by the way, is occasionally overrated! :-)
Dave Finkelnburg in Idaho, eager to be back in the studio after
doing pottery paperwork for a couple days
----- Original Message -----
From: "e wilson farrington"
I remember all the discussion a while back about techniques for firing
large platters, etc, that involved different types of clay patties, coils,
etc. to aid with the movement of the clay during firing. I know you're not
supposed to fire large items across two kilns shelves; so my problem is
this: I have large sculptures that measure about 22" wide X 31" long at the
base. They are hollow and the base is open in that there is no bottom, just
the bottom edges of the walls like an upside-down pot. My electric kiln will
accommodate them, but I can't find a shelf big enough to keep the pieces
from spanning at least two shelves. I understand I may be able to order a
very large shelf, ($$$$ and prone to cracking, not to mention heavy).
Unfortunately I did not think this far ahead while I was into making the
pieces. (I do that a lot- those creative juices get going and reality be
damned...) So, any suggestions? I was thinking about some combination of
clay patties, coils stuffed between the shelves...hell, I don't know.
HELP!!!
Thanks!--Willie
Earl Brunner on tue 31 dec 02
In my gas downdraft kiln, a full shelf in the arrangement would
seriously interfere with circulation. What I have done, and this might
not apply to your situation is to either put the larger shelf at the top
of the load so that it doesn't really interfere with most of the
circulation in the stack, or I will use smaller square shelves to span
the crack. I am dealing with a different issue though, most of my
larger diameter pots or platters have a smaller foot with significant
overhang beyond the rim.
>
>>They are hollow and the base is open in that there is no bottom, just the
>
> bottom edges of the walls like an upside-down pot. My electric kiln will
> accommodate them, but I can't find a shelf big enough to keep the pieces
> from spanning at least two shelves.
>
>
| |
|
