Earth and Fire Pottery on mon 7 mar 05
Hello all-
Does anyone have experience with using supports in a slip cast mold that is wide and somewhat thin? Our concern is that a piece that big will be prone to cracking during the forming and firing process. What materials are used for this? I'm working on a collaboration that will be about 20" x 29", and about 5/8' thick around the outside diameter. There are many kinds of metals that might work. We're planning to put these right into the mold.
I would appriciate any input. See you in a week and a few days in Baltimore! gregg
Gregg Allen Lindsley
Earth and Fire Pottery
Instructor - Mendocino College
Board Member Potters Council
Mail only: PO Box 402
Cobb, Ca. 95426
UPS etc.: 10325 Brookside Drive
Whispering Pines, Ca. 95426
e mail: gerrg@yahoo.com
http://www.earthandfirepottery.net
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John Rodgers on tue 8 mar 05
Gerry,
I'm a bit confused by your question.
You refer to
>>supports in a slip cast mold<<
and then to
>>cracking during the forming and firing process<<
As an old slip caster and mold maker, these are two different things.
Moldes are cast and made from the cold setting of plaster. Things of
clay are formed and fired
As said for the mold(s) - which is most typically made from plaster -
often they will have re-inforcing materials added for some strenght.
Fiberglas cloth is added sometimes, and so is hemp fiber or jute. I'm
sure there are other materials that could be used. I order hemp fiber by
the 50 lb bag from Polytek, a mold materials supply house. This is added
while the plaster is still liquid before it sets. I have on occasion
used stainless steel rods in places where a mold was thin and strength
of the mold was an issue. The stainless doesn't rust over time and crud
up the mold nor affect moisture absorption nor drying.
I have never seen anything put in a slip casting as any kind of
re-inforcement within the piece itself. The slip clay shrinks as it
dries, then shrinks again as it fires. This would preclude anything
being within the clay in the slip cast piece itself. It would crack for
sure due to differential shrinkage between the clay and the
re-inforcement. However, I have often seen, and used myself, external
slip cast cones and other slip cast supports to aid in the firing.
For example, I had a large porcelain angel to fire, and the wings would
sag during the firing. I made a mold for a tall thin cone, and would
cast them at the same time the angel was cast from the same bucket of
slip(this was important) so as to be sure that all things were equal.
The angel casting and the cones would dry together and shrink the same
percentage. I would put the angel in the kiln, then stand the tall cone
under the wing of the angel to support the wing. The cone was
deliberately made 1 inch shorter than the height of the wing and I would
put ceramic fiber between the top of the cone and the wing itself. When
fired, the cone and the angel would shrink the same, and at temperature
the wing, softened by the heat and sticking way out, would not sag
under gravity because the support was in place and there would be no
cracking or warping.
There is an engineering matter in the design of any large object that
will be fired. It is that from top to bottom there must be sufficient
material at any point to give sufficient strength at any point, to carry
whatever load there is above that point. The design of the piece will
dictate what material thickness is required. Shape has a significant
inpact. Example: an egg is terrifically difficult to crack when pressed
at both end simultaneously, but try it on the sides and it can be broken
easily. If a form is analyzed for it's inherent geometry, it can be
determined what basic form will give the greatest support for a give
thickness of material. Cones can carry a significant load though the
walls of the cone may be quite thin, for example. Of course the goal is
to transfer all loads from above down through the base to the surface of
the material the pieces is sitting on.
There are many considerations. Durability after completion of the
project is another. That alone may require more material than is needed
just for support in firing. You certainly would not want a beautiful
work to be broken in handling after it has sufficient strength to get
through the firing.
I hope this is of some help to you in your quest.
Good luck.
Regards,
John Rodgers
Chelsea, AL
Earth and Fire Pottery wrote:
>Hello all-
>
> Does anyone have experience with using supports in a slip cast mold that is wide and somewhat thin? Our concern is that a piece that big will be prone to cracking during the forming and firing process. What materials are used for this? I'm working on a collaboration that will be about 20" x 29", and about 5/8' thick around the outside diameter. There are many kinds of metals that might work. We're planning to put these right into the mold.
> I would appriciate any input. See you in a week and a few days in Baltimore! gregg
>
>
>Gregg Allen Lindsley
>Earth and Fire Pottery
>Instructor - Mendocino College
>Board Member Potters Council
>Mail only: PO Box 402
> Cobb, Ca. 95426
>UPS etc.: 10325 Brookside Drive
> Whispering Pines, Ca. 95426
>e mail: gerrg@yahoo.com
>http://www.earthandfirepottery.net
>
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> Yahoo! Netrospective: 100 Moments of the Web
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Ditmar on tue 8 mar 05
Nothing quite that size, but I've had no problems with pieces up to 12" diam
and 1/16" to 1/8" thick. Some with considerable 3d relief and detail. I
merely added paper fiber to the slip. Basically a soupy paper clay.
Handling it before firing....well that's another story. Needless to say the
pieces are fragile.
Plan to lose some.
Ditmar
> Does anyone have experience with using supports in a slip cast mold that
is wide and somewhat thin? Our concern is that a piece that big will be
prone to cracking during the forming and firing process. What materials are
used for this? I'm working on a collaboration that will be about 20" x 29",
and about 5/8' thick around the outside diameter. There are many kinds of
metals that might work. We're planning to put these right into the mold.
> I would appriciate any input. See you in a week and a few days in
Baltimore! gregg
Snail Scott on wed 9 mar 05
At 10:22 AM 3/8/2005 -0600, John R wrote:
>There is an engineering matter in the design of any large object that
>will be fired. It is that from top to bottom there must be sufficient
>material at any point to give sufficient strength at any point, to carry
>whatever load there is above that point. The design of the piece will
>dictate what material thickness is required. Shape has a significant
>inpact. Example: an egg is terrifically difficult to crack when pressed
>at both end simultaneously, but try it on the sides and it can be broken
>easily.
Well put, John!
When I teach sculpture, I call this the 'Toilet-
Paper-Tube Principle': An empty toilet-paper roll
stood on end can support several bricks, while if
the same tube is laid on its side, it can be crushed
with your pinky finger. Cylinders (and cones, as
you noted) are strong when their walls are vertical,
as any loads are carried directly down. The same
shape laid horizontally has almost no resistance
to collapse.
The shape of the connection between the form and the
kiln shelf is especially important, since it bears
all the weight of the clay above it. If the form
touches down perpendicular to the shelf (i.e.
vertically) it is far less likely to warp or collapse
than if that edge is undercut or curved. Upper
portions of the form can tolerate more 'sideways
toilet-roll' shapes, since they bear less weight.
For small work, the load is negligible relative to
the thickness of the clay wall, but when doing larger
work the effect is magnified.
Seated human figures are a classic example. Thighs
and butts curve under in just the wrong way to be
structurally sound as a hollow clay shell. (Actual
humans have an internal skeleton, which conveniently
carries the weight straight down - very different.)
So, I often reinforce these pieces with internal
buttresses and walls, especially in spots such as
where the vertical line of the torso meets the upper
leg. By continuing that vertical line invisibly inside
the piece, the load of the upper figure is carried
straight down to the shelf, without relying on the
visible outer wall for any significant support. This
principle applies equally to other forms of similar
geometry, and the larger the piece, the more important
it becomes.
For slip-casting, it's difficult to get access to the
inside of the piece to add reinforcement unless the
piece is cut in sections and joined afterward. If this
is actually the case, though, you might try pouring
out slabs of casting slip onto a flat mold, and
attaching them inside before final assembly of the
piece. It seems unlikely that this would be feasable
for most shapes, though. It may be better to avoid
warping by leaving the piece in the mold as long as
possible so that it's dry enough to support itself
without warping while wet, and then underfire it
slightly to avoid pyroplasticity.
This need to create structurally self-supporting forms
is especially important in designing slip-cast forms,
precisely because of these issues. I learned this the
hard way a number of years ago, when I did a limited
edition of porcelain teapots shaped like dinosaurs
hatching out of eggs (for the local natural-history
museum shop.) Even on such a small shape, the plasticity
of the porcelain and the under-curve of the egg shape
combined to create an annoyingly large loss rate due to
warping and cracking.
-Snail
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