Bill Merrill on thu 12 jan 12
One of the ways to avoid large platters or slabs from getting drying
cracks in the center is to pound the clay with your fists or heavy
paddle. Look at the Onngi potters who pound their bases of large pots
with a heavy wooden paddle. Doing this helps compress the particles of
the slab base before it is rolled out. Throwing slabs ( not on a wheel)
of clay on a cement or plywood surface can stretch and thin out the clay
before rolling thinner.
If you are making large tiles and fire them in an electric kiln keep the
slabs away from the elements and kiln wall. You can get splits on the
outside rims if you do. If I am making platters nearly the size of a
28" electric kiln. I use the lower elements mostly at first and have the
platters on the top tier of the kiln. When doing much larger pieces, if
you bisque in a gas kiln that seems to alleviate any breakage. Most gas
kilns have 9" walls and cool evenly if the damper is kept closed and the
kiln is left to cool on its own. Any kiln with considerable mass will
cool at a slower pace than a fiber or thin walled kiln.
Another thing that can help with cracking is too put mullite/kyanite or
silica sand under the slabs to prevent the pieces from sticking to the
kiln shelves during a bisque or glaze fire. The clay shrinks and the
kiln shelf doesn't and the Kyanite/Mullite can be used over, even if
used in high fire.
Bill Merrill
Vince Pitelka on thu 12 jan 12
Bill Merrill wrote:
"Another thing that can help with cracking is too put mullite/kyanite or
silica sand under the slabs to prevent the pieces from sticking to the kiln
shelves during a bisque or glaze fire. The clay shrinks and the kiln shelf
doesn't and the Kyanite/Mullite can be used over, even if
used in high fire."
Hi Bill -
Most of that advice was right on, as is the case with your advice in
general. I don't how much work you have done with large flat slab forms,
but the above is a recipe for real trouble with most such flat slab forms.
As you know, of all ceramic shapes, flat slabs are least able to withstand
uneven thermal expansion. Sand on the kiln shelf can be a good idea with
very large sculptural forms in a glaze firing when you know the piece will
shift position due to shrinkage, but otherwise it is generally a mistake
because it insulates the underside of the slab from changes in heat. There
is always a trade-off, but the piece certainly is not going to stick the
kiln shelf during a bisque firing, and it will likely crack from the outer
edge inwards if the outer edge heats or cools much faster than the center.
That is the risk.
As a practical and proven alternative, I recommend raising the slab forms u=
p
on wading. It can be a network of balls of wadding, or parallel rows of
coils of wadding. It should never be a criss-crossing grid of wadding
because that creates closed spaces and causes the same problem. With round
slab forms I have often used a sunburst arrangement of coils of wadding, bu=
t
I do not have them meet in the center, in order to open up more space for
passage of air. The objective is to get the whole piece to heat and cool
evenly, and lifting the piece up on wadding can make all the difference in
the world. Needless to say, the damp, plastic wadding is placed on the kil=
n
shelf, and the slab piece placed on the wadding and very lightly pressed
down so that it contacts all the wadding and is well-supported.
There would be no reason to ever use expensive alumina-clay wadding like we
use in salt and soda. In a bisque firing that is going to have a good
preheat, any scrap claybody will do. In any faster firing, especially a
glaze firing, a 50-50 mixture of flint and any scrap clay will make a great
wadding for any normal reduction or oxidation firing, either bisque-firing
or glaze-firing. The flint opens up the body so it can be fired very
quickly without popping.
I am sure that you already know much of what I describe above, but I includ=
e
it for the benefit of those who initially asked the question about firing
large tiles.
In the bisque-firing, slab tiles can also be fired on edge almost
vertically, leaning gently against the wall of the kiln, with wadding
spacers between subsequent tiles, so you can lean a whole series of them
against each other, using wadding to maintain spacing.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net
http://iweb.tntech.edu/wpitelka/
Steve Mills on fri 13 jan 12
Hi Vince,=3D20
I use wads or strips of Ceramic Fibre for the same purpose, cut from 1 inch=
b=3D
lanket, and often doubled up.=3D20
I've been using the same strips and wads for some years now, I just keep th=
e=3D
m in a box by the Kiln.=3D20
Steve M
Steve Mills
Bath
UK
Sent from my iPod
On 13 Jan 2012, at 03:45, Vince Pitelka wrote:
>=3D20
>=3D20
> Hi Bill -
> Most of that advice was right on, as is the case with your advice in
> general. I don't how much work you have done with large flat slab forms,
> but the above is a recipe for real trouble with most such flat slab forms=
.=3D
> As you know, of all ceramic shapes, flat slabs are least able to withstan=
d=3D
> uneven thermal expansion. Sand on the kiln shelf can be a good idea with
> very large sculptural forms in a glaze firing when you know the piece wil=
l=3D
> shift position due to shrinkage, but otherwise it is generally a mistake
> because it insulates the underside of the slab from changes in heat. Ther=
e=3D
> is always a trade-off, but the piece certainly is not going to stick the
> kiln shelf during a bisque firing, and it will likely crack from the oute=
r=3D
> edge inwards if the outer edge heats or cools much faster than the center=
.=3D
> That is the risk.
>=3D20
> As a practical and proven alternative, I recommend raising the slab forms=
u=3D
p
> on wading. It can be a network of balls of wadding, or parallel rows of
> coils of wadding. It should never be a criss-crossing grid of wadding
> because that creates closed spaces and causes the same problem. With rou=
n=3D
d
> slab forms I have often used a sunburst arrangement of coils of wadding, =
b=3D
ut
> I do not have them meet in the center, in order to open up more space for
> passage of air. The objective is to get the whole piece to heat and cool
> evenly, and lifting the piece up on wadding can make all the difference i=
n=3D
> the world. Needless to say, the damp, plastic wadding is placed on the k=
i=3D
ln
> shelf, and the slab piece placed on the wadding and very lightly pressed
> down so that it contacts all the wadding and is well-supported.
>=3D20
> There would be no reason to ever use expensive alumina-clay wadding like =
w=3D
e
> use in salt and soda. In a bisque firing that is going to have a good
> preheat, any scrap claybody will do. In any faster firing, especially a
> glaze firing, a 50-50 mixture of flint and any scrap clay will make a gre=
a=3D
t
> wadding for any normal reduction or oxidation firing, either bisque-firin=
g=3D
> or glaze-firing. The flint opens up the body so it can be fired very
> quickly without popping.
>=3D20
> I am sure that you already know much of what I describe above, but I incl=
u=3D
de
> it for the benefit of those who initially asked the question about firing
> large tiles.
>=3D20
> In the bisque-firing, slab tiles can also be fired on edge almost
> vertically, leaning gently against the wall of the kiln, with wadding
> spacers between subsequent tiles, so you can lean a whole series of them
> against each other, using wadding to maintain spacing.
> - Vince
>=3D20
> Vince Pitelka
> Appalachian Center for Craft
> Tennessee Tech University
> vpitelka@dtccom.net
> http://iweb.tntech.edu/wpitelka/
Vince Pitelka on fri 13 jan 12
Bill Merrill wrote:
"I regularly fire 36" flat slabs in my gas kiln and put Kyanite under them
which seems to act like roller bearings. My kiln cools slowly and I use
5/16" Advancer shelves. The slabs end up not warping or cracking. I have
not had a slab break or crack in over 15 Years. It's all in how you fire
and cool, the thickness of your slabs, the thickness of your shelves, etc.
I don't open the stoneware kiln until the kiln is at ambient temperature,
which can be in the case of high fire, 5 days. In school situations 5 days
is too long to tie up a kiln."
Hi Bill-
I am glad that this has worked for you. I acknowledge and respect your
experience and expertise, but I have seen so many slab pieces crack when
fired on sand or grog, and have heard so many reports of same from people
here on Clayart. As you point out above, a five-day cooling cycle just is
not practical in an academic setting, and it is not necessary when the slab
is fired on wads. Getting circulation around the slab makes all the
difference, and the slab can easily move during shrinkage. It's a win-win
solution.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net
http://iweb.tntech.edu/wpitelka/
Bill Merrill on fri 13 jan 12
Hi Vince,
I regularly fire 36" flat slabs in my gas kiln and put Kyanite under
them which seems to act like roller bearings. My kiln cools slowly and
I use 5/16" Advancer shelves. The slabs end up not warping or cracking.
I have not had a slab break or crack in over 15 Years. It's all in how
you fire and cool, the thickness of your slabs, the thickness of your
shelves, etc. I don't open the stoneware kiln until the kiln is at
ambient temperature, which can be in the case of high fire, 5 days. In
school situations 5 days is too long to tie up a kiln. The Kyanite can
be swept off the shelf and reused. Most clays will warp when fired on
their edge if they are as large as the ones I have made especially if
they are thin. My advice is for potters to try everything and find out
what works in your kiln and how your clay body responds to the fire .
My clay body has 8 ingredients in it with different particle sizes in
each clay.=3D20
For those who make multiple feet on their pots ( similar to Shang
dynasty, 13th century bronzes , Chou dynasty, 6th century bronzes) or
like the Rabbit pots of Ken Ferguson, fire those kind of pieces on a
slab with Kyanite under the slab on the kiln shelf. The slab and pot
will shrink with the slab and the feet won't have a chance to catch on
the shelf and cause a crack between the foot and pot. Reuse the Kyanite.
I am only writing what works for me and am not trying to bloviate
elaborately, just relating what works for me.=3D20
Keep making pots, the best ones are yet to be made.
Bill
-----Original Message-----
From: Clayart [mailto:Clayart@LSV.CERAMICS.ORG] On Behalf Of Vince
Pitelka
Sent: Thursday, January 12, 2012 7:46 PM
To: Clayart@LSV.CERAMICS.ORG
Subject: Re: tile/slab breakage.
Bill Merrill wrote:
"Another thing that can help with cracking is too put mullite/kyanite or
silica sand under the slabs to prevent the pieces from sticking to the
kiln shelves during a bisque or glaze fire. The clay shrinks and the
kiln shelf doesn't and the Kyanite/Mullite can be used over, even if
used in high fire."
Hi Bill -
Most of that advice was right on, as is the case with your advice in
general. I don't how much work you have done with large flat slab
forms, but the above is a recipe for real trouble with most such flat
slab forms.
As you know, of all ceramic shapes, flat slabs are least able to
withstand uneven thermal expansion. Sand on the kiln shelf can be a
good idea with very large sculptural forms in a glaze firing when you
know the piece will shift position due to shrinkage, but otherwise it is
generally a mistake because it insulates the underside of the slab from
changes in heat. There is always a trade-off, but the piece certainly is
not going to stick the kiln shelf during a bisque firing, and it will
likely crack from the outer edge inwards if the outer edge heats or
cools much faster than the center.
That is the risk.
As a practical and proven alternative, I recommend raising the slab
forms up on wading. It can be a network of balls of wadding, or
parallel rows of coils of wadding. It should never be a criss-crossing
grid of wadding because that creates closed spaces and causes the same
problem. With round slab forms I have often used a sunburst arrangement
of coils of wadding, but I do not have them meet in the center, in order
to open up more space for passage of air. The objective is to get the
whole piece to heat and cool evenly, and lifting the piece up on wadding
can make all the difference in the world. Needless to say, the damp,
plastic wadding is placed on the kiln shelf, and the slab piece placed
on the wadding and very lightly pressed down so that it contacts all the
wadding and is well-supported.
There would be no reason to ever use expensive alumina-clay wadding like
we use in salt and soda. In a bisque firing that is going to have a
good preheat, any scrap claybody will do. In any faster firing,
especially a glaze firing, a 50-50 mixture of flint and any scrap clay
will make a great wadding for any normal reduction or oxidation firing,
either bisque-firing or glaze-firing. The flint opens up the body so it
can be fired very quickly without popping.
I am sure that you already know much of what I describe above, but I
include it for the benefit of those who initially asked the question
about firing large tiles.
In the bisque-firing, slab tiles can also be fired on edge almost
vertically, leaning gently against the wall of the kiln, with wadding
spacers between subsequent tiles, so you can lean a whole series of them
against each other, using wadding to maintain spacing.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net
http://iweb.tntech.edu/wpitelka/
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