firing 14" bottom diameter pots

updated sun 14 apr 02

vince pitelka on fri 5 apr 02

> I realize that with a 13% shrinkage, my pots will be
> losing more than 1 3/4" in diameter from their 14" wet
> diameter by the time they're out of the last firing.
> That's a lot of movement.
> So I'm looking for the safest way to bisque (^06) and
> glaze fire (^9R) wide-bottomed pots to avoid cracking.

Curtis -
Many people will advise you to fire on a bed of sand of grog, but I strongly
advise against that. It does allow the bottom to move, but it insulates it
from the heat, which can cause far more serious problems. Just fire the
piece on wadding. Instead of small round wads like we use in salt kilns,
for large pots I use a starburst pattern of 3/8-inch coils. Use a mix of
60% sand or grog and 40% clay (ball clay or fire clay or stoneware clay or a
mix). This mix can be used in a bisque or a glaze firing, and there is no
danger of it popping, because the high percentage of tempering materials
allows gases to escape easily. Be sure to set the pot in place while the
coils are still damp, so that the pot will settle down and be very evenly
supported.
Best wishes -
- Vince

Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/

Fredrick Paget on fri 5 apr 02

Roll out a half inch slab of the same clay big enough to hold the pot , dry
it and put it under the piece when you fire it. The carrier slab will
shrink at the same rate as the piece and keep it safe. Sprinkle some coarse
silica sand under the carrier if it makes you feel better about the lousy
shelves.This method is used in firing big heavy sanitary ware and sewer
pipes to avoid cracking and it works.
Fred


>I realize that with a 13% shrinkage, my pots will be
>losing more than 1 3/4" in diameter from their 14" wet
>diameter by the time they're out of the last firing.
>That's a lot of movement.
>
>So I'm looking for the safest way to bisque (^06) and
>glaze fire (^9R) wide-bottomed pots to avoid cracking.
>
>The 10 lbs of stoneware body is thrown, unaltered, and
>with a uniform 1/2" floor, 14" in diameter, wet. !/8" to
>3/16" of the floor will be trimmed away, leaving the
>bottom 3/8" thick like the walls. I plan on only one
>foot ring near the perimeter along with a 1 1/2" solid
>circle in the center as a support to prevent sagging
>during firing. Does that seem to be a reasonable
>approach? ....................snip........
Curt Nelson

From Fred Paget, Marin County, California, USA

Curtis Nelson on fri 5 apr 02

I realize that with a 13% shrinkage, my pots will be
losing more than 1 3/4" in diameter from their 14" wet
diameter by the time they're out of the last firing.
That's a lot of movement.

So I'm looking for the safest way to bisque (^06) and
glaze fire (^9R) wide-bottomed pots to avoid cracking.

The 10 lbs of stoneware body is thrown, unaltered, and
with a uniform 1/2" floor, 14" in diameter, wet. !/8" to
3/16" of the floor will be trimmed away, leaving the
bottom 3/8" thick like the walls. I plan on only one
foot ring near the perimeter along with a 1 1/2" solid
circle in the center as a support to prevent sagging
during firing. Does that seem to be a reasonable
approach?

The walls are 4" tall, only slightly flared outward, and
have a heavy bead at the lip. Walls are 3/8" thick.
Visualize a large pasta bowl or a dog water bowl.

Firing will be done in a community college studio. That
is, many of the shelves are bowed and warped and the
layers of kiln wash are bumpy and many.

I recall reading that shelves in this condition are
problematic with large-surface-contact pots. Can the
situation be overcome with a careful sprinkling of
silica on the shelf? How about sprinkling alumina?

Or should kaolin/alumina wads be used? If wadding is
best, (I've never used or even seen wads), how big a wad
should be used? What's a reasonable number to use and
where should they be placed . . . only around the
perimeter in contact with the foot ring, or all across
the floor?

And are wads applied moist as you load the kiln, then
kind of gently squished down with the pot to insure an
even support? And then fired while not really
completely dry?

Thanks -

Curt Nelson

Snail Scott on fri 5 apr 02

At 10:04 AM 4/5/02 -0500, you wrote:
>So I'm looking for the safest way to bisque (^06) and
>glaze fire (^9R) wide-bottomed pots to avoid cracking...
>Firing will be done in a community college studio. That
>is, many of the shelves are bowed and warped and the
>layers of kiln wash are bumpy and many.


For lightweight work, it's less of an issue, but
for heavier work, the friction with the shelf
increases proportionally. This being the case, a
wide-diameter footring would better than a small
one, as it will distribute the weight of the pot
more effectively (fewer p.s.i.). So you can see
that it's really more a matter of the weight-to-
support-area ratio than just a matter of either
size or weight. (Most functional pottery doesn't
weigh enough to make it worth modifying the feet
accordingly; I'm just describing the general
principle.)

For bisque firing, the sad state of the shelves
probably won't natter much, unless the studio tech
in charge of loading decides to take advantage of
all that lovely space inside your bowls to load a
ton of other work inside. For the vitrification
firing, though, you are right to be concerned.

I recommend making firing slabs (sort of like a
Japanese 'hama') to support the work while firing.
I use this method on most of my large sculpture,
to reduce the friction with the shelf by better
distributing the load.

Simply make (by throwing or rolling) a slab big
enough to support the object, with a little extra
edge. Make it thick ehough to be sturdy - bigger
work will need a thicker slab. Then make sure the
slab is loaded into the kiln underneath your work.
It will shrink as your work shrinks, but without
the same amount of friction with the shelf, since
it's got more surface area. It also provides your
work with a flatter support - not terribly critical
in bisque, but very handy in the vitrification
firing, especially if your clay is prone to warping.
The slab follows the piece throughout the firing
process, through bisque and on to vitrification.
For very heavy work, I may put sand under the slab,
but for the work you've described, that won't be
necessary unless the shelves are extraordinarily
bad. The clay in the slab does go to waste (unless
you need garden pavers), but it's a lot better
than letting your work get messed up.

-Snail

iandol on sun 7 apr 02

Dear Vince,

I will not disagree with the way you put this. Yes, a bed of sand will =
act as an insulator and may lead to unwanted effects. Depending on the =
temperature to which these platters will be fired, raising them on wads, =
even as thin as one sixteenth of an inch can also have a deleterious =
effect on a wide platter. If the clay becomes pyroplastic, as many do =
when they vitrify, there could be an unacceptable degree of warping.

A light sprinkle of 100# alumina sand will lift the base away from the =
shelf, allow circulation and roll as the clay shrinks. Area of contact =
is minuscule and there will be almost no excessive insulation.

Best regards,

Ivor Lewis. Redhill, South Australia.

vince pitelka on mon 8 apr 02

Ivor wrote:
"I will not disagree with the way you put this. Yes, a bed of sand will act
as an insulator and may lead to unwanted effects. Depending on the
temperature to which these platters will be fired, raising them on wads,
even as thin as one sixteenth of an inch can also have a deleterious effect
on a wide platter. If the clay becomes pyroplastic, as many do when they
vitrify, there could be an unacceptable degree of warping."

Ivor -
Actually, with proper placement of wadding that is not true at all. One
just has to use common sense in the amount of wadding. With the starburst
pattern of coils under a platter, bowl, or jar/vase, or the parallel coils
beneath a flat slab piece, I have never had any problem of wadding, and I
have fired all kinds of claybodies this way.

Ivor wrote:
"A light sprinkle of 100# alumina sand will lift the base away from the
shelf, allow circulation and roll as the clay shrinks. Area of contact is
minuscule and there will be almost no excessive insulation."

In my experience, except in the case of very large pieces, movement is not
nearly as serious a problem as the circulation of heat and atmosphere
beneath the piece. Wadding will always accomplish that much more
efficiently than a bed (or a scattering) of sand or grog.
Best wishes -
- Vince

Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/

LOGAN OPLINGER on tue 9 apr 02

Dear Ivor and Vince,

I've been following this thread since it started, and at first what you were saying Vince made sense, to paraphrase - that a support of clay coils under the pot radiating out from the center in a star pattern would provide for the best circulation of heated air (combustion gasses).

And then I caught your comment Ivor to Vince,

>A light sprinkle of 100# alumina sand will lift the base away from the shelf, allow circulation and roll as the clay shrinks. Area of contact is minuscule and there will be almost no excessive insulation.<

A light went on. Am I missing something here? Don't we want the area of contact between the shelf and the bottom of the pot to be as much as possible, to allow heat to be conducted from the shelf to the bottom of the pot as efficiently as possible? Wouldn't a thin layer of silica or alumina sand about 1 mm. deep allow for the most efficient transfer of heat, yet provide the most support to prevent warping and also allow the clay to move as it shrinks.

I understand that the bottom of an open platter or bowl will receive most of its heat from the top down. But wouldn't a closed shape such as a casserole w/ lid or a large shape taller than it is wider depend more upon heat conducted through the shelf to the bottom, regardless of the type of firing (electric or wood)?

I humbly await your replies,

Logan Oplinger


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Dannon Rhudy on tue 9 apr 02

...following this thread since it started
.... Am I missing something here? Don't we want the area of contact
to be as much as possible.....

Indeed, we don't. Especially for large, wide pieces such as
platters, the heat from the shelf keeps the bottom really hot
while the edge is cooling. This often results in cracked
work. There are a variety of ways to moderate this, but lifting
the piece off the shelf enough to get some circulation is
a good technique. If a piece is footed, the foot ring will
help to moderate the transfer of heat to the bottom of the
work, and a notched foot ring will do even better. But for
large flat work, and for platters without foot rings or with
very shallow ones, finding a secure way to lift them a bit
off the shelf is very helpful in preventing rim-to-center
cracking.

regards

Dannon Rhudy

vince pitelka on tue 9 apr 02

Logan wrote"
> "A light went on. Am I missing something here? Don't we want the area of
contact between the shelf and the bottom of the pot to be as much as
possible, to allow heat to be conducted from the shelf to the bottom of the
pot as efficiently as possible? Wouldn't a thin layer of silica or alumina
sand about 1 mm. deep allow for the most efficient transfer of heat, yet
provide the most support to prevent warping and also allow the clay to move
as it shrinks."

Logan -
I am not sure where you got this idea. The shelves do not radiate any more
heat than the pots do. The heat is transfered through air from the elements
or the burners, and it heats up both the shelves and the pots. Because the
shelves are thicker, they heat up and cool down much slower than the pots,
and as Ron Roy pointed out, that is where the problem lies. After a piece
has been fired and is cooling down, the outer edges or the rim cool fast,
while the shelf holds the heat, and thus the middle cools down slower,
causing thermal shock and resulting cracking. If you elevate the piece off
the shelf with wadding you eliminate this problem. Note that this is a
problem only with large flat objects, or the bottoms of very large vases or
jars. On other work there is no reason at all to elevate the wares on
wadding (except in salt, soda, and some wood firing).
Best wishes -
- Vince

Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/

Lori Leary on wed 10 apr 02

Ivor wrote:

I would be pleased if you would describe this "Starburst of Wads".



Hmmm....sounds like some of that ole Post-Modern stuff to me.....

Lori L.
lleary@epix.net
Mountaintop, PA

Gail Dapogny on wed 10 apr 02

Vince,
I became interested in reading about your wadding technique not so much
because of cracking -- I fire lots of platters which have foot rings, and
they seem to be just fine -- but because you mentioned firing across a
seam between two kiln shelves, something I would never have dared to do --
seems like a crack just waiting to happen. I take it, if you are doing
this, that you place wadding right over that seam? Also, is your wadding
maybe only 1/8 - 1/4 inch thick? Do you have a pretty high success rate
with placing such pots across the seam? Thanks.
---Gail


>If you elevate the piece off the shelf with wadding you eliminate this
>problem. Note that this is a problem only with large flat objects, or the
>bottoms of very large vases or
jars.
>- Vince

Gail Dapogny
1154 Olden Road
Ann Arbor, MI 48103-3005
(734) 665-9816
gdapogny@umich.edu

vince pitelka on wed 10 apr 02

> -- but because you mentioned firing across a
> seam between two kiln shelves, something I would never have dared to do --
> seems like a crack just waiting to happen. I take it, if you are doing
> this, that you place wadding right over that seam? Also, is your wadding
> maybe only 1/8 - 1/4 inch thick? Do you have a pretty high success rate
> with placing such pots across the seam?

Gail -
If the pot is going to crack for some other reason, then nothing is going to
help, but if the problem is truly from lack of air circulation under the pot
and/or limited movement when the piece shrinks in firing, then placing it
on wadding will help a lot. When you use wadding, it makes no difference at
all whether the piece is fired on a single shelf, or spanning two
half-shelves. I have fired hundreds of large platters and bowls this way,
always spanning multiple shelves. When I use coils of wadding I roll them
about 3/8" thick.
Best wishes -
- Vince

iandol on wed 10 apr 02

Dear Vince=20

I would be pleased if you would describe this "Starburst of Wads".

In my imagination, a Starburst is a firework display which sends showers =
of sparks radially from a central point. I have difficulty in relating =
this to placement of wads of refractory under a pot.

Again I do not dispute your experience. But I did qualify my reply with =
a note about excess pyroplasticity as a caution. When I was firing Salt =
Glaze some ten or so years ago I was a regular user of wads to ensure =
pentration of the vapour under the pots to get a good finish. Only had =
problems when I let the fire get awy from my control or I had an over =
fluxed clay.

Best regards,

Ivor.

Marvpots@AOL.COM on thu 11 apr 02

Hi Vince:
Like your book, your explanations, so generously given, are a great source
of
educational information to those of us with far less experience and
knowledge
than yours.

Again, continued thanks!

Marvin Flowerman
marvpots@aol.com

vince pitelka on thu 11 apr 02

Ivor said:
"I would be pleased if you would describe this "Starburst of Wads". In my
imagination, a Starburst is a firework display which sends showers of sparks
radially from a central point. I have difficulty in relating this to
placement of wads of refractory under a pot."

I believe I said a starburst of coils - pretty simple - just coils radiating
outwards from a central point. The spacing is determined depending on the
pyroplasticity of your clay. When properly done, there is never any warpage
or slumping ever, unless the clay is way over-vitrified and starting to
glassify.
Best wishes -
- Vince

Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/

WHC228@AOL.COM on fri 12 apr 02

Good morning!
Many years ago I spent a month at Kohler, where they make sinks and toilets.
All of their objects are large and need to be fired on what they called
"setters."
They would make a piece of clay that looked like a slab slightly larger than
the piece that it sat on.
Since the shelves that these pieces sat on were often slightly warped or
uneven, they used coils of clay under the setters to level everything up.
The setters shrunk at the same rate as the piece preventing cracks and
warps.
When making extra large pieces I routinely use that method to insure
success.
Bill Campbell

iandol on fri 12 apr 02

Dear Vince,

I think we are running into that business of Cultures separated by a =
Common Language thing again.

My imagination comes up with the image of curved bicycle spokes when I =
read <coils radiating outwards from a central point.>>.

However I know you must mean straight rods of clay, not coils like =
sections of a wire electric radiator element

I also gather that you are not contemplating a glazed base for this =
situation.=20

Best regards,

Ivor.