David Hendley on tue 14 sep 04
In a bisque firing, anything you can do to equalize the heat, both rising
and cooling, when firing large bowls is good.
If you fire slow enough you won't have trouble on the rising side
of the equation.
Cooling is another matter, and where I always used to have trouble
with cracks.
The problem is that the rims cool faster than the bases. At the silica
phase change temperature, part of the bowl contracts and part of it
does not - and you have a crack. A crack with this cause will go from
the rim towards the center of the bowl.
The base stays warmer because it is in the center of the kiln, it is
sitting on a thick, heat-retaining kiln shelf, and it is usually thicker
in cross section than the rim. That's three different things keeping it
warmer.
Conversely, the rim cools faster because it is thinner and it is close to
the heat-losing kiln wall.
To add to the problem, the run-of-the-mill, average electric kiln is not
adequately insulated to slow the cooling enough.
So, that leaves the potter with four plans of attack:
1. Make the base of the bowl cool faster. You can do this by, as Brian
does, setting the bowl up on small spacers, so it is not in contact with
the kiln shelf. This lets the base cool faster, since it is not in contact
with the heat-retaining kiln shelf.
2. Make the rim of the bowl cool slower. You can do this stacking
things around the rim to serve as heat sinks. Remember that a
"heat sink" is a "heat retainer" during cooling. The more mass the
better. Pots are OK, kiln posts or bricks are even better, but not,
I think, usually needed. This means that during cooling, all these
heated things are radiating their heat back to the rim of the bowl.
The area around the base of the bowl is empty, so there is nothing
to radiate the heat back at those points.
3. Slow down the cooling by turning the elements back on during
cool down, just before the higher silica phase change temperature
(dull red heat). Since the elements are in the kiln wall, close to the
rim, this will bring the temperature of the rim back up even with the
temperature of the base.
4. Slow down the cooling by adding more insulation to the kiln.
I think quite a bit more insulation would be needed if this, alone,
is to fix the problem. My kiln has 3" thick, rather than the standard
2 1/2" thick walls, and it is still woefully under-insulated.
So, pick your plan of action. If you have cracking large bowls,
I would suggest trying more than one technique. I use #2 (slowing
rim cooling with heat retainers) and #3 (turning the elements
on during cool down). I turn all the elements on high for thirty
minutes, about 3 hours after shut down at cone 06.
Firing a bowl upside down would also help equalize the cooling.
However, I don't like it or recommend it because of the stress
it puts on the rim. Especially for a large bowl, which will have to
span 2 kiln shelves, as they are usually at least slightly not exactly
on an even plane.
David Hendley
david@farmpots.com
http://www.farmpots.com
----- Original Message -----
> Are you saying to put pots around the rim ???? I thought heat
sinks
> were used to pull heat away from an object. My thoughts are not to pull
heat
> away from the rim but to get the heavy bottom to cool faster. What i do is
> space
> the bottom from the shelf to promote air movement on the bottom surface.
Since
> i started doing this i have not lost any big forms. I hope i am
> understanding you
> correctly. Putting pots around the rim would act like a blanket and hold
> the heat
> around the rim so it would cool slower ... is that what you are
> saying....Dude !!!!!!!
> What about putting the pot upside down during the bisq. fire. Do you
> think that would
> have the same effect as the above mentioned... Thanks
Brian Haviland on wed 15 sep 04
Hi David
Thanks for all the information. I will try setting my computer
controlled kiln for a slower cool-down when i bisq. fire. This is an
approach i never thought of. I only use a slow cool program when
i glaze fire my pots. I am learning more on the silica phases and
changes at certain temps and things are starting to come together
as far as lost pots over 12 lbs. Plates never looked better thanks to
compression lesson from clayart last year. If i'm ever down texas way
i will definatley be a visitor to your little hacienda / pottery / home..
Thanks again !!!!!
Brian Haviland
At 09:47 PM 9/14/04 -0500, you wrote:
>In a bisque firing, anything you can do to equalize the heat, both rising
>and cooling, when firing large bowls is good.
>If you fire slow enough you won't have trouble on the rising side
>of the equation.
>Cooling is another matter, and where I always used to have trouble
>with cracks.
>The problem is that the rims cool faster than the bases. At the silica
>phase change temperature, part of the bowl contracts and part of it
>does not - and you have a crack. A crack with this cause will go from
>the rim towards the center of the bowl.
>.
>
>David Hendley
>david@farmpots.com
>http://www.farmpots.com
>
>
>
>
>
>
>
>----- Original Message -----
> > Are you saying to put pots around the rim ???? I thought heat
>sinks
> > were used to pull heat away from an object. My thoughts are not to pull
>heat
> > away from the rim but to get the heavy bottom to cool faster. What i do is
> > space
> > the bottom from the shelf to promote air movement on the bottom surface.
>Since
> > i started doing this i have not lost any big forms. I hope i am
> > understanding you
> > correctly. Putting pots around the rim would act like a blanket and hold
> > the heat
> > around the rim so it would cool slower ... is that what you are
> > saying....Dude !!!!!!!
> > What about putting the pot upside down during the bisq. fire. Do you
> > think that would
> > have the same effect as the above mentioned... Thanks
>
>______________________________________________________________________________
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>
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Brian Haviland
Fountain city, Indiana
Haviland Stone Pottery & Raku
bnhavil@greentokai.com
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Gail Phillips on wed 15 sep 04
Okay, but how large is 'large'? At what diameter do you start giving your pieces this treatment?
Thanks for all of the info!
- Gail Phillips, Praying Mantis Pottery (.com), recently redesigned.
-------------- Original message from David Hendley : --------------
> In a bisque firing, anything you can do to equalize the heat, both rising
> and cooling, when firing large bowls is good.
william schran on thu 16 sep 04
David wrote:>Firing a bowl upside down would also help equalize the cooling.
However, I don't like it or recommend it because of the stress
it puts on the rim.<
How about firing the upside down bowl on a bed of fine grog?
Bill
Ron Roy on sat 18 sep 04
I have left Davids post intact and I agree completely with his suggestions.
I have a couple of additions however - what a surprise!
The problem is made worse because some pots are left thick on the bottom
and thin at the rim - this will make the problem harder to solve. Keeping
the rims thicker not only slows the cooling but makes them stronger.
Trimming more clay from the foot and lower sides also helps - and if
balance is important in your work you will find it advantageous to take
more clay from the foot and leave more on the rim.
Bisque to a higher temperature - it makes the ware stronger.
Keeping the problem pots at the top of the kiln is a big help - because
that part of the kiln cools slower.
The reason I don't like firing containers upside down is the chance that
the clay will not oxidize properly in the bisque firing - with some types
of bodies. There is no way to tell when the next load of clay you get will
have some extra carbon - even if you are mixing your own. The same is true
of stacking rim to rim.
Firing ware upside down is a way to slow down the rim during cooling and
prevent bisque dunting - but you would need to take into account the risk
of not having "clean" ware out of the bisque firing.
The biggest problem is with bodies with a large amount of free silica
(quartz) and this is the case with porcelains in particular. Luckily carbon
in these types of bodies is not much of an issue because there is little
iron and it is the fluxing action of the reduced iron that is the main
problem. What I am trying to point out is - firing porcelain upside down is
less risky in overfiring terms due to improperly fired bisque - so firing
on the rim to stop bisque dunting becomes a possibility.
RR
>In a bisque firing, anything you can do to equalize the heat, both rising
>and cooling, when firing large bowls is good.
>If you fire slow enough you won't have trouble on the rising side
>of the equation.
>Cooling is another matter, and where I always used to have trouble
>with cracks.
>The problem is that the rims cool faster than the bases. At the silica
>phase change temperature, part of the bowl contracts and part of it
>does not - and you have a crack. A crack with this cause will go from
>the rim towards the center of the bowl.
>
>The base stays warmer because it is in the center of the kiln, it is
>sitting on a thick, heat-retaining kiln shelf, and it is usually thicker
>in cross section than the rim. That's three different things keeping it
>warmer.
>Conversely, the rim cools faster because it is thinner and it is close to
>the heat-losing kiln wall.
>To add to the problem, the run-of-the-mill, average electric kiln is not
>adequately insulated to slow the cooling enough.
>
>So, that leaves the potter with four plans of attack:
>1. Make the base of the bowl cool faster. You can do this by, as Brian
>does, setting the bowl up on small spacers, so it is not in contact with
>the kiln shelf. This lets the base cool faster, since it is not in contact
>with the heat-retaining kiln shelf.
>
>2. Make the rim of the bowl cool slower. You can do this stacking
>things around the rim to serve as heat sinks. Remember that a
>"heat sink" is a "heat retainer" during cooling. The more mass the
>better. Pots are OK, kiln posts or bricks are even better, but not,
>I think, usually needed. This means that during cooling, all these
>heated things are radiating their heat back to the rim of the bowl.
>The area around the base of the bowl is empty, so there is nothing
>to radiate the heat back at those points.
>
>3. Slow down the cooling by turning the elements back on during
>cool down, just before the higher silica phase change temperature
>(dull red heat). Since the elements are in the kiln wall, close to the
>rim, this will bring the temperature of the rim back up even with the
>temperature of the base.
>
>4. Slow down the cooling by adding more insulation to the kiln.
>I think quite a bit more insulation would be needed if this, alone,
>is to fix the problem. My kiln has 3" thick, rather than the standard
>2 1/2" thick walls, and it is still woefully under-insulated.
>
>So, pick your plan of action. If you have cracking large bowls,
>I would suggest trying more than one technique. I use #2 (slowing
>rim cooling with heat retainers) and #3 (turning the elements
>on during cool down). I turn all the elements on high for thirty
>minutes, about 3 hours after shut down at cone 06.
>
>Firing a bowl upside down would also help equalize the cooling.
>However, I don't like it or recommend it because of the stress
>it puts on the rim. Especially for a large bowl, which will have to
>span 2 kiln shelves, as they are usually at least slightly not exactly
>on an even plane.
>
>David Hendley
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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