David Hendley on wed 13 oct 04
I also think about Ron Roy when I fire off a load of bisque.
I don't want to quibble about what works for other people,
but here is my experience with bisque firing, with a hard-earned
and important lesson.
I had gotten cheap and lazy.
After decades of bisque firing, I thought I knew what cone 06
looked like, so I stopped using a cone and fired according to
time and atmosphere color.
Gradually, the temperature dropped. Hey, what's wrong with
saving a few bucks worth of electricity?
All went fine until I started making some large bowls (16"
diameter and greater). More than half cracked. Cooling cracks
which ran up the side from foot to rim.
This went on for months.
Many potters I queried had suggestions, such as adding
things to the claybody, firing slower, not nesting the bowls, etc.
Finally, Ron Roy asked the right question: Are you bisqueing to
a full cone 06?
Well, no, I didn't know exactly how hot I was firing.
Of course, I put a cone 06 in the next firing and the problem
was solved.
See, there really is a pretty great difference in the strength of ware
fired at C 08 vs. C06 - at least with my claybody. The stronger
clay can take the phase changes on the way down, but the
flimsy soft bisque cannot.
It was when I was having this problem that I also developed
my techniques for slowly and evenly cooling the bisque
kiln, which I sent into Clayart last month.
I will attach that message below, in case some missed it. It really
has cut my losses of large pieces in the bisque kiln.
So, that makes two very important considerations for successful
bisque firings, especially when firing large bowls:
1 - bisque to a full cone 06.
2 - stack the kiln and monitor the cool-down so it is slow and even.
Add to that number 3, which is Ron Roy's favorite tip:
3 - fire slow, especially when starting, to burn out all the organics.
Any aesthetic considerations, such as how fast the glaze is
absorbed, or economy considerations, such as firing faster and
cooler to save electricity, take a back seat to these technical facts
of life.
David Hendley
david@farmpots.com
http://www.farmpots.com
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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
Ron Roy on sat 16 oct 04
Hi David,
I - like you learned the hard way - but with 18" platters.
I finally discovered - my soft fire brick dampers had scrubbed down over
the years - like 600 firings - in my gas kiln - leaving a 1/2 inch gap. So
I thought the damper was closed properly when it was still open more than a
bit.
So cold air was being pulled through the kiln all during cooling. It was a
slow cooling kiln - took at least 3 days - but that did not stop the cold
draft from doing it's dirty work.
Stacking bowls and platters rim to rim would do the job as well - but -
like firing them upside down - any gases would be trapped inside and that
could be a factor leading to overfiring of clay from reduced iron.
It does not take many loses to negate any savings from firing lower - soft
bisque has no advantages in my book - and in fact - makes it harder to
control glaze thickness.
RR
>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
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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