Lili Krakowski on mon 6 oct 08
Deborah writes:
" I have this notion that for a glaze
firing, I should set the kiln on slow. Some of the students
responsible for firing want to set the kiln on fast. My thought
is
that slow allows for the glaze to do all the things it needs to
do.
Have I intuitively managed to get it right? Or have I developed a
theory that has no basis in fact?"
Basically I am with you. Slow is better. But there are many
factors to
consider.
If you are once firing then you must go slowly at the beginning
as you actually are bisque firing. All that water, all those
gases
should come out before the clay tightens up. Then there is
that silica inversion which I always must look up, because I
never can
remember it--but one must go through that slowly both on the
way up, and on the way down.
Even in glaze firing one should go slowly at the beginning I
think. UNLESS
the pots in the kiln are thoroughly dry. IF the pots are wet, if
the bisque is
wet, then steam can cause fissures in the glaze and other
problems.
Cardew reminds us 1/3 of kiln heat goes into the walls, the
shelves, the
posts. That IS something to consider. Obviously if one goes too
slowly
a lot of the heat will dissipate to the outside of the kiln. I
am not
talking here about the big huge solid as air-raid shelters kilns
but the average "hobby" kiln.
I "candle" my kilns overnight to allow everything to warm up and
to dry properly. In my opinion that allows for faster firing
later on. And of course one should hold at maximum temp., "soak"
for good results. With these digital kilns firing down is
relatively
easy, and that helps glazes heal well.
You did not tell us what type kiln, size, cones, nor what the
speed is.
So my answer is a sorta vague one. The criterion really should be
how the glazes turn out...
Lili Krakowski
Be of good courage
Tony Ferguson on mon 6 oct 08
This issue with firing speed is this:
1. the nature of your glazes, amount of organics burning out (and the needed time to burn out), off gassing, etc.
2. the nature of your claybody
3. the relationship between the glaze and claybody.
4. the kiln atmosphere, how it heats, evenly, or not, etc.
The only way to know is to test. I have learned to cut down my firing time by half. If you are thinking conservatively, then you would want to fire your kiln as fast as you could to save energy/time without sacrificing the quality of your work.
Test it. Drop your firing time in increments. When the results or work change to your disliking, then you know how fast you can fire.
Tony
Take Care,
Tony Ferguson
Artist...Clay, Web, Photo, Video
...where the sky meets the lake...
http://www.tonyferguson.net
Ron Roy on fri 10 oct 08
Hi Debborah,
You can speed things up at certain stages and slow down at the right places
and get the best results but it takes knowing when to go fast and when to
slow down.
If you don't want to figure that out you should settle for a slower type
firing - if there is no raw clay in a glaze firing you can go quite fast
(150C- 300F) per hour - so a cone 6 firing would then take 8 hours - but
the firing would be uneven and clay and glazes would not bind together as
well as in a slower firing. If you wanted to fire that fast in the
beginning to say 1100C ( 2000F) that would take 7.3 hours and then slow
down to 50C (120F) till 1200C then your firing would take 9.3 hours - add a
20 minute soak while the cone falls - about 10 hours.
I would caution that all glazes should be dry before starting such a
firing. Also - my experience says it's unnecessary to slow down for the
quartz inversion on the way up. If you understand how that works you will
see that the same forces are not a problem on the way up - but can be on
the way down.
There are good reasons to slow cool at certain stages - sometimes to help a
crawling glaze or pinholing to heal over - do that at the top of the firing
- and later to develop crystals between 1000C (1832F) and 800C (1472) -
that will extend a firing quite a bit. You can however schedule things so
that you are there during the important parts - like ramp changes, cone
watching and shut off times.
RR
>Deborah writes:
>
>" I have this notion that for a glaze
>firing, I should set the kiln on slow. Some of the students
>responsible for firing want to set the kiln on fast. My thought
>is
>that slow allows for the glaze to do all the things it needs to
>do.
>
>Have I intuitively managed to get it right? Or have I developed a
>theory that has no basis in fact?"
Ron Roy
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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