Jeff Abney on thu 30 sep 04
I'm sure this is an extremely elementary topic to most folks on Clayart,
but I'm hoping someone can take the time to answer a few questions for me
regarding reduction. I have previously used only an electric kiln except
for raku, so I'm not sure at all of the process for achieving reduction
using a gas kiln.
I have a soft brick kiln with an expanded metal lid (lined with fiber
blanket). It is approximately 15-cubic feet in size, with a flue on the
top, and a venturi gas burner (propane) on the side at the bottom. The
burner was purchased from Marc Ward, so I'm quite confident that the
pressure he recommended matches the orifice in the burner so that I have
enough BTU's to reach at least raku temperature.
I have not yet attempted a cone 10 reduction firing -- is this possible
with the kiln I have described? I understand the concept of reduction, but
I'm not quite sure of the procedures involved to do something like this
example -- I have a glaze recipe that calls for reduction to begin early
(around cone 012) and be maintained throughout the firing, up to cone 10.
Then a (two-hour oxidizing soak is called for.
So, if you're still with me,
1.) Is it possible to do this with the kiln I have?
2.) How do I make reduction begin to happen?
3.) How do I maintain reduction and still have the kiln rise in temperature?
4.) How do I do the oxidizing soak after reaching temperature?
5.) Should I seal the kiln as much as possible during cool-down?
If anyone is willing to tackle this, thanks in advance!
Rod Wuetherick on thu 30 sep 04
1.) Is it possible to do this with the kiln I have?
I would suspect that yes is the answer.
2.) How do I make reduction begin to happen?
4 ways
Most influential
Restricting the exit flue
- with a piece of shelf etc.
Increasing the amount of gas
- Wouldn't recommend this method with a fiber kiln. As there would be
little back pressure.
Less influential
restricting primary air
- affects flame length and shape
- lots of primary air short flame
- less primary air long flame
restricting secondary air
- on smaller kilns fully blocked secondary air ports can drastically
improve the stability of the atmosphere. You will often need to block these
on smaller kilns to achieve a neutral atmosphere.
3.) How do I maintain reduction and still have the kiln rise in temperature?
Carefully! You would use your pyrometer to watch for temperature drops
adjusting the above mentioned things as required.
generally a heavy reduction will cause your burners to whine and you will
get a 12-14" flame out of your spy hole.
4.) How do I do the oxidizing soak after reaching temperature?
Back off all of the above mentioned - reduce gas and wait. Watch your
pyrometer to make sure you are not gaining temperature but holding at
specific temperature.
There is quite a bit of "Yes it works - No it don't" controversy over
oxidizing at the end of a firing. If you have a kiln that is quite large
over 40 cubic feet you MAY notice some difference. Smaller kilns the chamber
clears out instantly anyway. Please understand though your mileage may vary
and some potters swear their glazes improve with oxidation and I do not
doubt them. I will say with the glazes I use I have never noticed a
significant enough difference to bother.
5.) Should I seal the kiln as much as possible during cool-down?
You should try but with that fiber top you are going to cool down like a
rocket. I personally have not fired many fiber kilns and have always used
IFB/hard brick kilns. i would suspect that many fiber folks fire down
depending on the work and glazes they are using. I do know that true matte
glazes are next to impossible to get in a fiber kiln unless you fire down.
Though if you are firing true matte glazes many factors come into play that
it takes some experience and lots of good detailed firing logs to get it
down pat. Just for the record an under fired glaze recipe is not a matt
glaze - it's an under fired glaze ;)
Good luck and hope this helps you a bit.
Cheers,
Rod
Snail Scott on sat 2 oct 04
At 02:02 PM 9/30/2004 -0400, you wrote:
>...a few questions for me
>regarding reduction...^10...
>1.) Is it possible to do this with the kiln I have?
Higher temperatures depend on the amount of insulation
and the power of the burners.
>2.) How do I make reduction begin to happen?
Restrict the amount of secondary air entering the kiln,
or tighten the damper, or increase the gas, or any
combination of these. Which is best will depend on
you particular kiln.
>3.) How do I maintain reduction and still have the kiln rise in temperature?
If the reduction is not too heavy, you will still get
temperature rise; just not as fast as in oxidation.
>4.) How do I do the oxidizing soak after reaching temperature?
Reverse whatever you did to get into reduction, i.e.
open up the damper, increase secondary air, or back
off the gas.
Remember, on most kilns these are fairly slight
adjustments. Take it easy at first!
>5.) Should I seal the kiln as much as possible during cool-down?
Yes, if you want a slow cool. If you want to crash-
cool or reoxidize during cooling, leave it open. You
can also combine these: crash-cool at first, then
seal it up for the later phases of cooling.
-Snail Scott
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