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reduction/gas kilns

updated fri 11 nov 05


mel jacobson on thu 10 nov 05

there is a great deal of work to be done
with potters in understanding reduction, when
it happens, what smoke is....and we have
to destroy a great many ugly myths.

there are some very important factors when
reduction is necessary.
how much?
how do you do it?

i have learned over the years that heavy reduction, black
smoke staining your kiln with carbon, and anything black
coming from your stack is bad.
no need for it.

i like to measure back pressure at the center back peep hole.
use the same one for every firing. ignore the other peep holes.
use just one. the same one.

i never let the flame be longer than (about) three inches...i just measure
the back pressure...i don't want black soot coming out.

there are three things to change to get reduction.
primary air, gas pressure and damper insertion.

you should use all of them in concert.
reduce the amount of air a bit, close your damper a bit, and
adjust the gas pressure a bit.
if you increase the gas pressure the kiln has more pressure in it.
if you close down your damper, the kiln has more pressure in it.
if you cut down the primary air, the flame does not have the clean
blue gas gets dirty. all of these thing produce carbon
in the kiln. or unburned fuel.

early reduction for some shino is good. perhaps cone 011.
i even add wood at this time to get some smoke in the kiln as
there is little energy in the kiln at cone 011. you need a full
kiln of energy to get decent reduction.

i like to think that my kiln goes into lite reduction about cone 08.
i keep it that way for about three hours. i then increase it a
bit at cone 1. leave it alone til the firing is done.
i would say about 5.0 on the oxyprobe. i use lite reduction.
many like 6.5 on the oxyprobe...or good back pressure and black
soot around the kiln. i do not.

i always down fire at 1900 for several hours...oxidized. it evens
the kiln, adds crystals to the glaze. flattens everything out.

heavy reduction is a waste of fuel, does not increase the quality of the
pots and often destroys the clay body with black coring. finding a
perfect level of reduction using all three tools....pressure, primary air, and
damper works the best.

just shoving in the damper without considering what gas pressure you have
or what are your burners are doing is silly.
it is the combination of things that works best.

this allows you to fire the same no matter what the wind conditions are,
what the temp or barometric pressure is doing. you can increase or decrease
your pressure and damper setting to achieve the perfect firing schedule.

the oxyprobe is a great learning tool. even using one for a couple of firings
will teach you what is going on in your kiln. often a quarter of an inch
of the damper will change the amount of reduction dramatically.
i have seen experience potters jaws fall when i show them. `gd mel, that
kiln just dropped to 0 reduction and you moved the damper a half inch, hell
it can't be. and look at all the smoke.`

i have preached for years...`far too much gas is used in many cases
to achieve very average pots.` often the best cure for a stalled kiln
is dropping the gas pressure. most potters just keep turning it up.
i have seen stalled kilns with 15 inches of flame pouring out of a kiln
that is running on ten pounds of pressure. i open the damper, turn the
kiln to 2 pounds, blue up the flame and the kiln jumps 500 degrees in
ten minutes. and then gets finished in no time flat. re/adjust the
reduction and get a fine firing.

in kilns that i fire, i almost never have any flame coming from a low port.

i have, along with bob anderson fired many kilns at the farm using
no reduction on the way up at all, raced to cone 11, turned off the
kiln, down fired at 1900 in reduction and had a perfect kiln firing.
clay body lite brown, perfect reds, spotted iron patterns.
yes, reduction on the way down. old myth broken...about 30 times.

i have also said here many times...over the 40 years i have fired
my neighbors have never known it was firing. no flame,
no smoke...just a little heat wave, and they cannot see that.
just think how long i would have been firing a kiln in a wealthy
suburban setting that i live in....if i had flame and smoke coming
from my stack. the neighborhood grew around me...i was alone
when i built my house. but folks would never had tolerated a
smoking matter how long i have lived here...i had to fire
my kiln with prudence, and it works.

each potter finds a balance in style, function, aesthetics and work
habits. firing the kiln is the same. one method forever may not
be good for you. change things, experiment, keep a few records.

i have seen kiln logs that would make you all laugh.
firing 93
kiln stalled again for the 93rd time.
could not get the kiln to cone 8.

so they fire again the 94th time...same way.
silliness rules.
and then if you really want some your gas kiln
oxidized. see your new pots...things you would never believe
could be that nice.
reduction has become almost a sacred religion. sorry...i have
seen temmoku to die for...from my kiln, oxidize. an entire new
life force.

from mel/

Linda Ferzoco on thu 10 nov 05


Thanks for this lucid explanation of flame-fired kilns. I don't have
one yet, but I'm storing up this info in the little gray cells for
the time when I do. What you say makes a lot of sense to me.

Thanks, Linda

--- mel jacobson wrote:

> there is a great deal of work to be done
> with potters in understanding reduction, when
> it happens, what smoke is....and we have
> to destroy a great many ugly myths.