mel jacobson on tue 20 mar 12
in response to david's post this morning, here is an old post of mine
that is in nil's book.
it sort of describes the dance.
I remember writing a section for Nils Lou's book the "Art of Firing".
I wanted to illustrate to people that there is no simple answer to
firing. The variables are just too many. For example:
June 16th, at the farm, the sky is clear, hot sun about 88F,
barometer is high. Perfect conditions to fire a kiln. Wind, out of
the south at 8 miles per hour. Happy potters. Six months later, at
the farm, -16F and a wind and snow squall from the north at 20-30
miles per hour and you have to fire the kiln. Barometer is dropping
like a brick in a pond. Notice any difference? Burning incense and
chanting will not help the firing, it may make you feel better, but
you must consider all the variables. In fact, you may have to shut
it all down and wait for a better day. And, remember, that propane
tank is frozen. Another problem to solve.
Yes, good science and engineering. Good clear thinking. You cannot
take out your notebook from June and fire the same way every time.
Learn to drive your kiln, try everything, one burner, two burners,
full gas, low gas, can you move the heat across your kiln?...one burner
on low, one burner full bore. You learn your kiln as you fire. You
memorize what you do with experience. Can you fire your kiln with a
brick in the flame way, move it around, what happens, how do you get
heat to the top, to the bottom? Learn to control your kiln, not the
other way around....it then drives you. It is like driving an
18-wheeled truck; you have to know what you are doing. Some days you
drive over the mountains, the next day you are on the hot desert, and
have to back it up a mile. Then it starts to snow. Same for your
kiln, it changes every day, every load.
What happens when you fire a set of all flat dishes? What happens
when you fire a load of 20 inch bottles? Every load is not a perfect
mix of tall, short, thin and fat. How does your kiln react when you
have every kiln shelf you own in one firing? This is why there is no
precise recipe for firing any kiln. It all depends.
Nils Lou has some great thoughts about high gas pressure and what it
will do and where it will go. If you fire with gobs of pressure the
heat will go lower in the kiln, when you fire with low pressure it
seems to go up.
A tuning brick half way down the flame way will help divert the heat
to the top of the kiln. But, without doubt, you have to find the
sweet spot for that tuning brick. And perhaps it should be a T with
a small brick on top to make a T that you can turn and experiment
with. There is so much to learn, to memorize. It can only come with
time and many firings.
It is like my favorite theory...the dumbest people in the world are
those that make the same mistake, over and over and over. `hey mel,
we fired this kiln 68 times and it stalled at 1900F every time`.
If my kiln stalled even once, I would change things fast. And how
often is the cure for a stalled kiln...turn down the burners or the gas
pressure? Often. Rarely does a stalled kiln need more gas or pressure.
There is a wonderful theory that if your kiln stalls, take off a foot
of your stack, or add a foot to the stack. But, you must do
something. Turn down the gas, turn up the gas, open the damper, close
the damper and then see what happens. In my opinion you have to get
a big early shot of heat. Kiln shelves and furniture have to get
hot, then the pots, then the air inside the pots, then the air in the
kiln, the walls of the kiln. No sense mess'n around. Put the burners
on full, open the damper, get that kiln to 1750F as fast as you
can. Then light reduction and that can happen with just a slight
push in of the damper. Wait for a slight flame to come from the
center peephole.
You can get reduction several ways, in with the damper, open the gas
pressure some, and turn down the primary air on your burner. Often
it will be a combination of those things. I rarely turn down the
primary air; it just stalls the quality of the flame and makes it
dirty. Keep that flame bright blue.
A great way to study reduction is to turn your gas pressure up, close
the damper a quarter of an inch at a time and measure the length of
the flame from the peephole. My own rule of thumb is keeping the
flame about two to three inches long. It will dance, so it is an
average. The backpressure flame will be largest from high on the
kiln, hardly any backpressure at the lowest peephole near the
floor. I always use the mid level peep on the front of the
kiln. The same one, every time. That will let me know from firing to
firing if things are the same. If the kiln is really loaded with
pots and shelves, the backpressure will be different. Learn to read
that. Remember, ` you're kiln will be unique.`
Kerry Brooks had a kiln near a large wall, it was like 5 stories, if
the wind was from the east, and she could not fire. The heat would
not come up the stack, it would come right back down...If the wind
shifted, she would just shut down.
http://www.visi.com/~melpots/
clayart page below:
http://www.visi.com/~melpots/clayart.html
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