OT sort of; kiln firing and aviation

My wife is a potter, and last week we were firing the kiln. It's a gas
kiln, and one of the advantages of gas firing is the ability to get
reduction effects by controlling the atmosphere inside the kiln. We
were doing this, and since I have a greater understanding of chemistry,
I was trying to explain how the various kiln controls actually did their
magic (and it is magic, not engineering, mind you!). As I was doing so,
I started to see a number of aviation related parallels which might be
of interest.

Reduction is achieved by starving the kiln of oxygen, so that the
unburned fuel tries to pull the oxygen from the glazes themselves,
leaving metal ions which give the finished ware its color. The kiln
itself is an updraft kiln; this means that the burners are on the
bottom, and the exit vent is on the top. A couple of fire bricks
partially cover the exit vent, and act as a damper. The whole thing is
a cylinder about three feet wide, four feet tall, and sits on a stand a
foot or so high. It needs to be fired gradually so the ware doesn't
crack, and control inputs have delayed results, so learning how to
control the rate of firing is itself a trick which takes many firings to
learn. Each kiln is different, and one practices by doing test firings
with an empty kiln, or one filled with kiln furniture or test tiles.
It's like going around the pattern to practice landings, except that
each landing takes a full day. I suppose it's really more like learning
to land a glider. You get one shot for each tow. Weather is important
too; hot humid days (high density altitude) give noticably better
reduction effects due to the reduced amount of oxygen present in the air.

Anyway, normally a slight increase in gas pressure will create a quick
rise in atmosphere temperature, but the ware still has to catch up, and
the =rate= of temperature increase decreases after the spike. That is,
if we're going at 250 degrees per hour at a certain gas pressure, when
the kiln gets hot enough the rate will decrease to (say) 150 degrees per
hour and we'll want to goose the pressure up a bit or the kiln will
eventually stall. Having done so, the pyrometer will jump from (say)
750 to 875 in a few seconds, but the subsequent climb rate would be only
250 degrees per hour. So, you have to wait fifteen minutes or so to see
what new climb rate you have achieved. If you did it right, the spike
would put you at the temperature you would have been at had the kiln not
slowed down, and the subsequent speed would be the same as the previous
speed. You keep doing this as you fire the kiln up to temperature
(typically over 2000 degrees F) over the course of the 8 or 10 hours it
takes, with certain pauses for critical temperatures.

Reduction for the glaze in question begins at 1400 degrees or so, and we
closed the damper sufficiently to achieve this. The way it works is
that by restricting the ability of the fire to escape, the airflow is
reduced, thus the inflow of air is reduced, and fuel remains unburned,
which then reacts with the oxygen in the glazes (normally metal oxides)
instead. With fuel remaining unburned, we need to increase the fuel
flow in order that sufficient fuel is burned to keep energy flowing into
the kiln, and keep the temperature rising at the right rate. It's a
balancing act with delayed results which we are still learning. We
wanted to be in reduction from 1400 to 1900, which means we still needed
the temperature to rise, which at first was no problem, but as the
temperature increased, more and more power was necessary to maintain
altitude. Each time we increased the power, we had to change the
mixture so that we could remain in reduction, but there came a point
where I practially had the kiln going full throttle (I've never run it
that way before except as a test) with the damper wide open, and yet the
kiln was stalled. I was not only unable to maintain altitude, the
temperature was actually dropping while flames were roaring out the top
for a good two feet. I was behind the power curve.

I did not expect this. I am fairly new at firing gas kilns; I only have
about 100-200 hours of experience with them, and half of that is bisque
firing, which is not as exciting. But with glaze firing, each flight is
a new adventure.

To start climbing again I had to reduce the power, letting the gas burn
within the kiln rather than outside of it, and of course I had to change
the mixture; the same kind of dance I do in an airplane. We stayed at
least in a neutral atmosphere, although not the fire belching reduction
we were aiming for, and the temperature started rising again. Depending
on the glaze, reduction need not be accompanied by the fire belching we
had, but we're just learning what we need to do to get the results we
want, and with a few weeks between kiln loads, it's a slow process.

No doubt there are many other parallels to aviation in life, and some of
the ones here are rather obvious (after all, both are combustion
processes), but it just struck me that day how universal the analogies
can be.

BTW, the landing was a greaser.

Jose
(posted to r.a.student also because I thought it might be of interest,
but I don't go there. If you wnat me to see a reply, be sure to post to
r.a.piloting)
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