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gas law

updated wed 21 mar 12


David Finkelnburg on tue 20 mar 12

Dear Clayarters,
I have been seriously remiss in my absence from the list but life has
intervened and taken away my reading time. I miss you and your discussions.
Craig Donalsen sent a nice note below. He refers to Charles's law,
which simply says, "At constant pressure as gas temperature increases
volume also increases." Gas, in this case, can be any "gas" including air,
methane (natural gas), propane, water vapor, or as in the case of a
fuel-fired kiln, a combination of most of the above.
Mel has made the point, I am sure, that as the "gas" (kiln vapor)
temperature increases inside the kiln, the volume of that gas grows. It
just expands, like hot air in a balloon.
Example-leave a filled balloon out in the hot sun long enough and
watch it swell until it bursts! In that case pressure also rises, but in a
kiln what happens instead is velocity through the kiln increases because
the expanded gas becomes less dense and the driving force (denser ambient
temperature air outside the kiln) pushes the gas up the stack.
However, that usually is not enough and at the same damper setting
the amount of air coming in drops as the temperature climbs, even though
the amount of fuel coming in stays constant. The fuel flow is constant
(unless you adjust the fuel valve) because the fuel pressure is so much
higher than the air pressure that it gets first shot at getting into the
If your kiln is, for the sake of discussion, at 1,850 F and you set
the damper so there is only enough outside air coming in to just burn all
the fuel, yet there is enough air and fuel to cause the kiln temperature to
rise, you have the kiln at a rising neutral atmosphere. There is neither
oxidation, nor reduction, but the temperature is going up.
If you leave the kiln like that and make no adjustment whatsoever,
sooner or later you will discover the temperature has risen enough to
prevent some of the air needed to burn the fuel from getting into the kiln.
The kiln will now be in reduction. Left alone, eventually the kiln will
reach such heavy reduction that it will stall entirely and any temperature
rise will cease.
To prevent stalling one has two choices. One is to add more air by
periodically opening the damper slightly. The other is to reduce the gas
flow by periodically closing the gas valve slightly.
In my firings I typically adjust both gas and air at the same time,
as most of you fuel firers do. I add some gas and some air. I find an
Oxyprobe to be useful to show me the effect of my adjustments. There are
alternatives, including experience, watching the flame out the peeps and
burner ports, or air being drawn in at either or both locations, for
example. If you insert a small stick in a kiln above red heat and the stick
doesn't ignite you know for certain you are in such strong reduction there
is NO oxygen available to burn the wood. It may smoke as heat drives off
the volatile molecules from the wood but if it doesn't burst into open
flame, which it should above ~500F, heavy reduction is present.
Thanks for letting me relax and visit with you this evening. I won't
be able to check Clayart for a while (drat), for certain not until after
NCECA, but I hope to see many of you there (smile). Come see our
presentations Thursday! I am sure Craig will keep me informed of how this
thread progresses.
Good firing!
Dave Finkelnburg, PE
Democratic Candidate for Idaho State Senate, District 28
8767 W. Pocatello Cr. Rd.
Pocatello, ID 83201

On Mon, Mar 19, 2012 at 8:50 AM, Craig Donalson w=

> Hey Dave,
> Not sure if you are following Clayart these days but NOW would be a great
> time for a refresher on the
> facts of the gas law. Much discussion about turning down the gas as the
> temperature increases but
> no scientific explanation as to why.
> Teaching Ceramic Glazes at SFCC this Spring and so far getting some
> beautiful results with just
> local clay and whiting...or GB....or wood ash.
> Hope this finds you well.
> Best regards,
> Craig