| Answers from the CM Technical Staff
Q: In the article "Striving For Perfection: Energy, Efficiency &
Perfect Combustion," [available on this page] in the December 2007 issue, Hal Frenzel defined
oxidation and reduction atmospheres and characterized the associated
kiln exit-flame colors. In our studio, I've noticed that when backing
off on the gas-to-air ratio, flame color coming out the flue changes
from deep yellow to pale yellow to faint green, and then finally to
bluish.
There is a debate in our studio as to the meaning of these colors. One
school of thought believes that exit flames of any color indicates
unburned fuel emanating from the kiln and bursting into flame in the
outside air, thus indicating a lack of oxygen in the kiln (reduction
atmosphere). Another school of thought holds that only the yellow flame
indicates reduction. Any greenish or bluish tints in the flame indicate
oxidation. So which is right?-M. B. |
A: Interesting debate you are having in your studio-and good observation of the flame colors! ANY flame coming out of the flue of your kiln indicates reduction (lack of oxygen) inside the kiln. A neutral to oxidizing flame will occur at the outlet of the burners, well inside the kiln and before the kiln gases reach the flue exit. What you are seeing at the flue exit is indeed the fuel burning once it hits the air outside the kiln and gets the oxygen it needs to ignite. The color of the flame tells you much about the degree of reduction present. The blue flame indicates mild reduction. The deep yellow flame indicates heavy reduction. The other colors are variations between those two.
Dave Finkelnburg, CM Technical Editor
A Probing Question
Oxygen probes are the most accurate way to measure atmospheric
conditions in a kiln. They compare oxygen levels inside and outside the
kiln. A sensor, which reacts with oxygen above red-heat temperatures,
is half encased in an alumina tube that is inserted into the kiln, with
the open end outside the kiln to allow the ambient air to flow into the
sensor. The half of the sensor outside the tube is exposed to the kiln
atmosphere, and the half inside the tube is exposed to the ambient air
in the room. When each end of the sensor is attached to a lead, the
difference between oxygen levels will produce a small electrical
charge, which is readable by an electrical multimeter.
Some oxygen probes also read temperature, since one half of the circuit
is completed using one lead of a thermocouple. The diagram above
illustrates this structure.
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