ginny bivaletz on sun 23 apr 00
my question is " does the damper need to be wide open
when i am bisque firing in my downdraft kiln ?".
until i can get an oxyprobe i am only guessing what
the atmosphere is in the kiln. i have read that one
can over oxidize which is just a waste of gas. we do
a 13 hour bisque firing ,climbing at the rate of 180 F
per hour in the second half. to keep that rate we
have the gas turned up to 10 lbs. when we do our
10-12 hr glaze firing, climbing much slower at the
end, we don't need to go above 3 lbs. pressure.
am i giving it too much gas during bisque. would the
kiln go into reduction if i close the damper a bit?
the heat difference between top and bottom is greater
during bisque firings and i am wondering if all that
gas is just going up the chimney, i know that is is
important not to reduce during bisque firing, but can
one fire bisque in a neutral atmosphere? can one have
a neutral or oxidizing atmosphere and slight back
pressure at the peep(no flame,though) ? confused
ginnyB on orcas island, washington
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iandol on tue 25 apr 00
------------------
Gas Bisque Firing
In reply to Ginny B on Orcas Island, Washington
The quick answer is to cut down gas pressure to a minimum. From the start of=
a
firing choose the lowest gas setting that will give a steady flame when the
damper is almost closed. The damper setting should just keep the draft =
pulling
and there should be a negative pressure in the chamber. This can be checked =
at
the spy hole with a lighted wax taper or long Lucifer. The flame will drift =
into
the spy as air is drawn in. Then watch the pyrometer. Increase the gas flow =
by
half a pound pressure each hour if the rate of temperature increase slows =
down
or stalls. Each time you increase gas flow, check the internal pressure with=
the
taper at the spy and adjust the damper to just give an inward flow. It will =
pay
at your next firing to record the pyrometer reading every ten or fifteen =
mins
and plot this onto a graph.
I did this with my kiln for the first three firings. Now I can bisque to =
cone 08
in 8 hours and glost to cone 8 flat in 14. The graph is almost a straight =
line.
If you need more info, please contact me.
Ivor Lewis
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