Michael Wendt on sat 9 feb 08
James,
Efficiency generally increases as the cubic volume
of a kiln increases.
This is due to the relationship between
the surface area if a cube and the volume
of a cube.
Consider:
kiln A 1' x 1' x 1'
outside surface area is 6 square feet
volume is 1 cubic foot
kiln B 2' x 2' x 2'
outside surface area is 24 square feet
volume is 8 cubic feet
kiln C is 4' x 4' x 4'
outside surface area is 96 square feet
volume is 64 cubic feet
The relation then is
As area/volume is 6/1
Bs area/volume is 24/8 which reduces to 3/1
Cs area/volume is 96/64 which reduces to 3/2
The real problem is large kilns take a lot longer
to fill.
I fire 12 cubic foot Olympic style updrafts.
We can still fire a load every other day if
we need to but when it is slow, our order
turn around time is "order on Wednesday,
out on Saturday.
Try that with a 64 cubic foot kiln.
I recommend a size that will fire
1-2 weeks work at a time so there
is not too much delay between loads.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave.
Lewiston, Id 83501
U.S.A.
208-746-3724
wendtpot@lewiston.com
http://www.wendtpottery.com
http://UniquePorcelainDesigns.com
James Myers on sat 9 feb 08
Hi All,
I have a student who is thinking about building a small gas kiln. Does
anyone out there have any experience with the propane usage between say a 10
cubic foot kiln and a 40 cubic foot kiln? I'm wondering what the
relationship of fuel consumption is if you fire both kilns in same amount of
time, 7 hours to cone 10. The larger kiln would obviously need to have a
stronger flame as the firing progresses to finish at the same time as the
smaller. But you will have to fire 4 loads in the smaller kiln to produce
the same amount of work as the larger kiln.
Cheers
James Mitschmyer
Peterorough NH
Joseph Herbert on mon 11 feb 08
Michael Wendt wrote: We can still fire a load every other day if we need to
but when it is slow, our order turn around time is "order on Wednesday, out
on Saturday. Try that with a 64 cubic foot kiln.
A balance between being able to fire work of a certain (large) size and the
time required to fill a kiln with more normal sized work is required. From
the point of view of a shared studio space person, a large kiln can be a
detriment to learning. The Clay Studio in Philadelphia and the Karamu House
in Cleveland both have relatively large (70 - 100 cu. ft.) gas kilns. The
Clay Studio has rather large usage (lots of classes, studio time, resident
artists, visiting artist) but even so the kiln cycle time seemed like three
weeks. The Karamu House seemed like one firing per session (one class of 8
people). When one is trying to become familiar with glazes, since all
glazes are local, having so few cycles of firing means the learning curve is
forced flat. The rate of gaining experience is forced to be low since the
number of available trials is small and distributed over a long time.
One of the advantages of electric firing, at least in shared studio
situations, is the smaller kiln size. They fill faster and studio
technicians are more willing to fire them, even loosely loaded. In
addition, the size of a ware load and how it is stacked has a significant
effect on the result of the firing in a fuel kiln. This is less true in a
electric kiln. The mass of the material heated in the electric kiln makes a
difference in the heat up and cool down rates but not nearly so much an
effect as the alteration of the flow of gasses in a fuel kiln, empty vs.
full. Since the glazes in an electric kiln studio are not expected to show
the serendipitous effects that one finds in fuel firing, the small
variations in firing has almost no effect on the quality of the fired
glazes.
One of the outstanding (as in really memorable) features of the sole group
wood firing I have attended was amateurs with axes. This was a result of
the way the firing was planned and provisioned, but still... According to
reports, after I finished my shift and went home, there were drunken
amateurs with axes. No injuries, thankfully. This was 100 cu. ft. kiln in
eastern PA. It required the efforts of several groups of students to
provide enough ware to fill it and the labor of a significant fraction of
them to fire it. The size of the kiln and its configuration made firings
far from routine. The people associated with the studio did not want to
deal with the logistics of firing very often. In addition, the kiln was
fairly quirky, so there were few people who knew the accepted firing
technique. Perhaps if the kiln had been smaller, it could have been fired
more frequently, more people would have learned to fire it, and the results
(as opposed to just the process) would have been more satisfactory. That
studio's most used fuel kiln was a commercially produced, 32 cu. ft. gas
kiln that was fired weekly.
Joe
Joseph Herbert
Technical Writer
Irving, Texas
214-725-8305 (Cell)
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