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Wood Firing at the Appalachian
Center for Craft
At the Craft Center we recognize the importance of
preserving and encouraging soda, salt, and wood-firing traditions. The results
gained are available through no other firing process, and wood firing is part of an unbroken continuity of
technology dating back to the
Paleolithic era.
We have two wood kilns - the "hoggama," a hybrid
groundhog/anagama, and the
Casebeer/Dillon Bourry box kiln. We always have piles of miscellaneous
bricks around, and plenty of space to build small
experimental wood kilns. In support of wood firing, we have three
Stihl chainsaws, a 30-ton ram-force gas-powered log splitter, and plenty
of room adjacent to the kilns to store firewood under-roof. |
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The Casebeer-Dillon Bourry-box kiln was designed by Doug
Casebeer, head of the ceramics area at Anderson Ranch, and was built under
Doug's guidance by Jane Dillon and her students in 1988. It was featured
in a cover story by Jane in Ceramics Monthly in 1989. It is a
two-chamber climbing kiln with 40 cubic feet per chamber, and the second chamber
is salt. It features the Bourry firebox design, where the throat arch
leading to the ware chamber is located very low beneath the level of the
wood. It also features "hobs" rather than conventional
grate bars to hold the wood suspended above the coal bed. There is a primary Bourry firebox before the first chamber, and a
secondary one between the two chambers. This kiln is fired with edge trimmings
from a local hardwood mill. This kiln carries less ash into the ware chamber than the hoggama, and
thus gives less ash-slagging, but produces very nice flashing. We
usually preheat overnight with gas burners, and then bring the first
chamber up to cone 10 in about 24 hours. Since the outgoing flue
from the first chamber enters the second, by the time the first chamber is
done, the second is already around cone 1 or 2, and it only takes another
eight hours to bring it to maturity. |
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The "hoggama" was designed by Stephen
Robison in 1998 and built by Steph, me, and my students. It has
approximately 200 cubic feet of stacking space, and combines elements of both
the traditional groundhog wood kiln popular among Piedmont and
Appalachian folk potters, and the Japanese anagama. The primary
shortcoming of many groundhogs is uneven firing temperature from front to back
due to the low ceiling, long level kiln profile, and short wide chimney.
Common anagama designs often feature a floor that rises in several steps, one or
more side-stoking ports, and a taller, narrower chimney. We combined
the two designs and achieved exactly the desired results, a kiln which has good
draft, is easy to control, and fires evenly from front to back. Steph's
article on the hoggama appeared in Ceramics
Technical, issue #8 (1999) pages 50-56.
We load the kiln through the door in the front, and then brick up the
door to leave a one-square-foot stoking port. |
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The hoggama main firebox is fired with a mixture of pine,
oak, and poplar split firewood, and for quick heat rise we use hardwood planks from a
local furniture factory. In the second half of the firing we feed hardwood
edge trimmings through the side stoking port to bring up the temperature in the
rear. We generally preheat for six or eight hours with a fire out front, which draws
heat in
through the air ports. After moving the fire into the firebox we take
about 24 hours to get to cone 10 or 11 and hold it there for at least 36
hours. In the past we have occasionally fired as high as cone 14,
but seem to get better results with a lower maturation temperature. |
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We work in six-hour two-person shifts, one doing the
back-stoking and the other in front. Additional people are usually
on-hand, but we can fire this kiln efficiently with two people per shift. The kiln fires
very efficiently. During the last 36 hours of the
firing, if we back-stoke three times with a small handful of 36" edge
trimmings to each 8-10 minute-interval front-stoke of three or four chunks
of split firewood, temperature holds. If we back-stoke two times to
each 5-7 minute interval front-stoke, temperature climbs. If we
front-stoke more frequently, temperature climbs quick. |
| In a wood kiln of this type, ash slagging generally makes the
cones fall 1 to 1 1/2 cones earlier than in a gas
reduction kiln. We like to fire to cone 11 flat with cone 12 bending,
which translates to an actual firing of around cone 10. During firing
we minimize agitation of the coal bed in order
to avoid heavy ash slagging on the wares. At the close of the
firing we stoke the kiln generously, and as that stoke burns we quickly
mud up the stoking port and the front and crown ports.
We always arrange to finish the firing just after dusk, and once the
kiln is sealed up, there is generally a good crowd to watch that persistent flame at the stack.
We generally unload after two or three days of
cooling.
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If you have questions or comments about kilns,
facilities, or the clay program at the Appalachian Center for Craft, please contact
me.
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