http://sidestoke.com/index.html
This
site is concerned with anagamas and
other woodfired kilns,
woodfired pots and woodfirers themselves.
http://donkey32.proboards.com/index.cgi?board=discuss&action=display&thread=235#
"Pottery kilns need fairly even temperature distribution
and very tight control of oxygen levels within the kiln. While rocket
stoves aim for very clean firings, and pulling every last bit of heat
out before exhausting to the outdoors.
Bourry box kilns are similar
in nature to rocket kilns in that they are designed with a
downward/sideways flame in mind. You can see the basic design here: http://sidestoke.com/bourrybox/bourrybox.html
. I was curious however if there has been any experimentation toward
using something like the Bourry box firebox? In such a design the wood
is loaded up from the top and as is burns it falls though. This allows
you to add a substantial amount of wood at one time."
http://artistpotters.com/anagama/wf_kiln/anagama_diagram.jpg
http://sidestoke.com/bourrybox/bourrybox.html
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The bourry firebox
works on the downdraft principle. The flames are pulled down by
the draft of the kiln, rather than rising as they do in an open
fire, so the fire burns upside down. In the diagram, logs at the
bottom of the stack have burned for the longest time, and fall
onto the bed of embers when they can no longer support
themselves. New wood is added to the top of the stack through the
firebox door (coloured dark blue).
Air for combustion
enters through the woodpile (“primary air”) and the
secondary air is adjusted to maintain the right proportion of air
and fuel for the oxidising, neutral or reducing kiln atmosphere
required. The purpose of the mousehole air is to keep the embers
burning. If stoking is too enthusiastic or the embers stop
burning properly the entrance to the kiln chamber will quickly
clog up and the firebox will cease to operate efficiently.
At the start of a
firing a small fire is started on the floor of the ashpit, where
the embers later build up. When this fire is big enough, usually
when the temperature is in the 350 to 400 degrees C range, logs
are introduced across the hobs. After a transition period, the
firebox starts to operate properly, and all stoking is onto the
hobs.
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Other
views of the firebox can be seen here
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It
is desirable for the burning logs to stay on the hobs as long as
possible, especially if the type of wood used produces persistent
embers. Missing the hobs when stoking, so the fresh wood falls down
and partially blocks the throat arch, is a very bad thing to do. For
this reason the wood has to be cut to the right length, a fact which
can be seen as the main disadvantage of the bourry box. In practice,
especially with Australian hardwoods which produce robust embers, the
natural tendency to stoke more heavily if the temperature fails to
rise can easily result in a partially blocked throat arch. The
counter-intuitive, but correct, action to take is to stoke more
lightly if the kiln stalls.
The mousehole needs constant attention
to make sure it does not clog with ash. It is not necessary to rake
coals out all the time. Stirring regularly is usually all that is
required. It should never be necessary to remove any of the main bed
of embers, although it might be desirable to have a supply of wood of
a type which does not produce large amounts of embers. This wood can
be used for a while if the embers build up too much.
Sidestoke
home
This
is the plan of a kiln with a bourry
firebox. At
the suggested scale it is easily built using standard 9ins x 4.5ins x
3ins bricks, and easily fired. If made with dense firebricks it would
be good for salting. More diagrams and comments will be added later.
The flashy colors are not just for show as some of these areas need
to be identified. The plan can be modified provided certain
proportions are maintained. The diagrams below are based on the plan
set out by Janine King and Steve Harrison in their useful booklet
Layed Back Wood Firing,
the third
edition of which is now
available . Photos of a similar kiln being built can be seen here
(external
link).
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Stoking
is done through the area covered by the firebox lid. The firebox
door is used in the initial stages of firing while the fire is
very small. It is also necessary for cleaning out ash and for
maintaining the mousehole system.
The
mousehole has to be protected by some sort of structure to avoid
clogging and to distribute the air into the bed of embers.
The
number and total area of the exit flues (fig 5) are significant
and must be kept in proportion to the other critical areas if the
plan is modified.
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The
critical dimensions are
A =
cross sectional area of exit flues (fig 5)
B = cross sectional
area of chimney (fig 2)
C = bagwall area (fig 2)
D = throat
arch area (fig 4)
E = floor area of chamber (everything under
the arch) (fig 2)
V = volume of the firebox above the hobs
(fig 4)
W = volume of the kiln (the box capped by the arch)
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Here
are some critical proportions which have been successful
in moderately sized kilns, where in both cases E = 100
First
case
A = 8, B = 7, C = 17, D = 25 and V = 12% of W
Second
Case
A = 11.1, B = 8.3, C = 16.7, D = 26.2 and V = 17.9%
of W
To
make sensible use of these figures it should be kept in mind that the
part of the kiln where the pots are stacked should be approximately in
the shape of a cube.
In
practice, the throat arch area is easily diminished by a build-up
of embers with some woods, and in small kilns it could be made
proportionally larger. Link here
to
see a small double bourry box kiln where I have successfully used
the ratios
A = 7.6, B = 7, C = 14.2, D = 56.5 and V = 11.7% of
W
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Figure
6 shows how the chimney is tapered above the exit flues. The
damper can be made from an old kiln shelf. A sheet of metal will
do but will tend to warp and get stuck in position.
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