The bourry firebox: how it works
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.
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.