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experimental wood-fired earthen kiln

updated mon 31 mar 97


Kevin P. O'Hara on tue 25 mar 97

Hello everyone,

Well, I had fun this last weekend setting the hill on fire. I made a kiln
in the side of the hill by my house. I dug out the remains of an old
varmint hole and converted it into a firebox and dug a hold on top of the
hill to create a firing chamber. I finished off the top with a few stray
fire bricks and a bunch of red clay bricks. I then used a four foot section
of heating duct for a chimney. the firing chamber is only big enough for
about 2 or 3 average sized pots at most. I fired greenware in it twice this
weekend. The first firing was with only one pot and I was only able to just
barely reach 1500 F. The second time I increased the size of the firing
chamber and made a make-shift fire-grate to hold the wood so I could more
easily scoop out the coals and ash. The second firing heated up really
fast, but when I hit 1500F, I hit a wall and could not raise the temperature
anymore. At one point, I cleaned the ash and coals and stoked the fire and
it jumped to 1600F but it went back down to about 1450F. I was using a
pyrometer to measure temperature so it may have been hotter. I was not able
to create a peep hole, to view cones, in the side of the hill. After it had
burned down, I took the top off the kiln and filled it with dead leaves and
such and pit-fired the pots to give them some color. More fire ;-)

My question is this: Does anyone have any suggestions on how to get this
thing up to about cone 06 or 05, and does anybody know of any good reliable
cone 014 - 012 glazes ?

Someone I know here suggested firing with hard Coal. I haven't heard of
anyone firing pottery with Coal.

I took pictures, so I will document the adventure and post the experience on
my web site. Since I don't have a scanner in my back pocket, it will be
about a week or so before I can get them up on the Net.

Happy pottery and fire making.

Kevin P. O'Hara
Mudslinger Pottery

Gavin Stairs on wed 26 mar 97

>My question is this: Does anyone have any suggestions on how to get this
>thing up to about cone 06 or 05, and does anybody know of any good reliable
>cone 014 - 012 glazes ?

Hi Kevin,

I think you've discovered why the ogama style kiln was invented. The flame
temperature of your wood fire is around 900C, or just above 1600F. In order
to get a kiln hotter than that, you have to preheat the fuel and/or air.
This is what happens when you have a large space for the firebox, and place
the wood right inside. The heat from the previous firing can now heat the
wood and the pyrolysed wood fuel gasses, and the ash bed can heat the
incoming air, with the result that the flame temperature can slowly rise to
the porcelain firing range.

The ogama kiln (literally "honorable kiln", if I've got the japanese right),
and similar designs from other places, place the fire right in the pottery
chamber. It still takes a lot of firing to get such a kiln hot, though.
You can really see the value of this reverberatory heating of fuel and air
in the noborigamas, with several chambers climbing a hillside. It may take
a long time to fire the first chamber, which is ogama style, but the
subsequent chambers, which are preheated by the exhaust from the first, and
which are fired by stoking small wood directly into the hot air stream from
the already fired chambers below, go very quickly.

In european kilns which reach cone 10, the firing chamber may be separate
from the pottery chamber, but it is designed to reflect heat onto the fuel
in the same way as firing directly into the pottery chamber. The fire box
is made massive and spacious, with a generous ash pit below. The radiant
heat of the fire heats all the fire box, and the ash as well. Ash may be
sintered into a klinker by the intense heat, but it helps to preheat the
air, while the reflected heat from the fire box heats the wood before it
burns, both effects giving a hotter fire. A long flame also contributes to
this, because the flame gasses can be preheated even more before burning.
But it still works with shorter flames, as from coal and coke fires.

Incidentally, early blast furnaces used a similar system. The fuel,
charcoal or coke usually, is mixed with the furnace charge, and therefore
heats along with it. The air which burns it enters through long nozzles
which penetrate the fire itself, and are therefore very hot. So the air
becomes hot before it reacts wit the fuel, resulting in an intensely hot
reducing fire, which smelts the ore.

Gas fires are much hotter, and don't need this reverberatory heating. A
neutral natural gas fire in air can reach ~2000C (3650F), which is far
hotter than required to fire pottery. That's why gas kilns fire to cone 10+
much faster than wood kilns, and are easier to design. It's also why they
fire more evenly. The burning is all over before the hot gasses come into
contact with the ware, so the ware experiences fewer temperature
fluctuations from direct flame contact, and fewer variations in
reduction/oxidation condition from turbulent flames.