Ceramic for Stoves Part 3b- Test Firing Local Clays- Re-discovery of a "Natural" Kiln

Richard Boyt, June 2003

Again, greetings from Pottershop Holler, where the sky gets smaller as the trees grow taller.

Before I continue with ideas on primitive kiln and stove designs, I want to backtrack to fill a few holes
I've left in earlier writings. Sometimes I have called for equipment or materials that are not likely
to be available to a primitive stove maker. If a pick ax is not available for digging clay, you could use a
sharp forked stick. To remove excess water from the clay slurry, you could use sheets of newspaper,
cardboard, plastic, metal, tin cans, plates, tightly woven cloth, buckets, tubs, flat rocks, broad leaves,
animal skins, wooden boards and bowls-- anything that can hold the slurry and absorb and/or evaporate excess
water can substitute for the plaster-of-paris bats and bowls I previously called for. After we build a fair
sized kiln, we can make all the bats, bowls, and tubs we'll ever need.

I suggested that very early kilns may have been specially shaped bon-fires, and I suggest that Mother
Nature herself may have shown one way to build them. Lightning sets fire to the forest and finds the hollow
center of a tree. The tree then becomes the stove, the fuel, and the chimney. This combination creates a
reverberatory furnace with the heat bouncing back andforth between the glowing red hot walls. Very hot,
and a roaring torch of flame out the top. On a related note, I just heard over the BBC that in
Tasmania, the world's tallest tree "El Grande", some 74 meters (242') high and nearly 400 years old has
burned after accidentally catching fire while workers were clearing brush. I'll make an uninformed guess
that it was hollow with openings at its base and along the trunk where limbs had been discarded. It would be
very difficult to stop a fire like this, once started.

A few years ago, we harvested some big old declining oaks and found a few that were hollow and useful only
for firewood. We cut one to firewood lengths and propped a half-meter (18") chunk on end on several
rocks to make space at the bottom for a draft, and built a fire inside. It smoked and flamed a bit at
first, but once it got going, the entire interior glowed, but with no sound, no flame, no smoke. Just a
pale transparent violet haze dancing over the surface of the red-hot coals and an invisible steady hot
column of gasses coming out the top. We lit a few of these "stump stoves" for family gatherings, and with a
grill on top, cooked hamburgers; and with sharp pointed sticks, cooked hot dogs. The heat was so
controlled that I found I could roast marshmallows with my fingers without burning fingers or
marshmallows. More convenient, however with a short throwaway plastic fork that stays cool so the
marshmallows don't slide off. Try that over a bon-fire.

A chimney made of a stack of tin cans can give the fire quite a "kick". Remove the ends of several cans.
Crimp one end of each can with the jaws of a modified pair of pliers. Insert the crimped end of each can
into the uncrimped end of another until you have the height you want. Make an adapter out of a larger can
or metal pail to bridge the opening at the top of the stump. Light weight, easy on, easy off (more details
on how to make tin can chimneys later).

So it is growing dark and the party is over, and it is time to break camp. No need for flashlights-- just
drop in a couple of sticks. They burst into flame almost instantly, and a half meter (18") plume of
bright yellow flame races upward, lighting the landscape. Now, time to put the fire out. Two of us
lift the still burning, yet heavy stump stove off its supports and place it firmly on soft ground to cut off
all combustion air. Amazingly, it has been burning steadily for two hours, and the outside isn't even
warm. This leads me to conjecture that wood doesn't burn (Das, you'll like this!). When hot enough, wood
pyrolizes, giving off combustible gasses. It is these gasses that burn and in burning heat the wood to cause
it to give off more gasses which burn. I suggest that this is the much same effect as seen in boiling water.

Adding heat does not raise the temperature of the water, it only makes it boil more quickly. The
boiling off or evaporation of water vapor literally keeps the water from getting hotter. Same with wood.
Starting off at about 250 degrees C (500 degrees F), the boiling off of gasses keeps the wood from getting
hotter. Once completely pyrolized, the wood becomes char, but char does not burn. Like wood, it vaporizes
and the gasses burn to heat the char to drive off more gasses. I'd bet this is a considerable
over simplification, but observation suggests that this may be at least partly true.

Time to leave. Slosh the glowing insides with water--a toy water gun works for stubborn hot spots. Cap the
top with a hunk of well-weighted sheet metal sealed by a gasket of wet wood ash. Charcoal can really be
stubborn in not going out. If it can, it will find even the smallest leak of air. However, we used one
single stump stove for three parties before it finally burned out one side and collapsed. Save any char that
is left. We can use it later to make a lightweight ceramic insulation. Also save ash. We can use it to
make a high-potassium liquid that serves as a semi-glaze. I have used it to harden the surface of
lightweight ceramic insulation. More on that later.

If hollow stumps are in short supply, split a big solid chunk of wood, hew out the center, and bind the
outer pieces back together with bailing wire, big rubber bands cut from old inner tubes, or anything
else that will hold the wood pieces tightly together. Use wet wood ash or clay slurry to make airtight
gaskets where needed, or to plug open knot holes or beef up thin places. To get a really hot burn, stack
the stumps. High enough, and it ought to roar!

This whole idea of a stump stove is so simple that I think it likely that prehistoric people used it to
fire their pots. Careful examination of chunks of unburned char can show how it was burned. The
curvature of growth rings and the character of the burned surfaces suggest fire inside a cavity.
However, the few archeologists I have talked to admitted that they had not heard of the idea of very
early stump stoves.

I admit, I cut my stump stoves to length with a chain saw. Not much chance of that in a primitive
environment, but cross-cut saws and axes are not uncommon in underdeveloped countries. A chain saw
could cut vertical slots deep into the log to prepare a hole. A long drill bit could make a pilot hole, and
careful firing might widen it enough to make a stump stove. Perhaps the greatest value of a stump stove is
its demonstration of the principle of reverberatory geometry that can be used to create efficient cooking
stoves as well as kilns.

So we have a small, very primitive kin capable of firing clay. Many other kiln designs could do the
job, but most of them involve heavy, stationary structures and lots of work, materials, and techniques
not available to very primitive stove makers. I encourage readers to explore and report on other
designs for simple kilns. I confess that I'm still getting a lot of use out of my tabletop electric kiln.

Next entry should be Ceramics for Stoves: 3C- "Test Firing Local Clays Using a Primitive Kiln".

In the mean time, prepare several disks of plastic clay about 1 cm (1/2") thick and 3 cm (1.75") in
diameter. Weigh each one accurately, then dry and weigh again to determine the percentage weight of
water required to reach plasticity.

Sorry this goes slow, but I'm still figuring out how to make it all work. Be patient. I think I know
where I'm going. By the way, use a mirror when you look down the inside a hot tin can chimney, and oh
yes, don't worry-- you eyebrows will grow back.

Dick Boyt
20479 Panda Rd
Neosho, MO 64850

Note: See other articles by Richard Boyt

Ceramics for Stoves

Practical Tips For Potters Making Improved Cooking Stoves