Ceramic for Stoves Part 3a- Test firing local clays- primitive kilns

Richard Boyt, May 2003

Again, greetings from Pottershop Holler (old maps). Much of this was written under a week of tornado watches and warnings, but at this moment the sun is
brilliant and the sky a deep blue even to the horizons.

First, I want to thank the several people who have encouraged me to continue this journey.
It will be useful to know how the samples of clays collected in Part 2 will respond to the high temperatures that can be reached in the combustion chambers of stoves. I have the luxury of various electric kilns capable of reaching more than 1100 degrees C (2,000 degrees F) (cone 03), in as long or as short a period of time as I may wish. But that violates the goal of describing firing techniques that might be used by potters in pre-industrial rural areas of third world countries where 30 amps of 220 volts is only a dream. Yet, they do make and fire their pots, and they have for thousands of years.

Historically, I know of no major culture that has not made pottery. In fact, it can be contended that pre-historic societies made the discovery that heating clay to a temperature approaching red hot turns it into a kind of stone, and that food could then be cooked and/ or stored in pots of fired clay. This discovery may have been instrumental in permitting nomadic hunting tribes to practice agriculture and to settle into villages, towns, and cities. Firing their pots required learning how to build fires hot enough to heat the clay to temperatures high enough to cause it to at least partially vitrify (melt) so that water could not soak into it and turn it back to mud, and yet to heat it slowly enough to keep it from shattering.

To fire a ceramic pot requires a very hot stove (kiln), much hotter than a cook stove. It must have a cavity inside that is large enough to hold the pot. At the moment, however, our problem is to test fire the small clay disks prepared in part 2. I find that a well made bon-fire can produce a considerable quantity of glowing char. Carefully pre heating and then positioning the disks within a bed of glowing char will bring them to red heat. The temperature can then be raised by rapidly fanning the char.

While unfired clay that is dry has lost its water of plasticity, it still contains an appreciable quantity
of water that is chemically held. The formula for an ideal clay is Al203.2SiO2.2H2O. This water will be removed when the clay reaches about 350 degrees C (650 degrees F). If that water cannot escape rapidly enough from the clay as it is heated, it may build up enough pressure to explode with a sound much like that of popping popcorn, and your sample disk will be no more than scrap. Therefore, the disks must be heated slowly. How fast is too fast? The clay will let you know, and anticipating failures is the reason I suggested you make half a dozen disks of each clay to be tested.

After the coals burn out and the disks have cooled, test the disks for relative vitrification. A very simple test can be made by touching the tip of your damp tongue to the surface of a fired disk. Quick absorption of moisture means that the clay is not excessively vitrified, and could probably be safely fired to an even higher temperature. This might make it a candidate for use in a combustion chamber. No, or slow absorption indicates that it might not take a higher temperature. Obviously a disk that has melted, bloated, or distorted will likely only be useful in spaces that do not get very hot.

Having fired your several disks samples, you now have a pretty good idea of whether you can use the clay samples you have found, or if you need to look for a different source. In any case, you will need to dig, slurry, settle and/or strain about 5 kilos (11 pounds) of clay and start removing enough water by absorption and/or evaporation to cause it to reach a plastic consistency. You will need this clay later for further testing and for construction of a kiln in which you will be able to fire the ceramic parts for a cook stove.

Most literature I have read contends that the earliest pottery kilns were pits dug into the ground, lined with limbs and branches onto which pots were stacked. More branches were piled on top, followed by a shingling layer of pieces of broken pots. Set afire, it burns slowly, probably smoked a lot, and when I tried it, never heated the pots much more than enough to get rid of the chemically held water. My pots ended up being quite porous. That is OK for cooling water by evaporation, but not so good for cooking.

A somewhat later design is thought to be a kiln in a hill. It is a lot of work to make, but it can reach higher temperatures. Burrow into the side of the hill and cut a hole up through the top to serve as a chimney. Place the pots on a grate above the fuel so that they will be submerged in flame. Add fuel as needed and be prepared to spend hours or even days of stoking. I built one, but it was so slow in heating that after two days of warming up all the surrounding damp earth, I gave up. In later developments, kilns were built above ground with walls of stone, brick, and mud, and again a roof of shards from pots that hadn't made it.

But I'm looking for something quite different-- an inexpensive, light weight, easy-to-build, portable kiln, just big enough fire the ceramic parts for an inexpensive, light weight, easy-to-build, portable cook stove.

The next submission I'm working on (Part 3B) is a surprisingly simple (natural) design for a stove/kiln that I have stumbled upon. It is so simple and obvious that I think early people must have used it. Apparently no traces of this kiln design have been found, perhaps because it literally destroys itself in the firing.

I hope this proves useful.

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