Ceramics for Cookstoves 2: Testing Unfired (green) Clay
Richard Boyt, May 2003
Greetings from Pottershop Hollow in the SW Missouri Ozarks where last week tornadoes did their damndest.
Sunday's blow just missed us by a few miles as did the storms of the rest of the week. Two years ago the
farm got hit by several small tornadoes that uprooted over a thousand of our larger trees.
Today, there are many fine books and magazines on contemporary ceramics that describe exciting new
equipment, materials, and techniques. Here, however, I'm trying to limit discussion to simple,
non-sophisticated, even primitive ceramic procedures that could be used by third world socities in
A quick test for clay plasticity can be made on the small samples of clay gathered in Part one.
First,remove all obvious stone, gravel, and organic trash.
Then add or remove enough water to bring a fist-sized
sample to the softness and plasticity of Play-Dough.
Squeeze it in your fist so that some of it tries to squish out between your fingers. When you relax your
fist, a plastic clay should retain its squeezed shape and not crumble away.
Plasticity is good, however, even less plastic clays may have some use in stove
building, as they may prove to be very refractory.
Why plastic? To be usable, clay must be shaped, then be strong enough to hold that shape while drying. Why
refractory? It is also important for the clay to withstand the extremely high temperatures and repeated
thermal shocks it would experience in a stove's combustion chamber. Clays that are less plastic or
refractory, however may find use in other parts of a stove that take less heat. A low refractory clay
subjected to excessively high temperatures may bloat, bubble, slump, or even melt into a puddle.
Refine about half a kilo (one pound) of each of the two or three most promising clays you have dug.
Remove all obvious foreign matter.
Stir in enough water to turn the sample to a liquid slurry thin enough to pass through the mesh of a small strainer.
Discard the coarse material that fails to pass. As an alternative to the use of the strainer, make a very
thin watery slurry of clay, and let it settle for a few hours, allowing any sand and grit to settle to the
bottom. Carefully pour off the upper layers of clay and water, and strain them through a thin cloth (such
as cheesecloth) to remove any suspended organic matter.
Next, let the clean strained thin slurry sit quietly for several days (the longer the better), to
allow the clay to settle out. Carefully pour off the clear water that rises to the top. The settled clay
is now ready to be dried to a soft, plastic, non-sticky, moldable consistency. You can achieve
this through a combination of evaporation and absorption of the excess water. To reach the desired
consistency reasonably quickly, pour the slurry onto dry Plaster of Paris batts or into plaster bowls you
can make, or pour the slurry onto the surface of dry newspapers that will gradually absorb the excessive
moisture. This method of using water to seperate particle sizes is essentially the same as practiced by
Mother Nature to form the clay beds we dig.
When the clays attain a plastic consistency, form from each sample half a dozen round disks about 1-2mm (1/16
inch) thick, and 2.5 cm (1 inch) in diameter. Imprint upon the surface of each disk an identifying number
and the the impression of measurement marks of an embossed plastic ruler. Keep a written record. Dry
the disks slowly to avoid warping or cracking as they shrink. Cracks at the edges and curling indicate
uneven drying. Drying may be speeded up by placing the disks between two dry plaster batts or between
weighted multiple sheets of newspaper. If using newspapers, change the wet sheets, as needed. When
completely dry, re-measure the ruler marks to determine percentage of drying shrinkage. I have
found that my pottershop clays typically have about a twelve percent shrinkage. In general, clays made up
of very fine particles tend to be more plastic and shrink more than clays made up of larger coarse
particles. Shrinkage and low plasticity both can present problems in forming, drying, and firing the
I admit that I have seen only a very small part of the world, but what I have seen in Eastern Europe and
Northwest Africa encourages me to believe that I might be of some small help in promoting clean burning,
inexpensive, simple, efficient cook stoves. Is only a little, but with love.
The next entry (Part 3) is intended to discuss methods of firing local clays for stove construction. Hope
this proves useful.
20479 Panda Rd
Neosho, MO 64850
Note: See other articles by Richard Boyt