Practical Tips For Potters Making Improved Cooking Stoves,
Part 3-- Testing the Clay For Cooking Stoves
Richard Boyt, June 2003
Dear Ken Goyer, Rogerio Cameiro de Miranda, and others:
The following material is the third part of a condensation from the booklet "Practical Tips For Potters Making Improved Cooking Stoves. Prepared by Tim Jones, Illustrated by Debbie Riviere. Published by Hofman Systems Engineering b.v., PO Box 642, 3100 AP Schiedam, The Netherlands (1993).
"Pure clays with no sand or coarse particles are not suitable for the production of cooking stoves. Experience and research has shown that the mixture for the best stoves must be made up of fifty percent or less of pure clay. More than fifty percent pure clay and the number of stoves that crack during production or early in use is too high.
"A simple and very practical method to make sure that a clay mixture has more than fifty percent of non-clay material, is to make a mixture of clay to which is added as much sand, saw dust, rice husk, ground up fired clay [grog], or charcoal dust as possible, but still allows the mixture to be [plastic enough to be] worked.
"Normally the starting point for a mixture for making fired clay stoves is a sticky plastic clay. Take some clay and dry it. Crush it and place it in a clear glass container three quarters filled with clean water. Cover the ... container and shake until the clay is well mixed with water. Let it stand undisturbed for a day or two. The coarse particles, such as stones and sand will sink to the bottom. The soil [silt] will be in the middle and the clay at the top... . The amount of sand and its presence in a variety of sizes is important. A good mixture of different sized sand particles has been shown to be preferable as long as they are no bigger than 2 mm. If the clay takes several days to settle, then it will be a clay with very fine particles. It will probably shrink a lot and will result in... cracking as it dries. A simple test is to take a sample from each layer that looks different. Rubbing it between finger and thumb will give an idea of how much coarse material is in it.
"To give an idea of how well a clay holds together (its plasticity)... add some water [to a handful of clay taken from the ground], and knead until it becomes soft enough to leave a clear fingerprint when squeezed, but not so soft that it starts sticking to the hand. Roll the clay into a coil with a diameter of about 10mm and try to tie it in a simple knot. If the coil does not crack or fall apart, then it is good and plastic. If the clay cracks and falls apart, it may be useful for mixing with a more plastic clay.
"The more clay shrinks, the more problems with cracking during drying will be experienced. If the shrinkage is more than 12 percent and the clay sample shaken up in the glass container took a long time to settle, the drying, firing, and end-use losses will be too high for this mixture to be any good. [Note: the clay itself may be quite suitable, if mixed with a coarse material].
"When making cooking stoves and liners from clay, it is very important ...to know how much [the clay mixture] shrinks so that this can be allowed for when the stove is made. To measure for shrinkage, small bricks 120mm by 30mm by 20mm should be made. Marks are made on sides of the bricks exactly 100mm apart. The test bricks should then be allowed to dry slowly and evenly, being turned regularly. When completely dry, the length between the marks is measured again, and the drying shrinkage is calculated.
"The dry bricks are then fired in a kiln. The distance between the marks is then measured again and the total shrinkage is calculated. "Lime [limestone] in a clay will cause serious problems, causing the fired clay to crack, crumble, and fall apart. A simple test to show lime is to take a lump of clay straight from the ground and squeeze lime or lemon juice over it. Look carefully to see if any bubbles appear. A few bubbles, and there may not be a problem, but if the sample fizzes, don't use it.
"The porosity (amount of very small holes in the fired clay) of the pottery or ceramic cooking stoves is important. This is because the higher the porosity, the better the stove is at withstanding the heat shock conditions it will experience in use. If the porosity is too high, then the stove will not be physically strong enough to withstand everyday use. If the porosity is very low, then the stove will be physically strong, but more likely to crack in use. A high amount of porosity is therefore a good thing, as long as the stove can be transported and withstand everyday use. The samples used for clay shrinkage tests can also be used for the porosity test. Heat the bricks to drive off any moisture, weigh them, then... boil them [in water] for thirty minutes. Dry the outside of the bricks and weigh them again. The porosity can then be calculated from the difference between the dry and the wet weight. A percentage between fifteen and twenty-two percent is acceptable, but a range between seventeen percent and twenty percent would be better.
My reactions to the above quotations from the booklet
"Tips For Potters"
1) In making 50/50 mixtures of clay and non-clay materials, it is unclear whether these proportions are based on volume or weight. It would also be useful to know at what temperatures the firing should reach.
2) I suggest that leaving 2mm diameter sand or rock particles in the clay might be a bit large.
3) Tim Jones makes a good point about lime in the clay. In my experience limestone chunks as large as 2mm can cause serious trouble when they hydrate and expand after firing, which can cause pieces on the surface of the pot to pop out. Vinegar is also good for testing for limestone.
4) I am concerned that the clays I dug, dried, soaked with water, and let settle for three days showed no layering, except for the water that came to the top. I found, maybe a few dark specks about .2mm diameter near the bottom. It is obviously going to need additives. Its dry and fire (2,000 degree C) shrinkage tested out at ten percent, and porosity of less than one percent.
Is anybody out there finding and processing clay? If so, what are you finding? Do you have any tips on techniques you use that other stovers and I might find useful?
Comments, questions, and suggestions would be appreciated. These condensations are intended to parallel and supplement my submissions on "Ceramics For Stoves", which at the moment are slowed by working on "Tips", chimneys, and how to make an appropriate kiln for firing clay samples.
The next submission of condensations from the booklet should be Part 4 "Materials That Can be Added to Make
a Better Mixture". Let me know if this is useful enough for me to continue.
Neosho, MO 64850
Note: See other articles by Richard Boyt