| Boiling Point No. 02 - May 1982 |
A primary factor in designing ceramic stoves is the suitability of the clay available. Close observation of local potters will give clues as to how to improve its qualities as these people have learnt through traditions how to adapt the clay for the pots they make. Clay used for cooking pots is not necessarily going to be suitable for ceramic stoves but because clay cooking pots have to withstand localised thermal stress and a fair amount of pounding, it will come some way to meeting the demands and provides a good starting point.
Observation of a group of Voltaic potters working on the edge of the Sahara desert found them mixing the following with their clay: 15% grog (crushed fired pots), 15% millet husks, 5% charcoal and 5% ash (percentages of total volume). They also added a small amount of dried donkey dung to the clay used to shape the neck of the pot.
The strength of the final product will be a function of both firing temperatures and the addition. The addition of grog a is widespread practice. In a study of two Ethiopian pottery villages Roel Hakemulder (1980) writes:
"Broken bricks are acquired from the Nicola Brick factory....The potters consider the powdered brick to be indispensable for it makes the pots strong. The bricks are powdered in several ways; using mortar and pebble, beating them to pieces with metal staves and milling small pieces of brick between rocks. The powder is seived and the fine powder sprinkled on the clay after which the clay is pounded and kneaded.- There is no standard quantity of brick powder to be mixed with the clay; the potters judge by feeling, adding a little water whenever they think it is necessary."
Other potters add ground broken up fired pottery in various sizes. The addition of grog serves several functions, firstly in a very fine plastic clay it opens out the body making it stronger for building larger shapes and less likely to shrink and warp excessively which could cause cracking in drying and in firing. The type of firing typically used by potters i.e. bonfire firing will only just serve to harden the clay into a biscuit stage with temperatures ranging from 600 - 850 degrees all in one firing. Grog additions especially in the case of high fired particles such as bricks will strengthen the weaker low fired body.
Charcoal and ash additions also act to strengthen the clay. The addition of amorphous silica in the form of grass stem ash or rice husk ash is a common ingredient in the clay used by the Thai Bucket stoves makers in Thailand. There they add up to 40% ash to their mix making a non-plastic clay which has to be moulded into shape. The silica addition will lower the maturing temperature of the clay. The tiny honeycomb structure makes an incredibly light pot which has some insulating qualities. Charcoal additions make a more porous body and the finer and more even the distribution, the stronger it will be as small pores tend to stop cracks propagating. It also serves as a localised fuel especially useful if the fuel for the firing itself is scarce. The Voltaian addition of millet husks would serve to some extent the points mentioned above with an emphasis on the increased burning during the firing.
Dung is well known in mud mix for increasing the binding strength of the mix by breaking down some clay particles into a bacterial ooze . The reasons for adding dried dung to clay are debatable. In the Voltaic potters case, one can only hazard a guess that adding dung to the neck of a pot, would increase the burning and so raise the temperature locally, thus creating a stronger area of fired clay where it is most likely to get knocked.
Sand is also widely used as an addition. It is a plentiful and easily added form of silica. Its gritty texture adds strength to the wet clay when building shapes and in firing it helps to lower the sintering temperature of the clay because of its high silica content although this only happens with any real effect at temperatures above 900 degrees C. The addition of sand also lowers the porosity of the fired clay thus improving its resistance to thermal stress.
Most typical firings are in the range of 600 - 800 degrees where the maximum "positive dimensional change" occurs: that is to say the pores increase in number and size rather than contract and glassify as at a higher temperature. This, along with the fired grog addition and honeycomb structures seems to improve resistance to the effects of thermal cycling. However open pores on the surface of a pot will weaken the structure if water is allowed to penetrate and turn to steam on heating. Primitive potters seem to have an answer to this as well: they finish their pots with a highly polished, almost impermeable surface. Whilst in the "leather hard" stage, the pots are beaten and tempered into shape. This treatment consolidates the surface layers and aligns particles. The mica, prevelant in many primitive potters clays responds well to that treatment, helping the thermal shock resistance and surface texture. Sometimes a fine clay slip is applied before the whole outside surface is burnished by rubbing with bone or pebbles to a high gloss. This then closes the surface pores of the pot giving it a hard impermeable skin and honeycombed granular structure within. Charcoal, however, would burn through the burnishing in the firing, making a pitted surface.
Increased additions of all or some of the materials above, plus beating and burnishing should, with experiment produce an acceptable body for ceramic stoves. Here at Shinfield we are carrying out a testing programme to find suitable refractories for ceramic stove making. We would be interested to hear of any unusual additions readers have seen or heard of. In the future, "Boiling Point" will be carrying an article on clay testing methods, procedures and results from our refractories programme so far.
By Jenny Trussell
Biscuit ware - clay which has been fired once and is still porous. Firing temperature can vary between 600 and 1100 degrees C.
Burnishing - surface treatment of leather hard clay rendering it highly polished by rubbing with a smooth hard object.
Sintering temperature - the temperature at which the clay particles start to fuse together.
Slip - fine particles of clay suspended in water to form a liquid.
Tempering - beating and thus flattening clay particles to create a stronger surface.