| Boiling Point No. 31 - August 1993 |
Enabling people to make better use of their local resources is a vital part of development. Clay is an appropriate material for improved stoves in many areas because it is cheap, widely available and readily worked. Potters' skills are traditional.
But clay has drawbacks too. Its behaviour in firing is often unpredictable and cracking rates during firing or first use are unacceptably high. The qualities requried from a clay stove are not the same as those needed for pots, bricks or tiles. Stoves must withstand temperatures up to 1,000°C, be resistant to thermal shock and must remain accurate in shape to within tolerances of less than one centimetre after firing.
Among potters and ceramicists there are many theories and explanations of the cracking problem. Good training and experience reduce the problem but better understanding of the micro structure of clays is also needed. To achieve this ITDG collaborated in a four year programme of stove ceramic research, based at the University of Sheffield, involving field testing in several developing countries. Research indicated that cracking results from stresses arising from the alignment of clay platelets. This can be reduced by using equal proportions of clay and non clay constituents and by alternative methods of forming the clay. This issue supports these research findings and gives guidance for clay stove makers in how to measure clay/ non clay ratios.