| Boiling Point No. 17 - December 1988 |
By Chetr Eimjilkusol, lndustrial Ceramic Research & Development Centre, Dept of Science Service
Reviewed by Tim Jones of ITDG
This paper was presented at the FAO seminar on cookstove production held in Thailand in June '88 attended by people from Asia already involved in a variety of areas of stove design, testing, production and dissemination. Thailand, has seen the production of the Thai Bucket stove for many years and now the improved version has been introduced and is gradually gaining acceptance. These stoves have a thick, fired clay liner surrounded by the bucket shaped steel cladding. They burn charcoal though the improved version will accept wood and biomass waste as a fuel.
In his introduction Chetr Eimjilkusol explains how widespread the Thai bucket stoves are in Thailand. However, he claims that a large number of them are made with unsuitable clays which would be much improved by means of clay analysis and a more careful choice of clays and mixtures. He explains briefly the composition of clays and how they are formed, and gives an analysis of the ball clays that have been found most suitable from the north eastern area of Thailand. A high alumina content in his opinion indicates a suitable good quality ball clay which should be the basis of a preferred mixture. If the clay is washed in processing the alumina content rises. This does not affect the fired strength of the clay because of the low firing structure and could be a good indicator. He follows with an analysis of selected clays in Thailand and states that it is preferable to select a clay with alumina content above 20%. It seems that these high alumina clays are not really chosen for the production of stoves but are in a way wasted on other products that have no need of thermal shock durability. Substandard clays are used for the stoves no doubt because they are cheaper or more easily available! It seems likely that local stove producers are not familiar with the particular production methods and the most suitable clays for stoves and they lack technical advice and institutional support from the improved stove promoters.
The author states that the best stoves from Panomprai can last for years with continual use, because the potters, a) use a good clay with a high alumina content 24.5%; b) from this good clay they always prepare grog by mixing the clay with rice husk approximately 50:50 which is hand formed into biscuits which are fired and ground up and screened to a fine grog; c) they mix the grog at between 25-33% into the original clay without the addition of too much water; d) the component parts of the stove are formed by being beaten into shape; e) these are carefully dried in the shade and finally in the sun before being fired in a wood and straw fire at up to 800°C.
He says stoves made this way have low shrinkage, high impact strength, high toughness and thus possess good refractory properties. From the above example he draws the conclusion that a suitable stove material should have:
1) low shrinkage 2) high porosity 3) high strength 4) high refractoriness (not less than SK cone 28 when tested in the laboratory).
To achieve low shrinkage the body clay should have a moderate amount of alumina and grog. Porosity can be enhanced by grog and also organic materials like sawdust, and rice husk, but this has to be carefully controlled or the strength will decrease. In general a product with low shrinkage and high porosity will also have high refractoriness. At the moment the understanding of good clay selection and preparation is lacking and potters are not willing to go to the trouble of making grog.
Producers take short cuts to compete in the market where price is a major buying criterion. The physical changes which take place in the clay during firing are shown in a table.
His final conclusions are that to achieve a good ceramic product the following are needed:
1) a selection of good raw materials plus a proper formulation and final preparation of the clay.
2) in forming, drying, handling, suitable equipment and methods should be employed.
The Thais have a long history of ceramic production and a considerable knowledge of the use of the clay materials available in their country. With suitable testing facilities these can be used to overcome the problems in the manufacture of longer lasting ceramic stoves to benefit a community where fuelwood is fast becoming scarce.