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close this book Boiling Point No. 28- August 1992
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Clays for Stoves

by Peter Young, SHE Programme, ITDG Report of meeting, April 1992, University of Sheffield/ITDG


Participants T Ahmed - Pakistan; Amarasekera - Sri Lanka,

J Muriithi - Kenya, T Sharma - India, G Sizoomu - Uganda, A Sudjarwo - Indonesia, K Sulpya - Nepal, B Workalemahu Ethiopia, A Hood - Sudan, P Nyiti - Tanzania, V Abbott - Kenya Facilitators Dr Messer and Mr Gaspe (Sri Lanka) - University of Sheffield, P Young and T Flavell - SHE, ITDG T Jones - Consultant A Koopmans - Thailand

Objectives of ITDG/University of Sheffield, 3 year

Research Project

The majority of the several million improved cooking stoves disseminated over the last decade, excluding China, have been made wholly or mainly of ceramics ie. fired clay. ITDG's stove programme initiated clay testing studies with the University of Reading in 1980 and started the present research programme with the Glass & Ceramic Department of the University of Sheffield in 1989 with funding from the Overseas Development Administration. Mr A Gaspe has carried out the work under the direction of Dr Messer and in co-operation with P Young of ITDG.

The project objectives are to devise and evaluate a set of broadly accepted standards for clays and stove production and testing methods to help stove producers make more durable stoves.

The Sheffield Seminar

The seminar was arranged to report on progress of the research work and to provide an opportunity for leading clay stove workers from several countries to present their views and problems, to discuss the work of Gaspe and Messer and to plan the field test phase.

Fourteen short papers were presented by participants and the University of Sheffield, with the following titles (copies available from ITDG).

Reference List of Papers Presented:

* Report on Clay Stoves - Tanzania, P Nyiti

* Ceramic Stoves in the Sudan - Dr Ahmed Hassan Hood

* Ceramic Stove Programmes in Uganda - George Sizoomu

* Testing of Pottery Clays - A Koopmans

* Traditional & New Development of Pottery Stove in Java - Aryanto Sudjarwo

* The Thermal properties of materials used in cookstove manufacture and their constraints and limitations on enhancing overall cooking stove performance - Peter Young

* Brief summary of ceramic technology in Ethiopia and Ceramic stoves developed by the project Bekele Workalemahu

* Maendeleo Production in West Kenya - Viv Abbott Ceramic Stoves in NWFP/Pakistan and FECT project - Tanveer Ahmed

* Clays for Stoves in Kenya - I Muriithi Muchira Ceramic Liner Production Process for Improved Stoves: the Kenyan Case - J Muriithi Muchira

* Making of Clay Stoves, Biogas Plants & Burners in India-DrT N Sharma

* Ceramic Cookstove in Nepal: Problems and Prospects - K M Sulpya

* Clay Stoves in South China - Zhu Zhaoling

* The Kenya Ceramic Jiko: An example of Technology Transfer - Kennedy Masakhwe.


The 5 day programme consisted of formal presentations on :

- the results of the clay for stove research projects:

- clay mineralogy

- characteristics of clay and their relation to stove performance

- the clay/non clay ratio as a predictor of stove failure from thermal stress

- methods and procedures to measure the clay/non clay redo.

Practical sessions were organised each day for the participants to carry out clay/non clay ratio measurements on samples of clays they brought from their own projects. In addition they experimented with construction techniques that give a planar platelett alignment rather than the hoop type of alignment associated with throwing and jigger jollying which induces residual stress upon shrinking.

Discussion and Conclusion

The problem of thermal stress failure of stoves in service is extremely complex but it was discovered that a simple characteristic of the clay body to the clay/non clay ratio, correlates well with the known stove weakness. A ratio of about 1:1 is required if the material is to be reasonably strong. Liners which may not require such a high strength could have a lower value for the clay/non clay ratio of about 10:6.

Four possible reasons for this correlation have been suggested.

1. Drying shrinkages and fracture initiating flaws:

A higher proportion of non clay means the body shrinks less and limits the tendency to split or crack and develop fissures which could act as fracture - initiating flaws.

2. Porosity and its effect on the resistance to thermal stress:

Resistance to thermal stress is generally held to improve with an increase in porosity. Non clay particles can increase porosity by developing quasi-pores associated with quartz when it passes through inversion, it can shrink away from the surrounding clay to form a pore.

3. Anisotropic shrinkage and stress development:

Clay platelets line up with their larger plane faces parallel; this allows the slip associated with the clay's plasticity. It also means anisotropic shrinkage occurs ie. shrinkage rates differ perpendicular and parallel to the slip bands. In cylinders the anisotropic shrinkage upon drying and in firing induces residual stress, which can approach levels close to the stress level which causes failure. By increasing the non clay element it can help limit the extent to which the clay platelets align themselves.

4. Effects of clay processing and means of manufacture:

This is directly linked to 3 because the more the clay is processed, formed, rolled, extruded and squeezed, the greater the level of platelett alignment. Some processes like throwing or jigger jollying induce more platelet alignment than coiling. In this case anisotropic shrinkages will occur more in stoves made by jigger jollying and throwing. The reasons for increasing the non clay element apply are discussed in 3. However, an alternative approach is to use a manufacturing technique that limits platelett alignment or aligns them horizontally rather than vertically in the stove wall. Since throwing and jigger jollying are well established methods of making stoves, there are likely to be constraints associated with abandoning them in favour of techniques that are perhaps less productive, such as coiling.

Future Activities

The technique for measuring the clay non clay ratio was presented to the participants and it is now proposed that this is tested to determine how easy the procedure is to carry out under project conditions and to determine how well the correlation between the clay/non clay ratio and the stove durability fits a wider range of clays.

Terms of reference for a study on clays have been developed for interested researchers. In February 1993 a second seminar will be held either in Thailand or Sri Lanka to discuss the results. Applicants will be selected from researchers who have submitted papers that follow the terms of reference or present other interesting and valuable findings about 'clays for stoves'.

Details of the seminar findings, the method to measure the clay/non clay ratio and the scope of work for further research are obtainable from Peter Young, ITDG.