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close this book Boiling Point No. 07 - December 1984
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Testing of Samples from The Gambia

by Jon Loose


As part of the ITDG programme of cooperation with the Gambian National Stoves Programme we took delivery of 16 Kg of unprocessed subsoil clay from Mandinari and Faraba (both in The Gambia). This was in order to investigate problems of low survival rates when firing ceramic stoves and subsequent cracking of the stoves in use. Laboratory tests were carried out on specially prepared samples to simulate the stresses caused by the use of a ceramic stove for cooking.

The subsoil deposits in The Gambia are believed to be all low in what is technically called Clay. Soil is divided into three main components: Clay, Silt and Sand, which have strict definitions to soil scientists. These divisions are based on particle size but often also correspond to mineral differences. The Gambian soils have high levels of silt and sand and relatively little clay Silt reduces the plasticity of the soil and also reduces the strength of the fired clay. In general too much silt is to be avoided in ceramics. Quartz or mica sand is often added to a pottery 'clay' (which is already a mixture of clay, silt and sand components) in order to improve it for use. The sand content in the soil samples we examined from the Gambia contained very little quartz or mica but appeared to be structurally more like silt.

Particle Settling Tests

Particle settling tests (Boiling Point No. 6) showed that there was a smaller proportion of large, dense particles (eg quartz) in the Mandinari clay compared to other stove clays. The Faraba clay and stove clays from Nepal, Sri Lanka and India showed similar settling patterns to each other (see fig. 1). The indication of this test was that large dense particles such as sand would need to be added at least to the Mandinari clay.

Fig. 1 To show results of tests carried out by Robert Hausner (Neval samples), Bill Stewart (other countries) and Jon Loose (Gambian samples)

Thermal shock testing

Samples were tested to assess the change in strength after thermal shock treatment. This test simulated fatigue caused by the use of stoves. Thermal shock treatment consisted of heating the samples to 400C and then quenching in cold water repeated 20 times. The two samples of clay from the Gambia were both tested with and without additives, no samples contained more than one additive. The additives were quartz sand and rice husk ash. In the case of sand (fine, sieved white sand) 25% by volume was added to each of the clays. For the rice husk ash (greyish white containing very little carbon) 40% by volume was added to each of the clays.

Fig 2 Mean Energy to Break Sample

The standard ITDG procedure was used. This has been described in previous reports (see below and Boiling Point No. 3).The cylindrical samples were subjected to impact from a pendulum with the height of swing regularly increased until breakage occurred. Samples from each of the 6 treatments were fired to 750C and divided into two groups. One group was given thermal shock treatment and then tested for impact strength and the second group was tested for impact strength with no further treatment.

Results: Mean Energy to Break Samples-Joules x 10-3

Table 1a: Unshocked













Table 1b: Shocked













Clay, no additions

The fired and shocked strengths of the clays were extremely low as may be seen from the table but the Faraba clay was apparently weaker. Variability of the results and the low strengths do not allow us to draw any further conclusions here.

Addition of sand

Sand appears to be the most effective additive to these clays (of the two tested) as shown by the unshocked strength and retention of strength after shocking, a finding which is supported by the fact that sand is traditionally one of the commonest additives. Grog, which is crushed ceramic, is also commonly used and we have previously found it to have a similar effect. The Faraba clay with sand was stronger than Mandinari clay with sand although resistance to thermal shock was good with both.

Addition of rice husk ash

Rice husk ash had a similar effect to quartz sand in increasing strength of the clays but to a lesser extent. There was a significant drop in strength of the Mandinari clay with ash when it was shocked, this effect did not occur with the Faraba clay. This effect could be due to a weakening of the clay by excessive silica addition from the ash. Ash gave more variation in strength and therefore would give more failures but has the advantage of increasing the porosity of the clay and thus the insulating effect. It also decreases the density and hence the weight of the stove. Rice husk ash is used in this way by the Thai stove industry.


Both of the raw clays would benefit from the addition of sand or grog. The optimum proportion will have to be found by further work because excess additives will weaken the clay. The Faraba clay could be used with rice husk ash to give a lightweight ceramic material but the Mandinari clay may be too weak for this when ash is added. The Faraba clay is closest in particle size tests to other stove clays and also gives the strongest ceramic when mixed with sand.

Reference paper

1983 C.R. Chaplin

Proc. Indian Acad. Sci. (Eng. Sci.) Vol. 6,1 pp. 47-58

Wood burning stoves: Material selection and Thermal Shock testing of Fired ceramic bodies.