| Boiling Point No. 31 - August 1993 |
|Clay Testing - 5 Country Reports|
by Peter Young, SHE Programme, ITDG, UK
In April 1992, at the first ITDG International "Clays for Stoves" seminar at Sheffield University, Dr Messer and Dr Gaspe proposed that the C/NC ratio be accepted as a good predictor of stove failure resulting from thermal stress (see article on page 2 ). This was based on a surprisingly strong correlation found between the C/NC ratio and the reported tendency of the stoves to fracture when in use.
Over the past year testing has been carried out in Ethiopia, Kenya, Nepal, Thailand and Indonesia to evaluate the clay/non clay ratio test procedure. The results show the effectiveness of adjusting the C/ NC ratio to around 1:1 and whether this improves the durability of pottery stoves. The country papers were presented at the second "Clays for Stoves" seminar hosted by Dian Desa, ARECOP in Indonesia in April 1993. Below are summaries of 5 papers.
It seems clear that the C/CN ratio is an important factor to be taken into account when deciding the suitability of a clay for making stoves. However, the rather general rule that the C/ NC ratio must be lower than 1:1 is probably an over- generalisation and is not very helpful for many clay sources.
In Kenya a C/NC ratio above one seemed to be successful, while in Thailand C/NC ratio were as low as 0.7:1 and in Nicaragua as low as 0.4:1. It therefore seems sensible that when selecting clays a good starting point is a ratio of 1:1 but other clay ratio samples should also be considered.
The choice of types of additive that can be used to mix with clays in order to bring the C/NC ratio down to 1:1 is still somewhat confusing. So far very few tests have been carried out which exclude other compounding factors such as method of moulding. In Sri Lanka fine sand seems better than grog but sand and grog are better than just sand. In Thailand grog seems to be very effective. Since there is very little clear evidence it is likely that the decisions on which additives to use will be made according to cost and availability.
If lime or montmorillonite* clay mineral are present in the raw clay body the C/NC ratio is not a good predictor of the clay's suitability for making stoves. In such cases the clay is best abandoned and an alternative used.
Lime can be detected by dripping an acid solution onto a freshly cut clay surface. Bubbling indicates lime. Detecting the presence of montmillonite is much more difficult and can only be verified in a well equipped laboratory. However, as a simple guide very high shrinkage may be an indication of montmorillonite.
In Kenya and Nepal cutting through the bridge above the stove's entrance was found to be a very effective way of providing stress relief. This was also found to be effective in Sri Lanka and India. Sulpya in Nepal has recommended that the use of a detachable clay ring fitted over the bridge gap helps to reduce distortion in drying.
Another factor mentioned in the country reports is 'beating' - a technique used to shape cooking and water pots which is also being used to shape some stoves. Koopman concludes that beating is likely to induce greater particle alignment and therefore more anistropic shrinkage and residual stress. This may be counteracted by its tendency to increase the strength of the pottery through closing up small fissures. * Montmorillonite is a clay allied to koalinite. It has finer grains and a weak cell structure, exceptionally plastic and exhibits high shrinkage
Full seminar proceedings can be obtained from Hofman Engineering Systems - Greenfields, P O Box 167, Chiangmai 50000, THAILAND.