|Boiling Point No. 22 - August 1990 (ITDG Boiling Point, 1990)|
by Tim Jones, ITDG, Ceramicist
This centre was set up during 1989 by the Sudan Energy Research Council (ERC) based in Khartoum. It is situated 11 km from Khartoum at Soba. The aim of the centre is to provide a service to entrepreneurs who are involved in the production of stoves with a ceramic component. It has the capability of advising on what are the best combinations of clays and biomass to produce the most durable inserts and complete clay stoves.
A major problem in Khartoum and most of the Sudan is the lack of suitable scrap metal to produce the claddings. This has been quite successfully solved by CARE's main producer who is using a thinner grade of steel found in the form of large tomato tins. These come into the Sudan from Egypt and the food is recanned in smaller tins for resale. This solves one producer's problems but there is still a serious constraint for all the others. The all-ceramic stove could be the answer by offering a fuel efficient stove which is cheaper and hence available to a larger cross section of the population.
A new ceramic stove is still in the process of development and a new paddle mould is being tested and its design modified for optimum performance. The requirements of all-ceramic stoves are different from those of ceramic lined stoves with regard to the materials they are made of. The fired clay has to take all the heat shock and support the weight of the pots without the support of the metal cladding. So the search is on for the best clay mixtures for this type of stove and for simpler metal bands that will extend the life of the stove but reduce the metal component.
The centre has prepared mixtures from the clays found in and around Khartoum, combined with varying percentages of biomass (sieved sawdust). The clays are crushed, sieved and mixed with water in small batches, three batches at a time. The resulting mixtures are allowed to dry for three days before being finally "wedged" by hand and made up into liners using a variety of production techniques. After firing, the liners are tested for their durability.
Considerable interest and demand was created by the CARE Sudan advertising campaign for the stove. The problem was, and still is, to meet this demand which is now starting to attract private entrepreneurs who know little or nothing about suitable clays and how to process them.
The testing centre was established in response to an increasing demand for more information about which clay mixtures would have the greatest durability. This was because there had been continual cracking during the production of the liners, and also during a very short period of time when they were installed in the stove and in use.
Clay testing is carried out at Soba by mixing small batches of clays and biomass, three at a time in half oil drums. They are mixed by hand and then made up into full size liners which are allowed to dry. The holes for the grate are cut, followed by further drying and then firing in the centre's kiln, which has been converted to run on a combination of waste oil and water.
After firing, the liners are subjected to destructive testing which involves putting them loosely in a metal cladding and lighting a strong charcoal fire in them twice a day, with a suitable time gap for complete cooling between lightings. This test is carried out for a maximum of seven days unless the liner fails before this time. The size and position of any cracks are recorded. Seven days was the maximum although experience showed that if a liner was going to fail, the faults occurred within the first four or five lightings. The stove is not considered to have failed until the cracks have caused the liner to become so inefficient that the stove is no longer burning the charcoal at full efficiency. This comes about when the holes in the grate become enlarged due to wide cracks and pieces falling off. The walls of the liner may also move out of alignment. This form of testing is faster than ''in use" testing and is more severe because the liner is not fully supported by the cladding and the cement mixture. A liner that does well under test is sure to last longer when properly supported by the cladding.
As different clays in a variety of mixtures are put through the testing procedure, knowledge is accrued that can be used to direct the initial formulation of the mixtures. Time is not wasted making mixtures that stand little chance of surviving. So far the most successful general combination has been that of a plastic, pure clay mixed with a coarse clay containing a higher percentage of coarse particles of sand, mica etc. These two clays are then combined with sawdust.
Results revealed how different clays respond when mixed together. Liners that when fired look perfect can crack within minutes of the first fire being lit, and others that just barely hold together and look very rough will survive the tests showing no signs of cracking.
As well as making and testing mixtures, the centre is involved in testing different production methods to see if the method of forming affects the final strength and durability of a liner. The different methods that are available at Soba are: hand forming with a simple metal mould, the paddle mould and the foot operated jigger jolly. They also have access to more mechanized production methods that use an electric pug mill and electrically driven jigger jolly. Testing is comparative. One clay mixture is used to produce inserts made by the different production methods. They are fired together, distributed around the kiln so that they experience a variety of temperatures and then destructively tested. There were indications that the liner produced by the mechanized methods were failing badly in the firing, but far more tests have to be done to see if there is a problem with this type of production when compared with the more manual methods.
It is hoped that the work of the centre will continue so that unanswered questions about mixtures and production methods can be solved and the stove programme in the Sudan can go on from strength to strength.