| Boiling Point No. 17 - December 1988 |
by Auke Koopmans, working with Hoffmans Systems Engineering b.v. in Thailand
During the past twelve years while working with potters producing products such as water jars, cooking pots, stove liners, roof tiles etc. quite a few production problems have been encountered for which solutions had to be found. Although not all of those solutions are applicable to production problems with stoves some of them are and can be used directly.
The problems in stove production vary from relatively simple problems such as cracking during drying to more complex ones like cracking of stoves after having been used only once or twice.
It was found that, in the case that serious problems occurred during the production, the potter almost always started looking for another clay source as this is the easiest way out. Changing the clay mixture is often not tried due to several reasons such as a lack of knowledge of what causes the problem and therefore also of how to solve it.
Besides, changing the clay mix is a time consuming process of trial and error.
Adjusting the clay mixture by changing the composition such as by adding other materials, by changing the mix or sometimes even by using a better method of preparing the clay can in many cases solve the problem and in general will be found to be cheaper. Although the problems are in almost all cases only technical the potter sometimes does not perceive them as such and claims that other non-technical reasons are the cause of his problems.
The following will give some examples found in the field and give some possible solutions on how to overcome those. It should be mentioned that, what might work in one case, does not necessarily give the same results in other cases as clay is a natural product. Large variations in clay compositions are found (the type of clay minerals present, the chemical composition, the particle size, just to name a few) even over short distances, which have pronounced effect on the suitability of it for certain products like stoves.
Clay preparation. Clay has to be prepared properly to get an even well mixed material from which relatively thin walled products like stoves can be made. Sometimes clay has to be mixed with other materials like sand while also sometimes stones are found in the clay.
Uneven mixing results in different shrinkage rates (cracks), weak spots (some parts crumble easily while other do not) while stones give problems during drying and firing as they do not shrink while the clay does (star shaped cracks).
A good method which is very effective in overcoming mixing problems is to build up a heap of clay in thin horizontal layers and then afterwards to cut or even scrape them off vertically in thin (about 1-2 mm) pieces for instance with a small bow wire. This cutting or scraping is useful in removing stones and at the same time results in good mixing in particular if it is repeated several times. Once the clay is prepared it should be kept covered as otherwise the top layer will dry out. Storing the clay for some time is also useful as moisture will be more evenly distributed after some time (say a few days).
Sometimes coarse grained clays have to be used as other clays are not available. In such a case forming is difficult as the clay does not easily stick together. The addition of organic fibrous materials can help in such a case. Finely chopped hay (dried grass) or straw, coconut coir and other fibrous materials aid in giving some bond while the addition of dung, etc. can help in breaking down larger particles. It should be noted that the latter is dependent on time and that no instant improvements can be expected although in a few cases a marked improvement in workability of the clay was found directly after mixing. Also here it is important that mixing is done thoroughly.
Forming. Sometimes hand thrown pieces but also coiled pieces crack during the first few hours after having been made, in which they are left to stiffen before the next production step, in particular if clays are used which are too wet and shrink a lot (above 8%).
Those cracks are formed due to stresses built up during drying where the bottom part sticks to the board on which the piece was formed (cracks starting from the bottom and going up vertically). Cutting them loose from the board helps and should be repeated every hour or so in the case of very wet clays.
Some potters prefer to work on the ground while others have to due to a lack of work benches. Clay is then stored on the ground and clay coils, in the case that the coiling technique is used, are laid down on the ground. With sandy soils often sand will stick to the clay and/or coils which can result in weak spots as the clay will not bond properly at those places where it is covered with sand (visible when you break it into pieces). The use of plastic or other materials like jute bags on which the clay is stored will help.
Some potters throw directly on the wheel and often the piece will distort in shape when removed from the throwing wheel. Even though it will be pushed back in its round shape, when dry the same distortion, such as oval pot seats, often will be found but less pronounced. This is caused by the so-called memory of clay. Only by throwing the products on boards and by leaving them there until stiff enough that they can be moved without losing their shape can this be remedied.
Drying. The same problem with shapes which should be round but are oval can also be caused by uneven drying. There is no specific sign which indicates such uneven drying other then that is often accompanied by localised cracking. Even in the case that the products are not put in the sun, uneven drying can occur as air movement (wind) is a powerful drying medium as it increases the drying velocity by removing water vapour from the surface and diffusion of water from the interior of the product can no longer keep pace with the removal rate.
In preventing uneven drying one should start with restricting air movements by using curtains or by covering the products in the initial stage while frequent turning around (and upside down) also helps, in particular where the products are dried in the sun.
Sometimes a stove will look completly dry but then it suddenly cracks. This is often the case when very fine grained clays are used which hamper the removal of moisture from the interior to the outside. If the stove does not crack during drying it will often show up after firing as a black core in the wall.
In the case that, even with slow and careful drying, still problems occur with cracks and uneven drying one might resort to changing the clay composition. Adding more grog or other material will improve the drying characteristics as it increases the amount and size of pores through which the water can migrate from the interior of the product to the outside. Research has shown that grog does give somewhat better results than sand and that the coarseness is less important than the amount added. However, in both cases the difference between sand and grog was found to be small.
For some materials like alum, ferric and calcium chloride it is known that they improve the drying characteristics. They also act as flocculants and promote the formation of larger pores. In some cases when these materials have been used depending upon on the clay type, direct drying in the full sun might be possible.
Firing. The firing process has a distinct effect on the quality of the stoves and some of the problems found, in particular with regard to low strength are caused by it. The main problem is the high cost of fuel which forces the potter to use as little of it as possible which results in low temperatures in the kiln (underfiring) while the kiln itself often is not very efficient with high heat losses to the top and sides. Increasing the efficiency is possible by improving the kiln (in Sri Lanka savings of 25-30% were found with kilns with higher walls and an insulation layer of ash incorporated in the walls) but this still leaves the problem of low strength.
There are some indications that low strength is not necessarily bad for a stove, depending on how the stove will be used. The low strength is thought to be beneficial in increasing the resistance against thermal shock but this has not yet been substantiated by research. Low strength however, causes higher breakage rates during transport while, in the case that the stove is used directly without insulation layer, the stove easily breaks when knocked against with cooking pots, wood sticks and so on.
Solving this can be done by firing at a higher temperature, by using more fuel or by using a clay in which a combustible like saw-dust, rice husks, etc. has been added. Adding such a material will increase the porosity and probably also the resistance against thermal shock but it has to be mixed properly and in relatively small quantities as otherwise it might even decrease instead of increase the strength.
The Thai bucket stove is a good example where the addition of large amounts of rice husk ash (containing a lot of carbon) to the clay (mixed with pugmills) helps in increasing the strength although the firing temperature is very low (around 750 C which temperature is also found in kilns in many other countries). In this case the strength probably comes from the carbon which causes a localized reduction atmosphere which is known to lower melting points.
Sometimes cracks and low strength are caused by too fast heating up, too fast cooling, stoves dried down too far, large stones or lime nodules in the clay and so on. Detecting such is not always easy but some hints can be given. Heating up cracks can be distinguished from cooling cracks by that heating up cracks often have opened up while they go around large grains like stones. Cooling cracks are in general closed and often go through large grains.
When stoves have been dried down very far (below the equilibrium moisture content) they often pick up moisture again during periods with higher humidity levels. This is shown by very small hairline cracks after firing. Drying down of stoves can be done (and in fact should be encouraged as it will save fuel) but then the stoves should be put afterwards directly in the kiln immediately followed by firing.
Problems after production
Sometimes after the stoves have been made and stored for some time the stoves crack or pieces chip off. This is caused by lime nodules (size over 1-2 mm) in the clay which during firing are converted to quick lime. During storage they can take moisture and the quick lime converts to slaked lime which causes expansion and can destroy the stove (visible as white spots in powder form). If such is found, the stoves should be dipped completely in water directly after firing which, in about 80-90% of the cases will solve the problem. Stoves are subjected during use to repeated heating up and cooling down cycles. A good stove can withstand this but cases are known where, during the first time being used. the stove cracked while sometimes the stove does not break directly but only after having been used for a few months. Solving such a problem is not easy as it can have many causes such as bad clay mixing, differential drying which builds up tension, localised overheating during firing and so on. Adding other materials like grog, sand or fibres does sometimes help while also firing at a lower temperature seems to have a beneficial effect but unfortunately not in all cases and then another clay source will have to be used where the same problem could probably occur.
Why lower firing temperatures do seem to have a beneficial effect is not known but it is thought that, because less vitrification (glass forming) takes place, localized expansion or contraction can be better absorbed by the ceramic material. Higher firing temperatures do have a positive effect on the strength of the ceramic material itself (important during transport etc.) and a compromise sometimes has to be found between mechanical strength and thermal shock resistance.
The addition of grog, sand and fibres gives more pores and therefore a more open structure to the ceramic material which is most probably why the addition (up to a certain amount) increases the thermal shock resistance.
In the literature the use of grog is often advocated while the use of sand is discouraged as it can cause problems due to the quartz inversion at about 570°C where a sudden expansion and contraction takes place during heating up and cooling. However during tests (in Sri Lanka) it was found that sand in general had a better effect than grog and could be used in larger quantities (10-20% for grog and up to 30% in the case of sand). The reason why sand has a better effect than grog is most probably the rounded shape and by being stronger then grog which is more irregular in shape. Very small cracks will not grow into larger ones but stop at hard grains or will go around it but because the shape is rounded will also be stopped. The insulation layer sometimes used around stoves can also crack and crumble during use. There are many reasons for this and maybe even more solutions. For such a layer more or less the same rules apply as for mud stoves. In Sri Lanka often use is made of ant hill clay as it contains a glue like material through which it has the same or even better bonding power as clay. In areas where clay and ant hill clay are difficult to come by, use is made of small amounts of cement (1 part cement and 6 parts sand). Sand mixed with wet slaked lime can also be used but such a mix ( 1 part lime and 3-4 parts of sand) tends to crumble easily in those places where it becomes hot. Adding 3-5% cement can help in such a case.
Note: A. Koopmans plans to carry out tests to determine whether high firing temperatures increase thermal shock resistance.