|Boiling Point No. 19 - August 1989 (ITDG - ITDG, 1989, 36 p.)|
by Tim Jones, ITDG
At the start of improved cookstove development in Sri Lanka prototype cookstoves were made by a rural potter who had considerable skill in the manipulation of clay. He could, with a certain amount of assistance, quickly produce the many modifications found necessary as the stove was optimised for maximum fuel efficiency and clean combustion. The end result was the Anagi 2 all ceramic, two pot, cooking stove.
The stove that is now being disseminated in very large numbers to households in and around Colombo resembles a piece of functional ceramic sculpture and its good looks help a lot with consumer acceptability. The stove is assembled from a total of 15 separate pieces, all of which have to be made extremely accurately if the stove is to function at maximum efficiency. The three main pieces of the stove, the fire box, tunnel and second pothole are all thrown on the wheel. These make up the body of the stove and are made and assembled with specially designed boards, tools and templates. The remaining 12 pieces are press-moulded using Plaster of Paris moulds.*
The reasons for using the moulds are threefold:
1 - they produce the components of the stove with consistent accuracy. This is particularly important when you want to produce three of a kind, eg the pot rests in the firebox and on the second pothole. These are produced three at a time with one mould;
2 - to produce the stoves in large enough quantities (6000 a month!), a production line technique had to be developed, which used the skilled potters to throw the 3 main pieces but left the assembly to hitherto unskilled workers. The moulds cast by a skilled potter ensure consistent quality;
3 - the moulds are very fast to use and can be used continuously all day and then allowed to dry at night. They will last for up to two months, when they have to be replaced as they become inaccurate through excessive wear. Six separate moulds are used to produce the twelve pieces for the stove and over the last three years of the project they have proved to be a great success.
It was decided to take the use of press-moulding a step further and to produce the main body of the stove using a large, two piece mould split down the middle, both halves of the mould being filled with clay. This is smoothed out to give an even wall thickness, and the two halves are then joined down the centre of the stove. The advantages of this method of production are that the external dimensions and finish are determined by the moulding process so that the stoves can be built without throwing skills and minimal training. The disadvantages are that the mould requires a very skilled person to make and so is expensive. A mould of the size required is a very heavy object to move around, especially when full of clay and is easily damaged because of this. Despite the disadvantages two moulds are being used by a traditional potter and his family in Sri Lanka to produce 7 stoves a day for 3 days, after which the moulds have to he dried for 2 days before being used for another three days. After the initial field trials the potter made his own decision to continue using the moulds in addition to throwing the components on the wheel. Another use of plaster mould arouse as an answer to a production problem in W Kenya where the pot rests for the inside of the ceramic firebox for the G.T.Z. Mandeleo cookstoves were not of uniform size and shape and so were far from satisfactory. This was easily solved by making a small Plaster of Paris press mould that produced three identical, correctly formed pot rests in half the time it took to hand form three sub-standard rests.
Experience to date indicates that plaster moulding has proved to be a very successful method for producing quickly and accurately the all important components needed for a fuel efficient, ceramic cookstove. We would of course be delighted to hear from anyone else with ideas or experience with Plaster of Paris moulds in the production of stoves.
* Plaster of Paris is anothr name for partially hydrated gypsum or calcium sulphate. Try a hardware shop or chemist - it is not very expensive.