Practical Tips for Potters Part 5 Making Up and Testing Mixtures, and Clay Preparation

Richard Boyt, June 2005

The following material is the fifth part of a seven-part condensation from the booklet "Practical Tips for Potters, Making Improved Cooking Stoves",

Prepared by Tim Jones, illustrated by Debbie Riviere, published by Hofman Systems Engineering b.v., PO Box 624, 3100 AP Schiedam, The Netherlands (1993)

Making up mixtures and testing them is a complicated and time consuming business, and it is easy to get lost in the process unless good and accurate records are kept. The usual mixture is of one or two clays with sand. It is better to use crushed and sieved dry ingredients as they are accurate to measure by volume. The test pieces should be small bowls or cylinders, not flat tiles, as the stresses set up in the round shapes of the stove have to be allowed for. If a mix experiences more than 10% breakage, you may need to change the composition of the mix. Try replacing the sand with grog or rice husk ash.

Mixtures should be tested for household use, but it is easier to test them at the workshop. The stoves need to have a strong fire lit in them twice a day for at least four or five days with a pot filled with water on top of the stove. Usually if the stove is going to crack, it will do so in the first few days.

There are two basic methods for preparing clay, the wet and the dry method.

With the wet method the clay from the ground is spread out in lumps to dry and then is soaked in water for several days, being well stirred until there are no lumps left. Additives such as sand or rice husks are added and then the liquid clay (slip) is poured through a sieve into a shallow tank where it is allowed to settle for several days. The resulting clear water is taken off and the remaining slip is placed in cotton sacks that are hung out to dry. It is then further mixed (wedged) so that it can be used for stove production.

The dry method uses all of the ingredients spread out on the floor in horizontal layers. The layers are watered and left to soak overnight. The clay is then dug vertically and set aside in small heaps which are then kneaded by hand (by foot). Alternately, the clay is dried, crushed, and sieved before any water is added. The disadvantages of this method are the amount of labor required it takes to crush and sieve the clay, and the dust produced which creates an unhealthy situation for the workers.

Once the clay mixture is prepared, it needs to be stored for as long as possible to allow bacteria to spread throughout the mixture. If the clay mixture develops a strong earthy smell, it means that bacteria is at work to provide a strength and plasticity in the clay able to withstand being made into stoves that will last. Keep the prepared clay in a cool damp corner of the workshop covered with wet sacks or plastic.

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My reactions to the above quotations from "Tips for Potters".

The liquid clay (slip) resulting from the wet method of preparing clay takes some time to remove the excess water. A week or two of removing the water that comes to the top can be followed by evaporation in shallow trays exposed to the sun and wind. I don't have access to rice husk ash, but I assume that wood ash might substitute.

The enclosed attachment contains drawings by Debbie Riviere for the "Tips for Potters" booklet. Additional useful information on the use of clay for stoves can be obtained from the Stoves Archives and Participants List.

Richard Boyt: "Practical Tips for Potters" Parts 1, 2, 3, and 4 (June, July, Aug 2003); "Ceramics for Stoves", parts 1 through 5a (May, June, Aug, Sept.

2003).

Dean Still, Ken Goyer, Damon Ogle, and Peter Scott of the Aprovecho Research Center: many entries of importance.

Hope this is helpful

Dick Boyt

Dick Boyt
rdboyt@yahoo.com
20479 Panda
Neosho, MO 64850
(417) 451-1728

Practical Tips For Potters Making Improved Cooking Stoves

Ceramics for Cookstoves

The Ten Can Stove, built by Richard Boyt (Feb 16,98)
Pictures of the TenCan Stove