Practical Tips For Potters Making Improved Cooking Stoves,
Part 4-- Materials That Can be Added to Make a Better Mixture
Richard Boyt, August 2003
The following material is the fourth part of a 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 642, 3100 AP Schiedam, The Netherlands (1993).
"A very sandy clay found alongside a plastic clay would be ideal, but usually sand as a separate ingredient has to be added. Sand opens up the clay to allow it to dry more evenly to prevent cracking. The only problem with sand is that it consists of silica, which goes through a process called the 'silica or quartz change.' This means that the sand grains expand quite suddenly as they get heated in the kiln, then shrink again at the same temperature as it cools down. If too much sand/ silica is in the mixture, cracks can occur during firing.
"Sieved fine sawdust, if added to a clay mixture, will do four things: it will open up the clay (make more pores), and help with the drying process by allowing the water to leave the drying stoves more easily and evenly. The sawdust burns out during the firing, and so increases the porosity of the fired clay, which is an advantage, as long as it is not too much and weakens the stove.
"The addition of rice husk or rice husk ash has been found to really help with the strength and durability of fired clay stoves. It is often preferable to [use] sawdust, if a choice is available.
"Grog is the name for fired clay that is crushed down to a powder and sieved. It helps the stoves to dry more evenly, without the problem of the 'silica change' that comes with the use of sand. It is a good use for broken pots that would otherwise be wasted. Other things that have proved to be good when added to the clay mixtures used to make cooking stoves are charcoal dust and any sieved ash.
My reactions to the above quotations from "Tips for Potters":
1: Sand, as an addition to a clay body can be a mixed blessing, and can be overdone. It certainly reduces plasticity, and poses the problem of a rather sudden two percent expansion at about 590 degrees C (1,100 F) as the temperature rises. As the clay cools, there is an equal contraction at the same temperature, so passing either way through this temperature should be done slowly. The problem is that thick sections of clay do not heat evenly, or cool evenly, and so internal stresses can build up that can cause fractures to occur, sometimes well after the finished clay object has been put into service.
2: Not having access to rice husks, or its ash, I've never tried it, but I have used shredded newspaper, shredded corrugated cardboard, and various sizes of sawdust and charcoal. Of them all, charcoal is the only one that does not shrink as it dries. This can be an advantage in arriving at a clay mix with relatively low drying shrinkage. The size of the charcoal particles may be an important variable. Dean Still says it should be very fine, but that breaks the cell walls of the wood that had been made into char. If the cell wall is lost, the cell itself could fill with clay. A coarse char might prove to provide more air pockets in the finished product.
3: Grog is a very good addition to the clay mix, but does not necessarily reduce the problem of silica expansion/ contraction, if the grog itself contained an excess of sand, which may be why the pot it was made from cracked in the first place.
Hope this makes sense. There's a lot I don't know, but I'm gaining on it!
The next entry, Part 5 will probably be "Mixing Up Clay Mixtures and Testing Them".
Neosho, MO 64850
Note: See other articles by Richard Boyt