Ceramic stove components - chemical bonding of clays
Crispin Pemberton-Pigott, New Dawn Engineering, Swaziland April 2006
Dear Clay-Friendly Stove Builders
While looking into the different means one might use to get a low expansion clay stove body I tripped over a possible explanation of why the low temperature 'clay' bricks made from termite mounds are so strong.
Stove components are usually fired at what ceramics people consider very low temperatures: 800 C is about the max. We call it terracotta and it is only barely a ceramic in the view of industrial advisors.
When I test fired termite mound material in Likasi last year I got very strong bricks at 400 degrees which is definitely not a ceramic bond at the molecular level. They are reddish and appear to be clay but it is now clear these bonds are chemical, not ceramic.
I chased the idea that the potassium collected in the grass that is hoovered up by the ants (not really termites) from the ground around the mound is accumulated in the mix. This is a flux and lowers the melting temperature of the clay minerals. Yes it helps, but there is more going on.
Next I chased the idea that adding lithium feldspar to the clay (to get the lithium content up) might produce a low expansion clay tolerant of thermal shock. This works well, however the firing temperatures are so high that it is not going to happen in a field kiln. Unless 1200 is reached, there is no substantive change to the ceramic minerals resulting in a really low expansion product (meaning it still cracks when heated rapidly on one side).
Now it appears that adding phosphate to sand+clay and firing it at relatively low temperatures will give a chemical bond something akin to the termite mound result. There might also be a natural glaze in the material that is melting and glueing the particles together - not sure. The expansion rate of this type of material appears to have been explored little. Everyone in formal industry immediately goes for high temp firing because it is easy in a real kiln.
Given the constraint of a low firing temperature and a relatively low temperature in use (about 600 C), there is a lot of room for investigation of chemical bonding of clay and sand with the goal of finding (perhaps) not an ultra low thermal expansion (ULTE) clay but an expansion-tolerant chemical bond that is flexible enough to accomodate the stresses encountered lighting or running a stove.
The stresses are heightened by the interest in insulative ceramic components. Obviously if the stove design calls for a high temperature on one side of a brick and room temperature on the other, huge stress can arise in between as one side expands and the other does not. Similarly if a vertical pipe is used with the flames coming out the top, sideways under a pot, the top of the pipe gets hot and the bottom remains cool. The pipe tends to get conical and cracks because the heat is not distributed evenly, inside to outside, top to bottom.
The standard answer of adding sawdust or other filler to create a little air pockets in a porus clay structure may not be doing what is assumed in terms of resisting cracking. If the firing temperature is not high enough to create 'real' ceramics, the bonding is chemical and the porus sponge is simply the density it needs to be to flex without cracking. In short, it is not a flexible or expansion tolerant ceramic. It is soil glued together.
Lastly, the possibility that components can be dry-moulded (with very low water content) may be a good way to increase the chemical bonding because there is no (or almost no) shrinkage with its consequent micro-cracking. If the bonding via low temperatute firing is predominantly chemical, then the little cracks expected to 'heal' during firing remain open, interfering with the chemical bond, as the part is not heated to a high enough temperature to melt and flow the clay minerals. Thus it may be very important to focus on maximizing chemical bonds (using calgonite, for example) while keeping the cracks away by pressing parts in a mould. Pot-spinning wet clay parts may be the worst way to make a strong, low temperature stove component. If so, it may explain why parts made in that way are so weak.
Perhaps a new era in clay stoves awaits.