[Stoves] Experimental clay for low cost stoves - a suggested mix

[Crispin Pemberton-Pigott]

Dear Firey Friends

Those of you experimenting with clay stoves may be interested to reproduce this mix to see how far you can go with the production of clay stoves that last a long time without fracturing from heat stress.

The plan is to make a mix that is workable on a wheel, which could also be pressed in a mould, and which can be moulded by hand.  The mix should also dry without fracturing.  The thermal expansion coefficient should be low enough to greatly reduce stress when the stove is lit and various parts are different temperatures - between 20 and 600 degrees, for example.

There is little in this project to follow the general rules for clay selection as found in books like "Clay Testing" by Gaspe, Messer & Young or Hugh Allen's book on JIKO making.  Instead of making a high thermal expansion material that copes with the stresses, the idea is to try to stop the stress from occurring in the first place. 

The name of this material is PK11.  The % figures are by dry mass.  Compensation must be made for moisture content.

45%    Lithium feldspar
  5%    Potassium feldspar
33.3% black plastic clay
10%    charcoal powder
  3.8% bentonite
  2.8% laterite

Water content of 16-18% of the final wet mass is recommended for 'throwing'.

The stove/parts should be dried normally, then heated to 105 for a few hours (the parts are usually quite thin) then raised to 600 for 3 hours followed by firing to 1150 C for a few minutes and cooled normally.

The resulting bulk density is about 1.3.  The sound is a metallic 'ring' when struck and from the test so far, 10-12mm is thick enough for most single pot stove applications.  It could be used for grates only or the whole stove.  It could be used for cylindrical combustion chambers.  I recommend conical rather than flat grates.

The cost of the ingredients in Maputo is 12 cents (US$) per Kg.

The thermal expansion is about 1.9*10^-6.  This may not mean much to most people but it can be described like this: heating PK11 to 600 C is about the same as heating regular stove clay to 170 C.  Most people will immediately understand that heating a ceramic pot irregularly to 170 C would not be expected to break it even if it was repeated hundreds of times.  Thus PK11 should perform well.

As clay workers well know, no laterite, bentonite or black plastic clay source is exactly the same, however this is a good start.  If you want, you can email me your clay analyses we and can make a recommendation on a mix to suit what you have.

Best regards
Crispin