Crispin Pemberton-Pigott, New Dawn Engineering, June 3, 2006
Dear Stove Builders
The first ProBEC Clay Ring Maker (CRM) is working and you can see it on the Bioenergylists website. The photos are changing daily as better ones are taken (thanks Tom). There is a sequence showing the entire cycle for making a clay ring.
These rings are used in the domestic Rocket Stove being produced by Andreas Michel in Malawi. This machine is to increase the production, make a uniform product and to reduce losses during drying and firing.
I have produced some rings from straight red laterite to test the compression and the results are very encouraging. This afternoon one ring was made with the composition:
11% charcoal powder
10% water added (to air-dry laterite) The water content is perhaps 15% - not important at this stage.
The mass will be in the region of 2050 gm after firing for a density of 1.35 depending on how much water is bound in the clay.
The spreadsheet tells me the charcoal powder content would have to be 34% to get a clay ring with a density of 1.0. Mixed 50-50 with the clay it would have a final density of something like 0.83.
The forming pressure is high enough to run the damp clay throughout the mix and the ring is strong enough to be picked off the mould with one hand and handled with ease.
As it has only 1/2 the water normally used to form a ring, this one should dry with a minimum of distortion or cracking. As it is not extruded at any time the mix is homogenous inside with no slip zones. The means it should shrink evenly in all dimensions (isotropically) and thus crack less.
Interestingly, adding water produced a higher density in some cases. Air dry laterite did not flow well enough to expel the air resulting in a porous ring. It is not clear if the clay will bind it well enough to be used like that. When adding charcoal powder (ground in a manual peanut butter maker) it can be pressed very hard with sufficient water to make it flow well and the porosity can still be set to any predetermined level because the charcoal burns out.
See also Programme for Biomass Energy Conservation in Southern Africa (ProBEC) www.ProBEC.org