Crispin Pemberton-Pigott, New Dawn Engineering, Swaziland, June 12, 2006
I fired two of the rings today for a brief time, perhaps 3 hours at temperature. No particular care was taken regarding the slow initial temperature rise.
At 1PM I put them one on top of the other into a small electric kiln that could ju-u-ust accomodate them. The inside of the kiln is about 250mm wide and the rings are 240mm. They were thus very close to the elements.
The temperature was set to 940 and the unit switched on. There was no ramping. The elements stayed on until the temperature reached 940 where it remained for a while and then it was switched off. The temperature of 940 was chosen because that is what a local brick maker uses in his kilns.
At about 5 PM I took them out with a dirty leather glove (see the black fingerprints) when they were still several hundred degrees. I put them out in the wind on the tailgate of the pickup.
The one I was interested in most is the 11% charcoal dust ring. It was well formed with about 15% water and should give an indication of the performance of the entire approach to making stove parts this way because:
1. The material is lousy, not really a good clay at all, with the non-clay portion being about 25%, a material that we know shrinks a lot when it dries and would normally be judged completely unsuitable for making ceramic stove parts.
2. The firing was done without checking if the water was really at a low level in the body (seems it was)
3. The firing was done at full power with no ramping - supposedly a ba-a-ad thing to do
4. The ring was subject to significant thermal shock both on firing and on cooling - gives an indication of how it will perform when a fire is lighted inside the ring.
Result: The Laterite+Charcoal ring has no visible cracks running through it. It has some tiny cracks on the surface by they are hard to distinguish with only my reading glasses.
It has a vitreous-sounding weak 'ring' to it. Definitely a transformation has taken place. The pure laterite one however, sounds like a badly made soil-cement brick (thunk-thunk). It may not have been held hot long enough. It had no charcoal to help fire through completely.
The pure Laterite ring has some hairline cracks on it. They may have occurred during rapid cooling and they may not be deep. Only destructive testing will tell.
The charcoal + laterite ring was formed at a mass of 2891 gm. When fired it weighed 2471gm. After firing it weighs 2025 gm for a density of 1.34
Putting these figures into the spreadsheet shows the total moisture content of the laterite including the crystal water bound in the clay was 11.8% which was liberated during the firing. The true moisture content of the laterite was initially 7.43% plus and the crystal water in it which was 4.37%. To this I added more water to bring the total liquid water content to 14.5% (or 18.9% if you include the crystal water.) It formed very well at 14.5% so I do not envisage there being any need to go higher.
Shrinkage: Before firing the height of the ring was 60.90mm and after cooling it is 60.49mm for a shrinkage of 0.67%.
Even with a poorly chosen material, a poorly managed drying regimen (left on a desk for a week), a rapid firing and cooling, together with a low ultimate temperature such as those reached in rural wood-fired kilns, a stable product can be produced without cracking at any stage.
The key to this is to form the material so that it has no slip-zone structure (homogenous), relatively dry (about 1/2 the water normally used) and to do it at high pressure with a suitable manual machine.
The computer tells me that if the water content was kept the same at each stage, adding 29% charcoal dust to the laterite instead of 11% would have produced a ring with a density of 1.00. That would be 829 gm of charcoal per ring.