fire brick vs cob vs compressed earth block

[cgd]

Starting the planning process, and I'm trying to sort out the direction to go. We are buying a hand-press compressed earth block machine for building our house, and I'm thinking that could be useful in making an oven too. Certainly for the base/foundation, but I'm wondering about the dome. I'm pretty sure I can cut the blocks before they cure, so it might be possible to do a dry stack dome. My guess is that the CEB is probably more like cob than brick in its heat retention. I'd use fire brick for the oven floor in any case. Any thoughts?

[Pusuken]

cgd, how did your experience go? I am looking for a used CEB press and interested in making an oven with the CEB (in addition to structures). Thanks,

[david s]

Starting the planning process, and I'm trying to sort out the direction to go. We are buying a hand-press compressed earth block machine for building our house, and I'm thinking that could be useful in making an oven too. Certainly for the base/foundation, but I'm wondering about the dome. I'm pretty sure I can cut the blocks before they cure, so it might be possible to do a dry stack dome. My guess is that the CEB is probably more like cob than brick in its heat retention. I'd use fire brick for the oven floor in any case. Any thoughts?

In terms of thermal conductivity I'd expect little difference between cob and compressed earth bricks. Also little difference between them both and fired brick. There would be a little extra shrinkage on firing the clay (around 3 % depending on clay composition, degree of vitrification and the temperature the brick is fired to), so the fired clay would be slightly denser which would increase the thermal conductivity a bit, but it's still essentially the same material.
The drawback with unfired clay whether cob or compressed brick is that it is not sintered so can therefore turn back into mud and it is not as strong as fired clay, therefore subject to abrasion damage.
Just in case you were thinking that firing the oven would sinter the clay, forget it. The sintering begins at 573 C and while the crown of the dome on the inside may exceed that the rest won't and the 500 to 650 C range is quite unstable with thermal expansion of some of the materials quite high. If this range is not taken really slowly in a controlled fashion the result is cracking so better not to overtire the oven in an attempt to sinter the unfired clay. I hope this explanation helps. The upside is that mud bricks are so easy to shape and should be free to obtain, so any rebuild down the track only costs your labour.
I built a kiln using mud bricks years ago and when firing it with forced air induction, we were able to get it up to around 1100 C. On dismantling the kiln the wares inside had fired ok, but the clay on the inside face of the mud bricks had melted in sheets about 3 mm thick. The fluxing effect from the iron in the clay at that temperature was enough to melt the silica in the mud. All quite instructive.

How I love fire.