After searching a bit and test my soil, I have determined that I have no usable amount of clay handy. I really want to build a RMH in our new cabin and wonder, Can you use packed earth instead of Cob?
I was thinking to build a hearth out of some cool green volcanic stone we have here, use mortar with that for the shell, and fill the bench around the pipe with either packed sandy loam type soil I have or use beach gravel. I can get truck loads of pea gravel sized stuff right off the beach.
I'm in the same situation where I am glacial scree and ancient riverbed good topsoil to 4 or 5 feet in some places (lucky me) and i'm going the route of dirtcrete basically 14 parts dirt one part portland cement this makes a suitable bench filler material you can increase the cenent portion to fit your needs and when you get down to your clay layer you can use that for the actual heater potion as it will be a lot more forgiving than than the dirtcrete which will crack under extreme heat. I'm excavating a problem spot in my garden that was part of an old leech field so it never really dries out and is kinda swampy to begin with. I figure a good use for it would be the greenhouse bench i'm planning on.
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Cob may or may not have been an engineered product, the key to it working so well is similar thermal expansion rates.
You may have some issues with mortar and volcanic ash. Prior to committing to and unproven formula, perhaps you could make a few small bricks of it, fire it in a wood fire and see what the results are. Might even embed a tin can in one of the test bricks to see if the thermal expansion on the steel wil cause separation, if it all holds up, give it a full scale shot.
Just hate to see someone work as hard and long only to find out it crumbles a few hot fires later.
Professor of Thermal and Electrical Engineering, Welding/metallurgy: Licenses: PE license, Mechanical license Variety of other "certifications" from industry groups such as Refrigeration Service Engineers Society http://www.rses.org/, ASHRE http://www.ashrae.org/ Ect.