Once it was level with the stove opening, then I put cardboard on top again and covered it with sand-dirt and more wet newspaper.
Then it was time to make the clay! Two parts sand, one part clay. As I dug deeper into this trench, I amazingly discovered sticky clay (dirt). It was not full on clay but definitely clayish. Then the dirt on top was sandish but still dirt. So as I mixed the clay I realized I need some real sand, real soon. So I went to the driveway with the push broom and swept up the dregs of the grit we use in the winter. It was sand left over with some grit.
So then I began wondering about thermal mass. There is a switch off between conductivity and insulation. You don't want any air pockets since that is insulation with no conductivity. But at the same time you don't want too much conductivity as that won't store up the heat. So you want a slow dissipation of stored heat. Luckily soil actually does this like clay as well. But unfortunately sand does have too much air pockets in it!
(Cracks = too much clay, crumbles = too much sand or silt, both = not enough mixing).So that sums up the dilemma. Because I had just mixed clay up north and watched a bunch of the videos on this - basically it's kind of like bread dough - you want it to stay together but not be dry.
Soaking the clay ahead of time, and/or grating it through a screen, improves workability, as it's the clay lumps that will take up most of your time and effort on the mixing tarp.Luckily this is exactly what I did - I threw my clay into a bucket of water as I found the clay. Not sure why I did that but I knew from working the clay up north - that you definitely want the stuff thoroughly wet.
For those who don't want to do any fiddling a tank could be placed into a cob bench which would be a pre-heater on the way to the hot water tank.So it turns out water is the best for both storing and conducting the heat as it uses convection to conduct the heat. And the above idea is exactly what I plan to do. I plan to just vent my exhaust directly into my water hole for drinking water!! So the water hole will store the heat, radiate the heat back out, and also I'll have heated water.
Of course beyond the home made clay - the other issues are - the size of the duct pipe, the amount of heat from a small rocket stove, the actual exhaust emissions - will there be back flow due to lack of exhaust power?
Of course this is just an experiment - I'm not "engineering" anything - I'm not measuring or estimating any BTUs or whatever. haha. The idea is to keep it as simple as possible and small as possible - therefore need as little as heat as possible. So I will have the tent super-insulated - with 3 feet of leaves on each side.
The above is of course just a "mock up" model and so the idea is that if this works with terrible clay conditions - so that heat and exhaust flows down the duct pipe - then it should be reproducible in much better clay conditions. The exhaust duct pipe for heating is just slightly elevated - it rises slightly - and I will have to dig out the end of the trench, so vent the exhaust. Normally a rocket mass heat has two duct pipes that circle back to the combustion unit - but this is a very small rocket stove and so should not need two pipes to vent off the heat and also the duct pipe is smaller.