Sand as the mass in a RMH

So I am finding not a lot of clay in my area - could I use sand, or just straight dirt (not humus), as the mass for the heater? Obviously cob is required to seal joints around the combustion chamber and barrel, but could the bulk of the mass be dry material? I was planning to clad the sides in timber anyway (not a fan of the cob look) so making that timber into a stronger box to support the load would not be an issue.

I know Paul used this approach in his 'movable' RMH, but I haven't seen or heard anything regarding the effectiveness or otherwise.

Also, whilst I am working from Wisner Corp's 6" plans, I want to make the bench wider so that it can serve as a day bed. I was planning to just space out the two flues to provide equal space between each of them and the two edges, is this likely to have any ill-effect?

Sand seems to rate higher as an insulator than it does as thermal mass, according to these charts:

Conductivity
http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html Sand, dry 0.15


Solids - Specific Heats
http://www.engineeringtoolbox.com/specific-heat-solids-d_154.html Sand, dry 0.19

Wet sand does better. Do you have any local rivers? You may be able to find some clay deposits on their banks.
I don't love this option, but did you know that cheap kitty litter is made of clay?

Reading that list though, dry clay is also listed as 0.15 - I can't imagine it would have much moisture left in it once the heater had been fired a few times. Or am I misreading it?

I think the real difference is the monolithic nature of a clay/cob mixture. Air is what kills your conductivity and no matter how much you compact sand it will still have millions of gaps between the grains. Clay is made of powdery fines that fill all those little gaps between the sand; in cob straw is just there for structure (like the fiber embedded in resinous fiberglass).

I might have to take a photo of whatever it is that is under my top soil - it almost looks like brown coal (it's not - we do have that in our valley, but it's not that shallow! Very densely packed brown/black stuff.

I feel that it might have the right sort of properties - perhaps I will need to devise some sort of heat absorption test using a probe thermometer.

Yes! Do that. Maybe a rock hound type on here can identify it for you. Maybe it can be broken up or crushed into a powder?
With what I've read about Australia's weather on the CBC, I'm just glad your valley is not underwater/on fire!

Funny you mention that - a week ago today we were 20km (13mi?) from a 65,000 hectare (150,000ac?) forest fire that was headed in our direction. April last year the entire farm (save the acre around the house) was underwater.

This stuff isn't rock - it's just really hard packed ... something. It crumbles in the hand easily enough. I will dig some up and take some photos.

My plan for a test is to fill a small can or saucepan to a measured depth, set the probe thermometer at a certain depth (say 1/3 from the surface) and then put it on the electric stove on a given setting, and measure temps every (say) 30 seconds. When it reaches the maximum displayed, I'll turn the stove off, and continue to measure until it goes back to room temp, or something near it. I figure I will repeat this for a few different substances, starting with everything at room temperature. Does this sound like a reasonable method?

Could be useful, if you have material to compare with your data.
But be sure to dry your material first! The water content will make your stuff look more conductive and appear to have more heat capacity.

Come up with any ideas, Phil?

Sorry Chris - been a busy week. Will get some answers this week hopefully.

I would think clay slipped sand, for lack of a better term, would be fine in non-structural cob. Just enough clay and dirt to make it monolithic, it would look like poured sandstone when done.

Any idea of an indicative ratio? Are we talking about 5% clay or something?

Try liter samples of 5, 10, 15% and see what happens when it dries. I have no idea how your dirt/clay will work. Mine would need about 10%.

Hi Phil-
When you say you are not finding clay, have you done a simple analysis on your soil? All soil has some clay, it's a matter of the particle size ratio of inorganics.... You can test by digging down in your yard to take a sample about a foot or so down (ideally 30") to get beyond the top layer which contains a lot of organics in the form of plant matter, etc. Let the soil dry out and crumble- you can screen if there are pebbles - just through a kitchen strainer or window screen. Put in a clear container; a glass gallon bottle is perfect. Add about half soil, half water, let slake 10-15 minutes, then shake well and let settle overnight. The faster it settles, the lower the clay content, and when it clears you will see the proportion of each particle size as it will settle in layers, sometimes even different colors if you're lucky! Some high clay soils will never settle completely and the water will not clear. More soils are classified as clay than not and as a cob ingredient most would work. If it truly is too low, you could beef it up by purchasing some bags of fireclay used in masonry mortar. It's cheap and if you can pick it up save even more. Most places that sell firebricks and cement blocks have it. You could also forage for "clay puddles" which is when a low area has flooded, drained and dried- in the same manner as the soil settling test. The smallest particles settle last and shrink more forming cracks and curling upwards at the edges. You can harvest them easily, as they rest on the fine silt layer and come right up. Another free resource is quarry dust- we have limestone here, or granite dust that collects from stone carving (your local headstone retailer) Either would be great in the mix.
Hope that helps.
http://pinterest.com/pin/317292736217091522/

Phil Hawkins wrote:This stuff isn't rock - it's just really hard packed ... something. It crumbles in the hand easily enough. I will dig some up and take some photos.

It might be old volcanic ash. We're sitting on top of about 300 feet of the stuff. It like mineral gravy when wet, and like weak concrete when dry.

Just when I thought I had time to conduct this experiment, I have found a water leak in my farm irrigation. Whilst I may not have clay, I feel I am about to be intimately familiar with mud and tree roots... sigh.