Using masonry rubble for thermal mass?

Another question about doing a RMH in the city. I prefer to use locally available materials, which around here are masonry bricks and rubble. Can the thermal mass around the exhaust ducting be infilled with cement and/or brick rubble (crushed)? And can the outside be a brick/cement wall? I know I can go out of the city to find clay somewhere, but why not use local materials if they are available? And what about around the base of the reburn chamber? Can that be cement (I'm assuming not because it could explode or at least crack in the heat, but I thought I would ask!)?

David Blackton : Any use of Cement concrete within the Rocket burner, and the first 6' of the thermal mass bench is Bad, The Lime in the Concrete starts to fail at 400 dF !
Famously paper starts to burn at 451 dF.

In a perfect world with all your supplies gathered close to hand, and having had a successful burn out of doors first and with the whole system hooked up, you would test
your lay out on your prepared floor, (cement or Wood ?) build your rocket Burner, and then layout your stovepipe one more time, and then start painting your sealed stove
pipe with Clay slip, this is to assure that you will have good heat transferring contact, fill in around your pie with Cob or concrete mix (after the first six feet ) and then when
you have guaranteed to yourself that you have good heat transferring contact You can dip each piece of rubble into a thin clay slip before embedding it into the Cob or the
concrete mix, again it is the small air bubbles that are or enemy, The Cob or the concrete should completely encase the stove pipe, about 2'' thick as it will be the support-
ing structure that carries the weight of your Thermal bench, your rubble is welcome as long as it is at least as heavy as Cob, except for decoration this woulD cut down on
the number of bricks available for use ! If one is prepared , and prepared to take the time, Cob is a friendlier material to work with, you will need to have rather stiff rubber
gloves to protect your hands, say two pair for every one that helps, and especially if you try to wear the same pair of clothes and shoes every day, you will need to replace
your Clothing, Cob which is sand and clay requires no gloves or special equipment rentals, and all Cob will wash right Out ! Guess which one i prefer to work with ! A.L.

OK, well I wasn't clear on how best to seal the stove if I am not using cement in that part of it. And what is clay slip? The basement floor is concrete with dirt underneath that. My plan is to utilize the fire bricks that I salvaged from the old coal furnace to build the heat riser. So what about packing just normal dirt (not topsoil or clay) around stove pipe, then topping that off with wet cement to take the gravity pressure off of the stove pipe? Or what if I just pack sand all around the pipe? A friend of mine buried an insulated swimming pool full of sand underneath his yard to store solar-thermal energy from the summer months for use in the winter months. The sand or dirt directly around the pipe would protect from the extreme heat, while the brick walls and concrete over the top would provide the structural support. Or maybe I just need to figure out how to make cob...

Can I just find a stream or pond and dig up some wild clay to use for this? I know a place where I used to find it...

David Blackton : WOW we really need a pottery Forum thread !Henry Ford the original, once famously said, they can have them (Ford's cars ) in any color they want, as long as
it's black!

You should be able to use any clay you can get your hands on as long as it's NOT Blue or blue gray, we need clay high in Silicates and Alumina, blue is condemned on sight !

The best explanation I ever heard for Clay slip is it is a good clay watered down to about the thickness of pancake batter !

A quick tutorial on grades of stove pipe, when i was going up there were some real thin grades of stove pipe that were usually said to have been blued, if the coating was left
intact and/or touched up with stove polish when it was taken down to be cleaned it was good enough to last as long as 3 -5 years, longer the farther away from the hottest
exhaust gases next to the stove ! if scratched it would rust in a season or less, Galvanized piping was to be used only for commercial operations and finally on oil burners down
stream from a draft corrector that mixed cellar air with he hot exhaust gases tempering them, It was also supposed to substitute warm cellar air for heat energy that would be
pulled out of the hot furnace after it had shut down simply by the 'draw'.

The stove pipe was soon found in many different homes too close to the furnaces hot exhaust gases, there had been a remarkable change in the galvanized coating on the
stove pipe, from shiny to dull this proved to be an indicator that trace amounts of Zinc had outgased from the Galvanizing, by the time it had turned dull, there had been a change
in the way that the galvanizing bonded to the pipe, and no more outgasing is possible, seeing the change is remarkable and we treat the first 6 feet of pipe to re-bond the
galvanizing !

There is also a heavier grade of stove pipes made for wood stoves that became popular in America after the First oil embargo of '73 !

So what grade of pipe should we use in our RMH? Counter-intuitively it is the thinest and lightest pipe that can be found, the stovepipe in the horizontal chimney should all be
considered as a sacrificial form, used to make the horizontal run out of cob, therefor the cheapest duct pipe which would be made as cold air return ducting for Commercial
Fossil Fuel Fired Forced-Air Furnaces, is the lightest and the cheapest which you will appreciate when you go to price the new stuff, I get mine from H.E.V.A.C. contractors who
I have repeatedly told I am making art with (O.K. a few know the truth,) all of them would rather give it to me that pay a tipping fee !

All of the brickwork of the Rocket burner can be dry stacked bricks, dipped in water 5 sec, and then dipped in the clay slip, always with your bricks best side in towards the
combustion chamber, as they fire you will get some glazing from the clay slip, however the Heat Riser should be made of laid up brick as a regular masonry project, to
promote long life and make sure that you don't destroy your good work when you set your barrel on top !

I will be off line and traveling several days, checking in irregularly, get 'the Book'- Pdf is cheaper, and faster with multiple copies ! Check out Bricks and Clay at >
traditionalovens.org[u] ... and go to [u]villagevideo.org/products,rmh/scenes/ to see good work done over wooden floors, and pay close attention to the 2nd(?) video clip preparing
(your home) for your new stove, that is enough to keep you out of trouble for a while, remember that most of the videos you see on U-Tube are full of crap, don't get caught
repeating someone else's mistakes ! for the craft ! Big AL!

Hi Allen whats wrong with blue gray clay?

I am only reporting i what i have been told here MOST of It is very low in Silicates and Alumina so that it gets condemned on sight, basically it has a too high expansion
and contraction rate ! Any one else want to weigh in here ? For the Craft Big Al !

Well, several years later, and I finally have the house to a point where I am getting ready to put in the RMH (I was rebuilding the roofs, backyard, masonry walls, etc.). I just found a whole pallet of new fire bricks for $1/each, and I also bought some burn barrels for $10. I've been having a really hard time finding clay though. I checked Home Depot, and they just stared at me, and finally someone sent me to an art store where it would have cost me thousands of dollars to buy enough clay for a RMH. I went to some masonry suppliers and tried some other construction materials wholesalers, and no one seemed to know what clay was. So I have two questions regarding the clay:

1. I found one place near me that sells "fire clay" (although they didn't know what just plain "clay" was). I got a few bags of this, but is that any different from just regular clay? Can I mix that with dirt or sand to make cob?

2. I did a soil test on my soil, and I can't figure out if it has any clay or not. Water seems to pool in our yard fairly well when it rain, so I had assumed the soil would have some clay, but when I did the glass jar test, it only has two layers, not three. Obviously the bottom 70% is sand/rocks, but the top layer I can't tell if it's silt of clay. On the one hand, everything settled out of the water in much less than an hour, which I've read is a strong indication that there is little or no clay, only silt. On the other hand though, I scooped off a bunch of this top layer and let it try out, then added a tiny bit of water and rolled it around in my hand until I was able to make little "snakes" out of it, which implies that that layer is clay, not silt. And of course the rain pooling in the yard also tells me it's clay. I'm afraid to carry in several tons of dirt into my house and build my whole project, only to find out that the dirt has no clay in it. Any thoughts on this? Also, assuming I can use the dirt from my yard, if it IS actually 30% clay, does that mean I don't need to add any clay at all? Or would I even have to add some sand into it to get the right ratio of sand to clay? Or can I just use it as is?

It would be nice to use the dirt from my yard for a number of reasons, not the least of which is that I have way to much dirt in my yard now that I've put in a 4' deep foundation wall all around my yard. Otherwise I might just be throwing away hundreds of bags of dirt, which I don't really even believe in!

David Blackton wrote:
1. I found one place near me that sells "fire clay" (although they didn't know what just plain "clay" was). I got a few bags of this, but is that any different from just regular clay? Can I mix that with dirt or sand to make cob?

Hi David
I've read about many people here using fire clay for the same reason but if you can harvest your own that would be even better as your going to need a fair amount of it once the thermal mass time comes and can start to really add up.

David Blackton wrote:
2. I did a soil test on my soil, and I can't figure out if it has any clay or not. Water seems to pool in our yard fairly well when it rain, so I had assumed the soil would have some clay, but when I did the glass jar test, it only has two layers, not three. Obviously the bottom 70% is sand/rocks, but the top layer I can't tell if it's silt of clay. On the one hand, everything settled out of the water in much less than an hour, which I've read is a strong indication that there is little or no clay, only silt. On the other hand though, I scooped off a bunch of this top layer and let it try out, then added a tiny bit of water and rolled it around in my hand until I was able to make little "snakes" out of it, which implies that that layer is clay, not silt. And of course the rain pooling in the yard also tells me it's clay. I'm afraid to carry in several tons of dirt into my house and build my whole project, only to find out that the dirt has no clay in it. Any thoughts on this? Also, assuming I can use the dirt from my yard, if it IS actually 30% clay, does that mean I don't need to add any clay at all? Or would I even have to add some sand into it to get the right ratio of sand to clay? Or can I just use it as is?

It would be nice to use the dirt from my yard for a number of reasons, not the least of which is that I have way to much dirt in my yard now that I've put in a 4' deep foundation wall all around my yard. Otherwise I might just be throwing away hundreds of bags of dirt, which I don't really even believe in!

Not sure where you heard that a soil sample has to have 3 layers but it doesn't. Just make sure your not using topsoil from your pile.
Making little snakes that don't crack too easily when folded is a good sign for clay. Another is to get a wet clump in your hands and squeeze - if it extrudes from between your fingers like linguini spaghetti it has clay in it. If it just gushes out of your hands with little to no structure its probably not clay.
What most people recommend is to make some test bricks with your soil. Vary the ratios of sand in each one, label and see how they turn out when dry.
At this point try breaking one in half. Does it have strength or does it just crumble apart?
If the plain soil sample cracks the most, its another sign you have clay (but too much). If it doesn't crack but has strength, then maybe you have what they call "ready mix".
Check out the permies cob forum as well.  cob forum   I'm sure there is lots more verbose info on how to identify clay

I know Donkey at proboards has lots of good info on identifying and working with clay also