Rocket wood stove using lava rock for heat riser insulation

Hello everyone,

I'm building a rocket wood stove for my shipping container shop.  I've got a 6" x 6" .25" wall J-tube design, using a metal barrel for the heat exchanger.

I'm at the point of gathering my insulation supplies for the heat riser.  I will fabricate another outer "insulation retaining wall" over the 6 x 6 square tube heat riser and pour the insulation material between the two.

I have been getting a lot of info off this forum board, so I thought that I would post a question which might be a resource for somebody else in the future.  My question is:

Is naturally collected lava rock a good choice for the purpose of insulation for the heat riser?  

The reason:  It's all around me and I would rather go out and get a bucket of this stuff vs. paying for something...

I have a screen which I could sift the lava rock through to get small pebble sizes and whatnot as I figure that would probably be the most desired size.  Well, good idea or bad idea?

Thanks!

Anyone?... I've done some lava rock sauna's before, but I'm hoping someone has done this for the heat riser insulation...

I doubt that most lava rock would be sufficiently insulating to make a really good heat riser. Unless it's the really frothy stuff (scoria) with lots of air pockets, I suspect it's going to be too dense and -- more importantly -- thermally conductive. A test (but only if you're really a bit on the foolhardy side): You know how you can pick up a glowing ember with your bare fingers? You can also heat perlite or vermiculite to a similar temperature (over 500 C) and hold it in the palm of your hand for a few seconds without getting burned. Try a small piece a lava rock and see how that goes. Have lots of cool water right there next to you and do the hot potato thing until you get a feel for it.

Scoria with a fireclay-sawdust binder might work pretty well. If you're in guinea pig mode I'd love to hear how this works out.

Phil Stevens wrote: A test (but only if you're really a bit on the foolhardy side): You know how you can pick up a glowing ember with your bare fingers? You can also heat perlite or vermiculite to a similar temperature (over 500 C) and hold it in the palm of your hand for a few seconds without getting burned. Try a small piece a lava rock and see how that goes. Have lots of cool water right there next to you and do the hot potato thing until you get a feel for it.

HAHA, I have not had any good luck picking up bare embers in my bare fingers, sounds like you are pulling my leg.  Are you just testing me?  

I can get some pretty good airy lava rock.  I live next to one of the thinnest layers of earths crust in the northern hemisphere, but I know what you mean by lava rock that is too dense, I see that here too.

Sorry, you lost me on the potato thing, I grow some nice creamy potato's to eat, but that's as far as it goes round here.  What is the potato thing?  

You never played hot potato? Like when you pull a baked spud out of the oven and bounce it quickly from one hand to the other?

And picking up embers is no big deal. It's the same principle as firewalking: the coals themselves don't have a lot of thermal mass and they're not very conductive, so you can touch them briefly with impunity. Try it and you'll see what I mean. But I'm not so sure I'd try it with lava rock unless it was a small chunk and I had a bucket of water close at hand....

YES!  I get the potato thing now.  I can be dense as a piece of Osmium at times, haha.  I have played the potato is hot and burning my fingers, but I need it out of the fire game a few times.

I can relate to the fire walking analogy, and further it into my own experience as aluminum feels hotter to the touch after welding than mild and stainless steel.  Not because it has a higher liquidus point. but because it is a much greater heat exchanger.  

The reasons that we used lava rock for the sauna were:
They do not explode in the fire
Heat retention was good
re-usabe
accessible and natural/free

I think I know where a big ol scoria lava rock field is and would be fun to check out again.  Looks like by definition scoria has a a specific gravity greater than 1 and will sink in water.  I think I can get a good bit of lava rock that will float too.  Seems like the right stuff.  I can probably pick some chanterelle mushrooms on the same trip.

I welded up my chimney and firebox tonight, so I can give it a light tomorrow and see what I've gotten myself into this time.