I built a ~6in rocket mass heater (core only currently) and I'd like to burn out the wooden core before installing it inside. The fire box and bottom of the heat tunnel are high density fire brick, the rest is perlite and clay mix. The last ~5 inches of the heat riser i had to make out of 1:1 clay/woodash as I ran out of perlite and didn't want to run up town for it.
I had extra woodash/clay mix, so I made a quick brick and tossed it in an open fire and by the time the chunk of wood it was sitting on burnt away, I didn't notice any major cracking in the extreme environment I put it in. Thinking it might be ok to burn the core out if I keep the air intake managed, or maybe it is better to cover the top of the heat riser in this case?
BTW, if it wasn't clear, this is my first rocket stove/heater, but with lots of reading and research mainly on this site, I'm pretty sure I have everything correct, or atleast close lol. The reason I want to burn it out right away is because the forecast is calling for rain tomorrow and would like to get the core inside before then.
Added photos for a visual. Don't mind the mess in the yard.
Anyway, the intake does have a couple of cracks, I think the clay sip was too thick, and the gap was too wide. I plan to add some more clay/ash mix around that area to seal it up and insulate the burn box intake some, then cob over it once it is inside and placed.
The last photo in this post is the ash/clay brick I made and tossed directly into a fire. Has a couple small cracks, but held up a lot better than I expected. Based on the jar "shake" test, the ground I found is 95%+ clay with a tiny bit of sand. It took a long time for the clay section to settle, so pretty sure it isn't clay silt.
Anyway, if there isn't a reply in a little while, I'll probably just move forward and burn the core out carefully and hope things don't get shocked too badly.
Update on this, core burnout was a fail, I used hard wood planks from a pallet and osb for the sides and the hard wood bowed out and cause the heat riser to crack all the way down. Back to the drawing board.
As long as the parts are not falling apart, I would say to wrap them in another layer of insulation, maybe with sections of snap-together duct for the riser, pack the wrapper snugly and try it. This would be a good opportunity to test the configuration with a barrel on top and a sheetmetal chimney connected (outdoors). If it draws and burns well, run it hard and check for deterioration at the cracks. Best case, it stabilizes (the fire will strengthen the clay parts) and you can use it in your final installation. Worst case, you either find that it doesn't draw well, or that it breaks down, and you can start over. Keep the parts in any case. If you don't want to burn this one further, break it down, scrape the reddened or smoked areas out, and the rest can be rewetted and reused.
Clay is always going to shrink on drying, even if the inner form doesn't swell, so I would advise burning out the form _for a clay mix_ while it is still damp. I did that with a wooden inner form in a cobrocket stove and it worked fine. The only caveat is that the form needs to be sturdy enough to stand for 5-10 minutes to dry out the nearest clay before the form collapses.
I did a cob core once where I had no time or materials to do it right and had to use cardboard as the inner form; the riser form burned out while the cob was still all wet and it sagged. If I had had a board or two to support the riser, it would have been okay. I took off the top 2/3 of the riser and rebuilt it around a piece of ducting, then fired it, pulling the duct out when the cob was stiff enough to support itself but still flexible.
Thanks for the tips. It cracked on both sides with the board bowed and knocked down one of sides of the riser, so I just picked out the charcoal and burning wood the best I could and threw it in the fire I have going. I have read the clay/perlite mix needs something to protect it, but I figured being the heat riser, the most damage would be during moving/installing. Oh well, live and learn. I'll be re-using the perlite/clay mix and probably dig up a bit more fire brick and try to get the clay slip a little thinner to use as a mortar between the bricks and cast a little less with the perlite for my 2nd attempt.
When the wall fell, it sort of crumbled, but it was also damp yet, so I suspect that is normal.
With the good experience with the woodash/clay mix, I'm half thinking about making a brick mold and make my own bricks and fire them (slower/more correctly) and build the core out of the home made bricks. So far didn't have good experience with the clay as is for a mortar since it cracked while air drying unless it cracked from the wood swelling. Anyway, maybe I should add a small amount of sand to it to help against cracks.
Oh forgot to mention, starting the fire was super easy with a hand torch I lit a small chunk of hard wood board and stuck it in the burn tunnel, instantly drafted and continued burning, out in the open it would have died out and smoked. Tossed a few chunks across the burn tunnel, hit em with the torch for 10-15 secs and they were burning. Around a min or two later the "dragon" came to life. Around 3 mintues and I had ~5 foot flames from the wood liner burning out. Slowing it down just made it smoke so I left it roar and covered the inlet a little, just enough to continue burning the smoke. Sizes seemed to be pretty good for everything till the heat riser wall fell.
Been a couple of hours and the wood was still burning! Outside of it warm to the touch. Anyway took apart the cool area and the perlite/clay mix was a bit crumbly, didn't hold together very well. I think I didn't put in quite enough clay. A few months ago I did some test pucks and they turned out well, so my memory must have been playing tricks on me and I ended up with the wrong mix.
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