I bought new 10-gallon and 30-gallon galvanized metal trash cans.
I put a concrete block in the bottom of the big one to raise the smaller one about 4″. I drilled 8 1/2″ holes around the bottom of the large one.
I put paper in the bottom, then filled the gap between the two cans with small sticks. The gap was only about 2″ all the way around, so not much room for the wood. I jammed in as much as I could, then put a couple of pieces of oak on the top. Next time, I think I should raise the small one another 6″ or so — that would give me more room to put larger pieces of wood underneath it, and also, since the sides of both slope outward as they come up, it would increase the size of the gap between them and make it easier to pack with wood. I’d like to get some larger pieces of oak in the bottom.
I packed the little one with small-diameter wood, mostly elm, put the top on it, and lit the paper through the holes in the bottom of the large can around 11am. I didn’t make any holes in the smaller can, trusting that gases would escape around the lid without building up enough pressure to cause an explosion.
It took a while to get burning, as the wood was right off our brush pile and not dry. I added a couple double handfuls of charcoal briquettes to the fire after maybe half an hour (of dubious merit with regards to carbon footprint, I know, but I wanted it to work). A lot of the wood had burned up after about 45 minutes, and at that point I added more 1/2″ to 1″ pieces. I put the top on it after about an hour.
I doubted the inner can got hot enough to make any char. I think raising the inner barrel and putting some oak underneath it would help quite a bit, and I’m not giving up on the system.
Four hours later, I took out the inner can and, sure enough, the wood was blackened, and some of it seemed to have become charcoal, but most not. I had put some old tomato vines on the top of the wood, and they seemed to be completely carbonized. Just not enough heat for the wood.
Later, I tried again, using the same wood in the inner can but adding more tomato vines. I raised the inner can another 5 inches with a concrete block, and made the fire with smallish pieces of oak. It got very hot, and I put the top on it after maybe 30 minutes because the steel of the outer can was glowing red and I didn’t want to burn a hole in it. There were some coals in the bottom, so it should have stayed hot for quite a while after I put the top on. I haven’t looked at the final result yet, but I’m much more optimistic this time.
Update: The second burn didn’t fully achieve the needed combination of temperature and time to turn the wood to charcoal. Only about half of it was converted to char.
Ending and Beginning 