The furnace shape and size seemed to work fine. We were burning about 3 lbs (about a gallon) of charcoal every 12 - 15 minutes. The total charge of ore was about 18 lbs - added in 3 lb lots. On the face of it, the furnace seemed to be working fine. We got liquid slag at the point your 'reciept' said we should. This material was pretty viscus, and still had considerable iron content. Over the second tap of slag, the material become more fluid, more glass like and far less dense, At that point I was pretty excited - as I knew the iron had to have gone somewhere.
We started developing a problem at that point.
First, the liquid slag started to pool up high enough to clog the air
intake. As the air was hitting the liquid slag it was solidifying in a
curved shape extending from the bottom. The hole for air moved towards
the top of our pipe - in fact deflecting the air up. This was despite poking
with a rod - eventually the air pipe was actually inside the slag pool.
I'm not sure why the 'bowl' formed so high - there was about 25 cm between the floor of the furnace and the air intlet. It may have been that the angle was not directed down enough. Note that Gus thinks the air * flow * should have been higher. We came to the air volume based on your notes and visible consumption rates. We were adding charcoal every time the level dropped down about 6 - 8 inches in the stack (the 12 - 15 minutes I mentioned). We tried 'full blast' for a minute or two, and you could see the fuel level dropping. My guess is at that air flow we would have been adding 3 lbs charcoal every 3 - 5 minutes. (as it was we used about 6 full bags) Our intake pipe was made of straight 1" ID steel (schedule 40) - if the end had been slightly crimped down to closer to 3/4" we might have had better delivery * pressure * . We had no way of measuring air pressure. Air volume may be estimated by timing how long it takes to fill an industrial garbage bag.
Right after the 2nd tap we noticed the start of a burn through on the side of the furnace, about 6 inches or so above the air inlet. Obviously the hot spot had moved from closer to the centre to along the furnace wall. This process would continue and excellerate over the rest of the firing. Eventually we ended up with a metal plate attached to the outside covering a hole that was about 6 x 8" - judging from the orange heat on the metal.
What seems to have happened is that the iron rich slag taped out eariler was becoming semi molten and then re-freezing when it hit the spot where the furnace wall burned out. The slag poured out near the end of the process was likely liquified furnace wall. In the end we ended up breaking down the furnace (which I would rather NOT have done - but too many hands where jumping in and things got out of control in the excitement). There was a large mass of 'gromp' attached to the furnace wall at the burn out point. I have not wieghted the end products yet - but expect that the metal is here - likely as semi processed grains.
Once I get a chance to recover the remains from the smashed (and unfortuneately kicked around) furnace remains I'll drop you another note. I'm hoping we may have something like spongy cast iron mixed in with the gromp. We did test over the hot surfaces of the grapefruit sized gromps with a magnet, and the pull is incosistant (which gives me some hope).
We did learn a lot in terms of physical process and set up. I feel the overall design of the furnace is sound - but needs some tuneing in details (wall thickness / air intake angle / air pressure)
Needless to say, I'm also pretty frustrated by the lack of a bloom. Its a major effort for me to mount one of these firings, in terms of time, effort and resources.