I remember using a chimenea and having extreme heat come out the top. enough to melt change and burn out the steel screens that stop sparks from flying out. This furnace design is inspired by that upward funnel of the heat.
This is the bottom before pouring the concrete. It is a coffee can inside of a plastic flour pot. the coffee can has holes for the three steel rods that support it and will hold up the crucible. both the coffee can and the pot have holes that allow the air input through the aluminum beer bottle. The bottle design is a deviation from the drawing above allowing for insulation in the bottom and a direct air input to the coal. With the concrete. The top half is simply a coffee can with a tall thin bottomless can inside of it. This will hold the crucible once i drill holes in the bottom of the coffee can to allow the heat through.