We tried to hold a metal casting workshop on April 7th  but the flowerpot furnace design didn't get the aluminum hot enough and we had a lot of dross form on top of the almost melted aluminum. We did get some molds made up and ready to go.
The furnace consists of half of an old propane bottle as the main fire chamber sitting on a large metal can as the air chamber below. There are about ten holes drilled in the bottom of the bottle that are all within the area of the air chamber.
For the blower, I have the fan system out of an old dishwasher that blew air through the dishwasher during the drying cycle. This gives a low speed but quite adequate air flow for the charcoal. The fan is ducted right into the air chamber.
The lid is heavy gauge stainless steel with a hole cut into the center to direct airflow out over the crucible.
When the fan is on the charcoal burns with a steady outgassing of burning sparks away from the coals. This doesn't leave the firebox but it tells you that the heat levels are adequate.
When we lit the charcoal and set the crucible in place, it took less than twenty minutes to get a nice molten state on the aluminum. The exhaust hole over the center of the furnace keeps a reducing atmosphere in the furnace to minimize the dross and scum formed on the molten aluminum.
As I poured the aluminum there was some initial resistance until the first of the Styrofoam gave way and vaporized. Some of the aluminum spilled over the edge and formed a bright shiny mass on the ground. I am sure there are improvements to be done to out molds and sprues to allow the aluminum to puddle on top before caving into the mold.
After the pour we left the mold to sit and smoke. Since we had used a heavy oil as the binder in the sand, this was burning off and releasing smoke from our mold. The sand separated easily from the aluminum when we were done.