Before I dive into my weekend's efforts, a quick note on nomenclature (so exciting, I know). I've been misappropriating a few terms as I've gone along, and should clarify. Smelting is extracting metal from ore, and inasmuch as soda cans aren't really "aluminum ore", I'm really just melting things. Also, I may have mistakenly conflated furnace (for melting) with forge (for heating to working temperature).
The first time we fired it up, starting the coals in a borrowed coal starter, the crucible seemed like it just couldn't reach a high enough temperature. After cooling and cleaning I noticed that the foil we'd thrown in as a test had melted, but only partially. Given the amount of time (and fuel) we gave it, this seemed strange. The body and lid of the furnace visibly darkened in color during use, leading me to believe that a good deal of the heat went into driving off residual water in the plaster. The body did have a week to set, but it was very thick and the lid had only set for a few days. A second reason that the aluminum did not melt may have been due to the crucible. As it is made of a silica-based ceramic, it doesn't conduct heat well and is likely better suited to a less directional electrical-element based furnace. Hoping that the initial firing had burned off any residual water, I decided to work on finding a steel crucible as well.
A second (better documented) attempt was made the following day. I thought for a while about where to find a serviceable steel crucible at short notice, and eventually decided to buy a large-mouth can of soup. The soup was tasty, and the can was steel. I've been calling them "tin cans" my entire life, but the magnet doesn't lie. While it looked bad enough after one use for me to throw it out, at a dollar or so a firing it's a reasonable approach in the short term.
I gathered together all of the supplies and got a friend over just in case of emergency then started in.
After dumping the started charcoal into the furnace, placing the crucible and replacing the lid, we turned the blower on and added the flux. For flux I obtained some Morton's Lite Salt, which is half KCl and half NaCl, which lines up well with the recommended flux for aluminum. Rather than pouring it in (the airflow kept tossing it back out), a teaspoon of the salts was folded into a pouch of aluminum foil and dropped in as a packet. After just a few minutes a metallic bead of aluminum was apparent at the bottom of the crucible, so we commenced loading in the scrap aluminum. A few charcoal briquettes were added when the melting slowed down, but far less than were burnt through the first time.
The air exits the furance vent fast enough to juggle bits of aluminum!
Using discarded cans as the sole source of aluminum did generate a lot of dross, as seen piled on the brick in the photo below, though a bit of additional flux tossed in at the end did seem to free it from the liquid. The two lumps of reclaimed aluminum are visible in the muffin tin. They were allowed to cool for about 20 minutes while my friend ran to the grocery for hot dogs and marshmallows, as the coals still had a good deal of heat left in them.
I'm planning on collecting all the supplies and their costs into a table for reference, just in case anyone else is thinking about taking a stab at this but is being held back by cost concerns. The next steps will be to build a mold flask for green sand casting, mixing up some green sand, and picking a few good objects to cast. I've got some ideas already!