A few successful furnace cycles have been run, and a handful of lessons have resulted. I was given a whole bunch of machine shop turnings to melt down (13.6 lbs of them) and learned that it takes a little more care than melting cans. Turnings (also called chip) have very large surface area compared to their volume, and their surfaces are saturated with crystal structure defects due to the machining. Taken together this means that they very readily react and oxidize at elevated temperatures. Not thinking about any of that, I went ahead and tossed a bunch of chip into a fresh steel can and fired up the furnace.
As evidenced in the above photo, this didn't work out great. A second attempt was made later with three thoughts in mind. 1) Allowing the crucible to reach temperature would thicken the protective oxides on the steel that prevent reaction with aluminum, 2) an existing bath of liquid aluminum would prevent rapid oxidation of the aluminum turnings if they could be effectively submerged, and 3) the residual machining oil on the chip might be a contributor to the reaction. After allowing a new crucible to reach temperature, and observing that a foil-and-salt flux packet readily melted into a bead, cans were fed in until a good layer of liquid aluminum was apparent. At that point I was able to feed in foil-wrapped packets of turnings which seemed to melt readily without adverse reaction. I haven't come up with an easy way to remove all the machining oil, but in the short term it doesn't seem to be an issue. It will take me quite a while to melt down all the turnings, as it doesn't look like I can just feed them in non-stop, but there is no shortage of cans.