So, to briefly review, the furnace that I’m going to build is essentially a bucket made of refractory concrete. The bucket will contain charcoal and a crucible: a smaller removable vessle that contains the actual molten metal.

What could possibly go wrong? Metal crucibles can fail in their welded joints or, if made too thin, simply burn through. Ceramic crucibles can crack. Both can be dropped during removal. So as a safe operations consideration, we should figure out how to deal with the containment failure situation.

(Apropos of nothing in particular, I once had a dream where the NPR guy, you know the one, said “NPR news reporting is financially supported by containment. Containment: the property that allows some things to be kept inside other things. For more information, log on to www.containment.com/npr.” Apparently I listen to NPR too much.)

If the crucible fails then the bottom of the furnace is going to be full of molten aluminum with hunks of burning charcoal floating in it. Obviously it’s going to be hard to get it out while still molten, and even harder once it solidifies. The solution is to not get into that situation in the first place; the furnace needs an emergency drain. We can put a hole, say 2 or 3 cm in diameter, in the bottom.

This safety system of course will only work if the drain is not plugged on either end. On the interior end, it seems unwise to assume that the cracked crucible is going to float on the spilled aluminum; perhaps it is only cracked halfway up and still too heavy to float. The crucible will have to rest on some sort of grate or plinth that permits access to the drain plug.

That then of course naturally leads to the question of “where does the molten aluminum go from there?” We’ll need an emergency containment system of some sort under the furnace. A hole with a bucket’s worth of sand at the bottom would do, or a cast-iron pot. The furnace cannot simply rest on the ground. And we certainly do not want the possibility of spilled molten metal on a concrete or cement floor, for the reasons described in the previous episode.