YES, that aluminum pot is a no-no!!!! As stated, the slump phase is too close to the temperature(s) reached not only by the molten lead, but the heat source under the pot. (As an example, I use a cast iron propane range, and the fingers that the pot rests on glow orange every now and then.....) Add to that the pressure from 100-200 pounds of lead, and if it lets go, it'll happen QUICK.......Bad things, indeed.....
Dan