Waaaayyyyyy back in the day when I was in high school, I did quite a bit of aluminum casting. I only needed to do a couple of pieces, but I really liked it, so helped out some of the other guys that were having trouble. As a result, I rammed up quite a few molds (or instructed others) and oversaw the melting and pouring of quite a bit of AL. Here is what we would do:
Start out with some pop cans, typically a #6 bulge crucible about half full of crushed cans, then add some extruded aluminum from old storm doors, etc. While this is starting to melt, we'd have a couple pieces of broken cast aluminum heating up on the top of the furnace lid. Once the metal in the crucible started to melt, we would add some preheated cast aluminum, and we would add it very slowly. Once it melted, we would skim off the dross, add more cast AL if necessary to fill the crucible. Once the full pot was ready to go, skim once more, then pour into ingot molds.
Ingots would get tested for 'gumminess' by drilling a test hole in the ingot. If it was deemed as material that would machine reasonably well, a sharpie was used to place a mark on the ingot. Ingots with marks were then saved for casting projects that would require machining later, such as pulleys for belt sanders. Unmarked ingots were used for casting simpler items like ash trays, hood ornaments, etc. that required little if any machining. It was a simple and rather inaccurate system, but it gave students the chance to pour a number of times by making ingots, and also gave some idea of the differences in alloys. As someone mentioned earlier, using more good cast aluminum in a pour usually resulted in the best ingots.
