I have done casting before but under lab conditions, reducing atmosphere but it still remains that evry time you melt Aluminium the surface of the pieces are scrap as the oxide will never melt.
You only have to see the prices paid for ally turnings, not worth a lot at all.
Each time you melt you produce more oxide scrap and are just wasting energy.
If you want to play then go ahead but you could end up paying more in propane than the excercise is worth!

Peter