I remelted aluminum cans as part of an “Engineering Day” demonstration in college. It went OK. Aluminum melts at around 1200F, so you don’t need a really hot fire. At 1200F, iron will just begin to glow a dull red if it is dark. In bright daylight, it will be black. So, if you do this outside and your iron/steel container gets red hot, you are probably way too hot.
We decided to use beverage cans so we would know the composition. Aluminum cans are made from grade 3004, which is fairly pure; with scrap from auto parts, you just don’t know.
We used a ceramic crucible and an electric furnace (we had those available). There is no reason you couldn’t use steel or cast iron (although you may get some iron pickup in the melt). Any rust present may be reduced to iron and contaminate the melt. A large amount of rust could be explosive, but if everything is clean, you should be OK.
The cans are covered with a lacquer on the inside (to prevent the contents from attacking the metal). This burns off around 500, but the fumes are not only very objectionable, but poisonous as well. Make sure you have plenty of ventilation. By the smell, you can tell its bad so you really don’t need any special knowledge; you just need to be able to smell. We burned the lacquer off in a separate operation, but I don’t know if that is really necessary.
You will get a bit of dross or slag on top, particularly if you are using cans (because of the large surface area). Aluminum is very reactive to air (regardless of temperature), but the oxide layer is effective at preventing a continuous reaction.
We used plaster of Paris molds, but we cured them in an oven at around 300F prior to use. You need to make sure anything you pour liquid metal into is dry, or it may come back out faster than it went in, and spraying liquid metal is a bad thing to be around.
Of course, wear heavy clothes. A leather welding apron and leather welding gloves are not a bad idea. A plastic face shield and goggles may also be called for. Sure, molten aluminum will go through any plastic as if it wasn’t there, but you are mostly trying to protect yourself from spatter and a two layer defense would probably do that.
One final word of caution. Hot aluminum does not look hot. It looks the same (shiny silver) hot or cold. Always assume it’s hot. Have extra buckets of cool water around and brush up on first aid for burns.