I think is down to method.
Just sticking a few lumps of alloy in an old pot, melting it and pouring into another old pot will get you anything, good bad or indifferent.
For a start iron is soluble in molten aluminium and the gasses cause bubbles.

You also need ge gassing tablets to throw in that cause all the dross, shit , crap and German routers to come to the top so it can be scooped off and thrown at next doors cat.

Material also plays a part just because you stuff one grade in it doesn't meant you get that grade out as it changes in the melt.
professional smelters melt a charge, run a test to see what's in it or more important what's missing and then lob in ear of bat, tongue of newt to make up.

Things like pistons when melted contain most of the original metal plus carbon from the burnt oil and deposits which doesn't help, clean alloy is hard to get in quantities.

One of the best grades is LM25 which is available in ingot form but rare to find as scrap items, when heat treated this is closed grained, machines nice and can even be bent slightly proving how ductile a casting can be.

Unfortunately to make good casting it then becomes another hobby at the expense of others.

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