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This was a wonderful post, thank you for taking the time to share it!

One question—I thought aluminum was really reactive with oxygen, and normally ever piece of it we see is coated with a thin film of the oxide. Does this present any special issues when heating it up to melt it?

I'm guessing the oxide is not a problem, it'll just float and you scoop it up. What I'd like to know is if more oxidation is taking place while the aluminum is being heated and when it is in liquid state. Heat should accelerate the process, right? And we need a shielding gas to keep the oxygen away while welding aluminum.

  > Twenty minutes later I have melted enough sprues and ingots that the
  > crucible is almost completely full. I take the lid off, skim the dross — the
  > bits of aluminum oxide and impurities that float — off the top and put it
  > in the ingot tray to cool; this will be trash. (Trying to recover aluminum
  > from aluminum oxide is not really worth it when aluminum is so plentiful.)

Correct; the oxide is not a problem. There's not much of it, it melts at a much higher temperature than the pure metal, and it floats, so you just scoop it off before you pour.

Whether oxidation or reduction is taking place during the melting process is determined by the amount of oxygen arriving via the air blast; too much oxygen and you get a hot oxidizing atmosphere, which oxidizes both my iron crucible and the top of the melt. Too little oxygen and you can actually get a reducing atmosphere, the furnace runs a bit colder, less heat escapes through the flue. So it's a tricky balance to strike and I'm still fine-tuning it.

I add potassium chloride and sodium carbonate to flux the melt and remove dissolved hydrogen; the KCl helps keep more aluminum oxide from forming apparently, though some home foundry sites report that they don't see a difference if they skip that step. Both chemicals are cheap, so I'm not too concerned.

It's only oxidizing at the air/metal interface. It's a very small amount and the oxide already in the basin is lighter than plain Aluminum so it floats to the top.

ever piece of it we see is coated with a thin film of the oxide.

The chemical composition of the oxide [0] is the same as the lenses on iPhone cameras, the so-called 'sapphire.'

[0] http://en.wikipedia.org/wiki/Corundum

It's not just so-called sapphire, it is sapphire (artificial, and a very thin layer of it but it really is the real deal).

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