I stumbled upon this video, which is about a guy who melts copper and when it is molten, adds cast iron and then aluminum. The latter is probably a magnesium rich alloy (> 50% Mg) as it reacts vigorously with bright white light when introduced in the copper-iron alloy. Al does not do that as its boiling point is way higher than the temperature of the alloy already in the crucible (1300 C according to his pyrometer readings), unlike Mg which already boils at 1100 C. Before pouring a lot of dross is formed (probably MgO and Al2O3), at the end it shows only 3kg alloy of total 5kg with 2kg of remaining crap. At the end of the pouring the alloy appears to be very oily and viscous. What are your ideas ?
A nice green colour out of the mould. Looked more orange after a polish. Would possibly develop a nice green natural patina over time. Wonder if the iron would make it rust? Im pretty sure that wasnt magnesium or there would have been a huge ball of burning white light that wouldnt have gone out until all the magnesium had burnt off. edit The autoignition temperature of magnesium is approximately 744 K (473 °C, 883 °F)
What would it otherwise be ? Aluminum does not spontaneously ignite when added to molten copper. I have done this before for making aluminum bronze and the Al metal just melts and dissolves in the Cu metal without flames and sparks.
If it isn't magnesium then maybe some kind of thermite reaction with the aluminum and oxygen in the dross.