On Feb 1, 9:49*am, Stealth Pilot
wrote:
...

I think that the same thing occurs with water believe it or not.
the fluid appears to dissociate into its component parts in the molten
aluminium.
the oxygen causes lots of oxide froth on the top of the crucible.
the hydrogen remains as a dissolved gas until the aluminium starts to
solidify whereupon it comes out of suspension as bubbles.
...


That sounds about right to me.

The solubility of a gas in molten metal increases with the temperature
of the gas. Aluminum, being a reactive metal, reacts readily with
the
oxygen creating dross and leaving the hydrogen in solution until
the metal cools whereupon the bubbles form.

Molten copper does not react readily with oxygen and oxygen bubbles
in cast copper are a problem when the melt is open to the air.

I've never heard of nitrogen bubbles in either metal so I assume the
solubility of nitrogen is very low for each.