Aluminium Oxide
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The density of aluminium oxide is close to that of the metal itself so oxide inclusions may not all float to the surface.
Aluminium is a common and very deleterious impurity in sand cast gunmetals and bronzes. As little as 0.01% is enough to cause leakage of castings, because aluminium oxide films and stringers become trapped in the solidifying casting.
Melt down under a fluid cover of ALBRAL 2 to minimise oxidation and cleanse the melt of aluminium oxide films, then deoxidise.
