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Iv no idea with white metals. but some metals can be strengthen by heating and quenching.

But its not something id advise but if you have spare white metal sprue, you could try.

I don't think there are any other ways. (not chemically or physically i can think of)

White metals IIRC have a low melting point so be extremely careful as well if its older and have lead in it it could be even worse.