transitive & intransitive verb .
To make or become tough. synonym: harden.
The Century Dictionary and Cyclopedia
Specifically, of copper, in its metallurgy, the elimination of the foreign metals as oxids and of copper oxid itself, changing the qualities of the cast from brittleness to ductility.
To grow tough or tougher.
To make tough or tougher.
the GNU version of the Collaborative International Dictionary of English