toughen

To make or become tough. synonym: harden.

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.

To grow or make tough, or tougher.

To make tough.

To become tough.

make tough or tougher