harden

Concise Oxford English Dictionary © 2008 Oxford University Press:
harden/ˈhɑːdn/

▶verb

• 1 make or become hard or harder.

  ■ (of prices of shares, commodities, etc.) rise and remain steady at a higher level.

• 2 (harden something off) inure a plant to cold by gradually increasing its exposure to the outside.

– derivatives
hardener noun.