harden
v. B1 Intermediate US //ˈhɑɹdən// UK //hˈɑːdən// hard·en
v. to become firm or solid, or to make something do this. You can also use it to describe someone becoming less kind or more determined.
v. to make or become tough, solid, or firm in physical texture or emotional resolve. Often describes the process of materials setting or the psychological shift toward a less sympathetic attitude.
The wet cement will harden in a few hours.
The government's stance began to harden as the negotiations reached their third week without a resolution.
As the molten lava cools and begins to harden into basalt, it creates the jagged landscapes characteristic of volcanic islands.
Usage
Can be used both transitively (to harden something) and intransitively (something hardens).