What is the verb of solid?

There isn't a direct verb form of "solid." "Solid" is an adjective, describing a state of matter.

Here are some verbs that relate to the concept of being solid:

* Solidify: To make or become solid.

* Congeal: To become thick and solid.

* Harden: To become firm and solid.

* Crystallize: To form crystals, which are solid structures.

* Set: To become firm or solid.

* Freeze: To become solid by cooling.

The best verb to use depends on the specific context and what you want to convey.