Please guide me for the part of speech of clean here in this extract:

He had schooled him in the evils that befall prophets; in those that come from the world, which are trifling, and those that come from the Lord and burn the prophet clean;
for he himse lf had been burned clean and burned clean again. He had learned by fire.

Excerpt from “The violent bear it away”, Flannery O'Connor.