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