I assume that you are asking what literary device is being used in this case. I would say that this is a metaphor. Danforth is comparing the trial process that they are conducting in Salem to a fire.
In a metaphor, an author compares one thing to another to help us understand the first thing better (or to see it in a different light). The author makes this comparison, but does not use the words "like" or "as." If Miller had said "Our trials are like a fire that melts..." that would be a simile.
As it is, Danforth is telling us that he sees the trials as a fire that will burn or melt away all lies and deception. This is a metaphor.