When Danforth confronts Proctor and warns him, "We burn a hot fire here; it melts down all concealment." What device does Miller employ?in The...

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.