Jamieson, Fausset and Brown on Daniel 3:19

19. visage ... changed--He had shown forbearance (Da 3:14, 15) as a favor to them, but now that they despise even his forbearance, anger "fills" him, and is betrayed in his whole countenance.

seven times more than it was wont--literally, "than it was (ever) seen to be heated." Seven is the perfect number; that is, it was made as hot as possible. Passion overdoes and defeats its own end, for the hotter the fire, the sooner were they likely to be put out of pain.