For thou shalt heap coals of fire upon his head, and the LORD shall reward thee. Thou shalt heap coals of fire upon his head - Not to consume, but to melt him into kindness; a metaphor taken from smelting metallic ores: -
So artists melt the sullen ore of lead,
By heaping coals of fire upon its head:
In the kind warmth the metal learns to glow,
And pure from dross the silver runs below.
S. Wesley.