Proverbs 25:22

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.