“When the flame changes color, the copper is pure.”
A friend of mine is a chemist and metallurgist and swordfighter and pyrotechnician. An interesting dude. He was physically illustrating a phrase to me that I’d brought up in conversation a couple days earlier. A phrase I’d known since childhood, under a number of different contexts. The Refiner’s Fire.
As my friend finished his sentence, the flame turned a bright green, as if on cue. He’d taken copper that was already 98% pure and made it about as pure as you can by putting it into a furnace like nothing I’d ever seen.