"Scale" is iron oxide that forms on the surface of the iron as you heat it. It's a sort of dark metallic grey. When you hammer the metal, the scale flakes away and sits on the anvil like an eggshell, pearl-gray and curved and delicate. When you put the piece back into the forge, you can see the scale that's left, curling in thin and shining yellow leaves around the hammermarks like a stream around rocks, living gold.
There's the forge itself, the way flame bends around the metal, the cobbled luminous refractory along the inside walls of the forge, the colors the flame takes on: a pale green if there's machine oil on the piece, deep luminous purple for some kinds of paint, blue when the flame is warming up. All the musical ringing notes of the anvil - I can tell whether a hit is true by the chord the anvil makes.
I wish I was a good enough photographer to capture these things.