LEXICONMaterials and finishes
Patina
A surface layer formed on metal by reaction with its environment over time. On steel it is iron oxide, and whether it protects the metal or slowly consumes it depends entirely on which oxide formed.
Stable versus active
Magnetite — black iron oxide, Fe₃O₄ — is dense, adherent and genuinely protective. Red-brown rust, hydrated iron(III) oxide, is porous, holds moisture against the metal and keeps the reaction running beneath itself. Ordinary speech calls both patina. Only the first is stable, and distinguishing them by eye and by touch is the most useful single skill when assessing anything made from reclaimed steel.
Temper colours
Heating clean steel in air grows a very thin oxide film that produces interference colours in a fixed sequence: pale straw near 200 °C, brown at about 250 °C, purple near 280 °C, blue at about 300 °C, grey above roughly 340 °C. On a salvaged component these bands record grinding, flame cutting or welding, marking exactly where heat entered the part. They are chemically stable and worth preserving — and easy to destroy with an abrasive pad in seconds.
