Patina, Rust, and Tarnish: How Color Changes as Metal Ages

A fresh metal surface shows the color of the metal itself - copper's warm pink, iron's blue-gray, silver's neutral shine. But metals are chemically restless: exposed to oxygen, water, sulfur, and pollutants, their surfaces slowly react, and the reaction products - oxides, carbonates, sulfides - form a new layer with a new color. Age doesn't fade a metal so much as re-skin it.

Those new colors come from the same handful of mechanisms behind all color. Most corrosion layers are chromophore compounds that absorb certain wavelengths - green copper carbonate, orange iron oxide, black silver sulfide. But when the layer is a very thin, transparent film, the color instead comes from interference, the same effect that colors soap bubbles - and its hue depends on the film's thickness. Both write the story of a metal's exposure onto its face.

Pick a metal and run time forward. Copper travels from bright pink through a dull brown to green verdigris; iron marches to orange rust; silver yellows then blackens with tarnish; aluminum - almost alone - stays bright. Each path is the metal's particular chemistry made visible. Age the sample and watch the color turn.

Heat steel and it paints itself. A transparent oxide film grows on the surface, thickening as the temperature rises, and light reflecting off its top and bottom interferes - so the steel cycles through temper colors: pale straw, then brown, purple, blue, and gray. Smiths read these to judge temperature and hardness; jewelers grow them on titanium on purpose. The demo computes the color from real thin-film interference as the oxide thickens.

Why does aluminum foil never corrode through while an iron nail rusts to dust? Both react with oxygen, but their oxides behave oppositely. Aluminum's is thin, transparent, and self-sealing - it forms instantly and stops further reaction, so the metal stays bright. Iron's rust is flaky and porous; it doesn't seal, so fresh metal keeps getting exposed and the rust grows and eats the metal. Watch both age.

Each weathering color is a specific compound with its own hue. The green of an old roof isn't "copper" - it's copper carbonate and sulfate. Orange rust is hydrated iron oxide; black tarnish is silver sulfide. Pick a corrosion product to see its color and formula.

The vocabulary of aging metal.

Six questions on patina, rust, tarnish, temper colors, and protective oxides. Instant feedback, no scores recorded - a wrong answer comes with a short explanation.

Aging color joins the physics of metals, thin films, and chromophores - here's where to go next.