Why the Statue of Liberty Is Green (And Pennies Aren't)

The Statue's green hue is patina—a protective copper carbonate layer formed by atmospheric reaction.

The Chemistry

1. Fresh copper: Shiny salmon-pink (1886 dedication).
2. Oxidation: Cu + O₂ → Cu₂O (red), then CuO (black).
3. Carbonation/sulfidation: CO₂, H₂O, SO₂ (urban pollution) → Cu₂(OH)₂CO₃ (malachite, green) + Cu₃(CO₃)₂(OH)₂ (azurite, blue) + Cu₄SO₄(OH)₆ (brochantite, green).
4. Stable patina: ~30 years to mature; now ~5 mm thick, adherent, protective.

Why Pennies Stay Copper-Colored Longer

• Modern pennies (since 1982) = 97.5% zinc core, 2.5% copper plating.
• Zinc sacrificially corrodes first, protecting copper.
• Only when zinc depletes (decades, or if plating breached) does copper patinate green.
• Pre-1982 solid copper pennies do turn green over time (check old jars).

Patina = Preservation

The green layer shields underlying copper from further corrosion. Without it, the 31-ton copper skin would have eroded significantly. Cleaning it would cause rapid re-corrosion—so it's never removed.