Copper Patina

When bare copper is exposed outdoors it reacts slowly with oxygen, moisture and airborne sulphur and carbon compounds. The surface first darkens to browns and then, over years, develops the familiar green or blue-green coating called patina. Once fully formed, the patina is tightly bonded and highly resistant to further atmospheric attack, which is one reason copper roofs last so long.

Colour sequence

New copper is salmon pink. In the first weeks, especially in wet weather, rainbow-like interference colours can appear before the surface settles into russet and then light and dark chocolate browns. A dark oxide stage can persist for years before green begins to show. Mill oils sometimes cause temporary dark streaks that rain washes away.

Chemistry

The early brown films are oxides and sulphides, which offer only modest protection. The mature green layer is mainly basic copper sulphate in urban and industrial air and basic copper carbonate in cleaner rural air; near the sea, chlorides also take part. These films are thin, on the order of 0.05 to 0.08 mm, and are described as self-healing: a scratch slowly repatinates.

How long it takes

Published estimates vary with climate:

• Industrial and seacoast locations: about 5 to 7 years (Australian and US sources), or about 5 years in marine air and 10 years in industrial air (New Zealand guidance).
• Rural or moderate locations: about 10 to 15 years or longer; some sources cite around 18 years, and more than 30 in very mild air.
• Arid regions: patina may never form; copper stays dark brown or bronze.

Horizontal and low-slope surfaces green first, steep slopes later and vertical walls last.

Managing appearance

Clear lacquers and oils slow colour change but need reapplication. Factory pre-patinated copper sheet is available for an immediate green look, though its colour can vary between batches. Handling with bare hands can leave marks, so installers often wear cotton gloves.