Why Does Copper Slowly Turn Green Outdoors?

Fresh copper sheet is reddish brown, yet old roofs are often blue-green. Copper does not simply react with oxygen in one step to make a single substance called “green copper oxide”. The change is a slow sequence of surface reactions.

Copper first reacts with oxygen, producing a thin layer dominated by cuprous oxide that can shift from reddish brown to darker colours. When moisture forms a very thin film on the surface, carbon dioxide, sulfur-containing pollutants or chloride ions near the sea can dissolve into it and take part in further reactions. Because time, humidity and local air differ, the resulting proportions of basic copper carbonates, basic copper sulfates and chloride-bearing copper compounds also vary. Patina is therefore not one substance with a fixed composition. On copper exposed for decades in urban air, the outer green layer often contains the basic copper sulfate mineral brochantite.

This is still corrosion. Unlike rust on iron, which is often loose and flaky, however, a mature copper patina can adhere compactly and slow the passage of water, oxygen and ions to the metal below. A green old roof is not necessarily losing copper at the same continuing rate. Its colour records a history of environment and reaction while also forming an imperfect but useful barrier.

A copper strip placed in salt and vinegar may change colour much faster, but that is an accelerated reaction under deliberately altered acidity, chloride concentration and exposure. It does not reproduce the composition of a patina formed outdoors over decades. After such an experiment, the strip should also be kept away from eyes and food.

https://pmc.ncbi.nlm.nih.gov/articles/PMC5503407/
https://hero.epa.gov/reference/3220924/