Copper patina · Verdigris
Verdigris or patina
Both are green, both sit on copper, and chemically one has nothing to do with the other. The distinction is not splitting hairs, because one of the two is poisonous.
What each one actually is
The damp cloth test
Run a damp cloth over the green area. If green comes away on the cloth and bare metal appears underneath, it was verdigris. If the surface stays exactly as it was, you are looking at patina.
The reason lies in how the two come about. Patina grows out of the metal, the copper itself turns into its compounds. The layer has no edge against a substrate because it is the substrate. Verdigris, by contrast, is a salt deposited on the surface, more coating than layer.
What that means on a building
When green appears on a copper roof or a copper façade, it is patina. It is wanted, it protects, and it belongs to the material. Clients watching a copper roof age for the first time still ask about verdigris regularly, because in everyday speech the word is used for any green deposit.
The answer is short: verdigris would need acetic acid, and there is none on a building envelope. What you are seeing is what copper has been doing for centuries.
Details of the compounds and how they form follow the German Copper Institute information sheet i.30 and general technical literature on copper corrosion.
Questions about verdigris
Is verdigris the same as patina?
No, they are two different substances. Patina consists of basic copper compounds, mostly carbonates and sulphates, with chlorides added near the coast. It forms through air, moisture and carbon dioxide. Verdigris is copper acetate and forms when copper comes into contact with acetic acid. Both are green; beyond that they have nothing in common.
How do I tell which one I am looking at?
By how the layer behaves. Patina sits firmly on the metal and cannot be wiped off, it has become part of the surface. Verdigris lies on top as a powder and comes away with a damp cloth. If green stays on the cloth, it was verdigris.
Is verdigris poisonous?
Copper acetate is one of the harmful copper salts. Swallowed or inhaled it is damaging; brief skin contact is considered harmless. Patina, by contrast, is non-toxic and irritates neither skin nor airways. That is why the distinction is more than splitting hairs.
Can verdigris form on a façade or a roof?
Practically never. It would need acetic acid, and that does not occur on building envelopes. What turns green on copper roofs and façades is patina. Verdigris is more likely to be met in the kitchen, on copperware that has been in contact with vinegar or acidic food.
Does patina have to be removed?
No, quite the opposite. The layer holds fast, is stable in itself and renews itself where it is damaged. It slows further attack by air and moisture and is the reason copper roofs last for centuries. Removing it takes away the metal’s protection.
Why are the two constantly confused?
Because they look alike, and because in everyday speech the word verdigris is used for any green deposit on copper. The technical term, however, means only the copper acetate. On a building it is patina, and that is also the chemically correct word.