Verdigris

In detail

Verdigris is the green or blue-green layer that forms on copper and copper alloys as they corrode, chemically a mixture of basic copper carbonates, sulphates or chlorides depending on what the metal has been exposed to. It is not dirt and it is not tarnish in the sense that silver tarnishes: it is the metal itself converted into a new compound, so removing it removes material. On architectural copper the layer is stable, adherent and protective, which is why a church roof is left alone. On plumbing fittings, cookware, instrument keys and decorative brass it means something different — moisture is being trapped against the metal, usually by a cloth left in contact, a damp cupboard, or a cleaning product that was not rinsed and dried off. The carpet institute's copper corrosion and brass stain entries are about the marks such fittings leave on a textile rather than about the metal, and both are short: detergent solution, warm water, professional. On the metal itself the trade guidance for copper alloys is that treatment must be matched to whether the piece is lacquered, patinated deliberately, or bare, because a polish appropriate to one destroys another.

What works

• Non-ionic detergent is effective against Verdigris Established practitioner practice

Why

• Verdigris is explained by Corrosion and passivation

Hacks and claims involving this

• Lemon and salt to clean copper Established practitioner practice