Due to copper's longevity as an exterior building material it is subjected to long-term man-made pollutants and can, in time, accumulate assorted debris including bitumastics, tar, soot, dirt, oil and bird droppings.
The major concern with cleaning copper surfaces is permanent damage to the copper. Historical information points to some successful cleaning methods.
To remove encrusted deposits on roof surfaces:
Walnut shell dust is blown from a nozzle at 30 psi at the edge of the crust. This forces the crust to lift off the surface without damage to the copper.
To clean unevenly patinated copper:
- Sponge bathe copper with a mixture of six parts concentrated phosphoric acid to one part concentrated nitric acid diluted by no less than 50 percent distilled water to a pH between 1 and 1.5. A thickening agent can be added to the acid as a buffer and to contain the acid and make its removal and disposal easier.
- Leave the acid solution on the copper for one minute, then lift off with sponges soaked in sodium bicarbonate solution. Neutralize any residual acid by rinsing the copper with fresh water and treating with a sodium bicarbonate paste at pH 10, rubbed on with sponges and flushed off with water.
- Apply ammonium oxalate as a second neutralizer to even out any residue left by the first neutralizer. Sponge with rinse water to wash away the residue.
- Wipe the cleaned copper, using parallel strokes, with a clean cotton cloth until no color shows on the cloth.
- Wipe again with a cloth saturated with mineral spirits (no less than 96 percent aliphatics) and continue wiping until no color shows on the cloth.
- Apply a thin coat of carnauba wax. While the wax wears off fairly soon, it allows the copper a chance to start its repatination uniformly.
The above procedure can be used when replacing portions of a patinated copper roof or in instances of an addition where a new copper roof abuts a fully patinated copper roof. Treating the existing copper roof allows it to patinate along with the new roof resulting in a uniform finish for both roofs.