Bronze disease
Bronze disease is the irreversible and nearly inexorable corrosion process occurring when chlorides come into contact with bronze or other copper bearing alloys. It occurs as a dark green or a lighter fuzzy green coating on copper, bronze and other copper bearing alloys generally due either to contamination by salt water or after burial in dirt (as chloride salts are generally present in soil to some degree). If not treated complete destruction of the 'infected' artifact is possible. Transfer of chlorides from the contaminated artifact to other artifacts can spread the condition.
Reaction
Initially copper is oxidised to the cuprous ion:
(1) Cu → Cu+ + e-
The cuprous ion reacts with the chloride ion to form the insoluble white coloured salt cuprous chloride:
(2) Cu+ + Cl- → CuCl
The cuprous chloride reacts with atmospheric moisture and oxygen to form a green cupric chloride/cupric hydroxide compound and hydrochloric acid:
(3) 4 CuCl + 4 H2O + O2 → CuCl2·3 Cu(OH)2 + 2 HCl
The remaining copper is oxidised by air to the cuprous ion:
(4) Cu → Cu+ + e-
The cuprous ion reacts with the chloride ion in the hydrochloric acid to form the insoluble white coloured salt cuprous chloride:
(5) Cu+ + Cl- → CuCl
The reaction then repeats from equation (3). It is the presence of two different white and green salts that lead to the fuzzy green appearance.
Treatment and prevention
Initial treatment can involve placing the object in a desiccating environment. Deprived of water the reaction cannot continue. However, re-exposure of the object to even atmospheric water will restart the process.
Surface treatment
Removal of the chlorides is essential. In practise this first involves physical cleaning (with a wooden or even metal pick) to remove the bulk of the chlorides and then chemical treatment. One such chemical treatment is soaking the object in a 2 - 5% sodium sesquicarbonate solution. The weaker solution may have no effect whereas the stronger solution is likely to strip the patina valued by collectors. It is important to not use tap water as commercial supplies often contain dissolved chlorides as part of the sterilisation process.
The specimen should be held in a dry environment and periodically inspected for recurrence of bronze disease.
Internal treatment
If chloride ions have penetrated beyond the surface more rigorous treatment is required.
This typically involves soaking in acetone to displace any water in the specimen. Then soaking in a benzotriazole (BTA)–ethanol solution to chelate the copper and make it unreactive. Pits and holes may be filled with zinc powder which is then painted over with shellac coloured to look like the specimen.
Prevention
Waxes prepared with BTA are available commercially, the idea being that the BTA will prevent any reaction by chelating the surface copper and the wax acting as a physical barrier reducing exposure to water, oxygen and chlorides. (However, coating an infected object with wax will not obviate the problem.) Storing the object in a completely dry or oxygen free environment will also prevent bronze disease as will isolation from contact with chlorides.