Bronze disease

Bronze disease is the name given to a form of corrosion of bronze and some other copper aiioys, in which iight biue-green outgrowths form on the surface (see Fig. 41). it is an especiaiiy obnoxious form of corrosion that particuiariy attacks ancient excavated bronze objects. Unless terminated by speciaiized treatment soon after excavation, bronze disease usually results in the complete corrosion and total destruction of the object. 

Bronze disease generally appears first in the form of bulky, bright blue-green, powdery spots that disfigure the surface of objects made from the alloy. The spots are quite easy to loosen and remove, but even after they have been removed, when the objects are exposed to the environment, the process continues and the spots recur in a short time. The spots are formed wherever copper atoms in the bronze react with chloride anions from salts in the soil and/or in groundwater. In dry places the growth 

FIGURE 41 Bronze disease. Bronze disease, one of the most serious corrosive processes besetting recovered bronze antiquities, resuits from the interaction of one component of the patina on ancient bronze, nameiy, cuprous chioride with atmospheric oxygen, in a damp environment. Smaii spots of a light green powder (composed mainiy of cuprous chioride) that grow rapidiy on the surface of the patina are indicative that the bronze disease process is active. Uniess the chemicai activity of cuprous chioride is inhibited by some conservation procedure, bronze disease generaiiy resuits in the eventual total destruction of bronze objects. 

The bronze disease corrosion process usually starts when copper atoms in bronze react with chloride ions under humid conditions, forming cuprous chloride, a bright blue-green, very unstable compound of copper  

Following this initial reaction, a number of hypotheses have been advanced to account for the ensuing decay process. The explanation that follows is one of several that have been proposed (Gilberg 1988 McLeod 1981). Part of the cuprous chloride initially formed reacts with oxygen in the air to form cupric chloride  

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Bronze disease necessitates immediate action to halt the process and remove the cause. For a long time, stabilization was sought by removal of the cuprous chloride by immersing the object in a solution of sodium sesquicarbonate. This process was, however, extremely time-consuming, frequentiy unsuccesshil, and often the cause of unpleasant discolorations of the patina. Objects affected by bronze disease are mostiy treated by immersion in, or surface appHcation of, 1 H-henzotriazole [95-14-7] C H N, a corrosion inhibitor for copper. A localized treatment is the excavation of cuprous chloride from the affected area until bare metal is obtained, followed by appHcation of moist, freshly precipitated silver oxide which serves to stabilize the chloride by formation of silver chloride. Subsequent storage in very dry conditions is generally recommended to prevent recurrence. 

Unless interrupted by preventive conservation treatment, the sequence of reactions of the bronze disease process recurs again and again, until all the copper in the alloy is converted to copper compounds and objects made from the alloy turn to waste. 

Clow, P. (1982), The Prevention of Bronze Disease in Archaeological Specimens, thesis, Portsmouth Polytechnic, Portsmouth. 

Gilberg, M. (1988), History of bronze disease and its treatment, in Daniels, V. (ed.), Early Advances in Conservation, British Museum, London. 

McLeod, I. D. (1981), Bronze disease An electrochemical explanation, Bull. Inst. Con-... 

Scott, D. A. (1990), Bronze disease A review of some chemical problems and the role of relative humidity, /. Am. Inst. Conserv. 29(2), 193-206. 

Moreover, the formation of copper trihydroxychlorides accelerates the corrosion of the remaining metal, and is accompanied by an important volume expansion, which results in a fragmentation of the object. This process known as bronze disease can be identified by the formation of spots or patches of a light green loose powder on the surface of the object (Fig. 5.5). 

Fig. 5.5 Light green powdery eruptions typical of bronze disease...

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