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 affected artefact is possible. Transfer of chlorides from the contaminated artefact to other artefacts can spread the condition.

Bronze disease is the chloride corrosion of cupreous (copper based) artefacts. It was originally thought to be caused by bacteria. It is contagious in that the chlorides which cause it can spread the condition if they are brought into contact with another cupreous object. Despite its name bronze disease can affect any copper-bearing alloy, not just bronze. It is not reserved for antique objects but can affect contemporary metals like modern cupro-nickel coins.

Bronze disease ranges from vivid green to pastel green. It is commonly present in all colours in this range due to the series of reactions that cause it and there may also be tiny, possibly microscopic, blue crystals. Bronze disease typically affects isolated patches of the object in severe cases being a visibly and tactilely raised bloom of microscopic crystals as well as being associated with pitting. The patches of bronze disease can be scraped off the surface using a fingernail or a wooden pick. These properties are all in comparison with verdigris, which is normally a duller shade, uniform across the whole of the affected object, and cannot be scratched off with wood or fingernails. Unlike bronze disease, verdigris serves to protect the metal.

As it relies upon the presence of chlorides, water, and oxygen, the absence of one of these three halts the progress, although any damage done is irreversible. Treatment for the condition typically involves physical removal of the chlorides (through scrubbing), chemical or electrochemical removal, and then isolating the object from oxygen, water, and future chloride contamination using an airtight container or a wax coating. These treatments may also remove any patina, loss of which is often seen as undesirable to collectors and conservators but is preferable to loss of the object.

Bronze disease is common or even ubiquitous on artefacts recovered from a marine environment due to the presence of chlorides in sea water. Coastal areas may also be hazardous due to salt carried in the atmosphere as well as the humidity. Absence of dissolved chlorides and oxygen in soil means buried objects may not be affected while interred (similarly, lack of soluble salts and oxygen means that buried metals may not develop a patina or that oxidation of the metal may be reversed). When an artefact is recovered, surface encrustations may hide and/or protect bronze disease.

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Wikipedia