BRONZE DISEASE: A REVIEW OF SOME CHEMICAL PROBLEMS AND THE ROLE OF RELATIVE HUMIDITY
DAVID A. SCOTT
THIS ARTICLE has attempted to review some of the relevant information concerning bronze disease and to suggest that more work is required to understand precisely all the variables involved in the process. Some of the more important points are summarized here.
The presence of paratacamite or atacamite on the surface of a bronze object does not necessarily mean that the object is undergoing active corrosion. Further research is required on the basic copper chlorides to understand in more detail their chemistry and their interrelationships with cupric complex species, particularly in the case of calumetite.
The location of cuprous chloride within the patina constituents can vary. In some cases it is adjacent to the metal surface, but in other examples it may overlie cuprite or be sandwiched between cuprite layers.
The problems of the appropriate RH for the storage of bronzes has been examined, and, for the majority of bronzes, an RH between 42% and 46% was found to be sufficient; cuprous chloride will not undergo chemical reaction at this humidity level, which already incorporates a margin of safety. More problematic objects may require lower levels. Testing this hypothesis is not easy, since bronzes are often treated and stored at low humidity in the conservation laboratory and then returned to display or storage after treatment; there the RH levels may be much higher, making continuous assessment of the situation difficult.
Since the cost of maintaining an RH of 39% is high, further museum work will be required to ascertain if the recommendation of 42% to 46% RH can be confirmed as soundly based.