BRONZE DISEASE: A REVIEW OF SOME CHEMICAL PROBLEMS AND THE ROLE OF RELATIVE HUMIDITY
DAVID A. SCOTT
2 THE PRESENCE OF CUPROUS CHLORIDE IN CORRODED COPPER ALLOYS
NOT ALL bronzes suffer from the presence of cuprous chloride, the mineral nantokite. The mineral form was first identified and named from the mines near Nantoko, Chile (Palache et al. 1951). Cuprous chloride can occur as massive granular lumps or, in laboratory-made samples, as tetrahedral crystals. The mineral is isotropic, with refractive index 1.930, and under the polarizing microscope may show anomalous birefringence, especially at the edges of the sample when mounted in Melt Mount (refractive index 1.66). The material usually occurs on copper alloys as a gray or gray-green translucent waxy solid that can easily be cut with a scalpel or a fingernail, since the hardness is only 2.5 on Moh's scale.
In the original bronze disease model proposed by Organ (1963), the reactive cuprous chloride, if present, is depicted as being formed at the interface between the bronze and the cuprite, which may grow over it. Other situations in which the author has observed cuprous chloride include the following:
- adjacent to the metal surface and under a layer of cuprite
- as isolated pits below the original surface of the object, often covered in warts of cuprite or malachite and frequently with cuprite adjacent to the copper or bronze
- above a cuprite layer. The cuprite layer is adjacent to the metal surface and the cuprous chloride is usually covered by malachite or basic copper chlorides.
- in the central core of the object, replacing previous metallic areas. In some cases these objects may be totally mineralized. If all the cuprous chloride has transformed, these mineralized objects will be chemically stable and require little conservation treatment in terms of stabilization, although they are physically very fragile.
Objects studied by the author show considerable variation in the extent to which surface disruption occurs with time. A number of bronze objects from Palestine are in the Petrie Collection, Department of Western Asia, Institute of Archaeology, University College, London. One of these objects, which had obviously suffered severe bronze disease and had been stored in uncontrolled conditions for more than 20 years, was examined and analyzed. The light green corrosion was identified as a mixture of atacamite and paratacamite, the most commonly found mixture of isomers in many ancient bronzes regardless of the location in which they are found. Most of the small objects in this collection illustrate the effects of disintegration in poor storage conditions over many decades. Little change is now evident, and many of the objects have stabilized themselves by reaction of the cuprous chloride with moisture. Some of these objects could be described as metastable, for they contain cuprous chloride at some depth and if cut or sectioned cuprous chloride in the unreacted state may still be found.
Objects, of course, cannot necessarily be left to stabilize themselves over long periods of time without disintegration and loss of material, which would be unacceptable. It should be recognized, however, that disfiguring light green corrosion excrescences on ancient bronzes do not necessarily imply that the bronzes are still unstable, even though they were excavated many years ago and kept in uncontrolled storage since that time. Some of these objects are reduced to fragmented heaps of light green powder, but those that survive may now be quite stable.
The ability of cuprous chloride to lie dormant until exposed to the atmosphere is unusual. In many other metals the presence of chlorides creates immediate instability, as in the case of iron, steel, and cast iron, in which the chlorides are water soluble and undergo chemical change quite quickly in burial environments. The relative stability and insolubility of cuprous chloride in unexposed pits create problems in the mechanical cleaning of ancient bronzes, since the exposure of such pits usually necessitates further stabilization measures or monitoring procedures. In some cases, especially where warty corrosion is present and it is desired to clean the warts down to the level of the rest of the patina, the exposure of cuprous chloride can create considerable problems with stability. In such cases aesthetic and practical decisions have to be made concerning the object. Is it better to leave the object with unaesthetic warty corrosion, or can the warts be cleaned and the object be either treated or kept in controlled storage and properly monitored?