AN EXAMINATION OF THE PATINA AND CORROSION MORPHOLOGY OF SOME ROMAN BRONZES
DAVID A. SCOTT
The aim of this paper is to examine, in detail, the corrosion and patina of some Roman bronzes in the collection of the J. Paul Getty Museum. Strictly speaking, “patina” and “corrosion” are different words for the same surface alteration, but here “patina” will be used to describe a smooth, continuous layer that preserves detail and shape, while “corrosion” is used to describe mineral deposits and surface accretions that do not form a continuous and smooth layer.
Patina formation and the corrosion of ancient bronze objects have been the subject of many previous studies, yet there are few detailed descriptions of the corrosion products of ancient bronzes that comprehensively document identity, mode of appearance, and corrosion micromorphology. Corrosion structures encountered in ancient bronze objects have many similarities to morphological processes that occur during the weathering of soils and rocks, yet the descriptive terms available, taken from metallic corrosion, are not adequate to describe the thick, complex mineral layers that are often observed on ancient bronzes.
The information that can be derived from mineralogical study of ancient bronzes has a direct impact on the degree to which surface cleaning of such objects can be carried out during conservation. One of the most currently favored and best-informed approaches to their surface cleaning is to examine the object carefully under a binocular low-power microscope and clean the artifact to reveal, if possible, an “original surface.” Such surfaces often retain surface detail derived from the object itself, such as spinning marks, inscriptions, and mold lines from casting. It is possible to find this interface preserved in corrosion products because copper alloys frequently exhibit a layered structure in their corrosion, and one of these layers often preserves surface detail, either in the cuprite, malachite, tin oxide, or other “marker layer” or “original surface” (Organ 1963, 1977). Some years ago, Jedrzejewska (1976) drew attention to ancient bronzes as “documents of the past” in their excavated, uncleaned state, and she suggested that those of little or no artistic value should perhaps be preserved, for future scientific study and experiment, in their virginal, uncleaned condition. Another suggestion made by Jedrzejewska (1976) was to leave small strips of corrosion, untouched by cleaning, in less visible places as evidence of the state of the object before conservation.
All of the artifacts examined in this paper could have been cleaned skillfully during conservation to a uniform and aesthetic patina layer. Surface cleaning was carried out, but many areas were left uncleaned, and, as a result, much evidence has been preserved that would otherwise have been lost completely. In some cases—the cleaning of coins and inlaid artifacts, for example—uniform mechanical cleaning to reveal detail is highly desirable, but in other cases, mechanical cleaning can directly remove evidence of archaeological association and of the complete history of the events that have occurred to the object since burial.
The archaeological conservator has a tendency to clean all bronzes mechanically, especially if they have a good patina layer, without appreciating the destruction of evidence that this process necessarily entails. In accord with the advice of Jedrzejewska, echoed recently by Chase (1993) and Bassett and Chase (1993), the Antiquities Conservation Department of the J. Paul Getty Museum has developed a cautious approach to the conservation of archaeological bronzes. As a result, much associated material is now being left uncleaned in areas of less visual importance to an overall appreciation of the object itself.