The composition and structure of corrosion products are important criteria in evaluating the authenticity of historical bronzes, and for this reason naturally-occurring copper corrosion products have frequently been the subject of scientific investigation. On the other hand, less attention has been paid to artificially induced corrosion, despite a well-documented, legitimate tradition for the patination of bronze statuary, and increasingly sophisticated efforts by forgers of antiquities to produce credible "archaeological" surfaces. The most reliable scientific method for detecting forged archaeological bronzes is the examination of their microstructure, but the procedure usually requires sampling, which in some cases is not feasible. The goals of a project currently underway at the Fairchild Center are to identify methods that effectively falsify archaeological corrosion, and to establish a methodology for characterizing the resultant patinas using the least invasive analytical techniques possible.

Simply stated, metal corrosion is a process of chemical dissolution. Cations migrate from the metal substrate and react with available anions to form the metal salts that constitute tarnish layers and corrosion crusts. The character and chemical makeup of the corrosion products depend on the nature of the substrate and the environment to which it is exposed. Bronzes and other copper alloys in indoor settings develop copper oxides on their surfaces. In open-air environments this initial growth of cuprite (Cu₂O) over tenorite (CuO) is usually obscured by green or blue secondary corrosion products, for the most part carbonates, sulfates, and chlorides. Over time, or if the outdoor environment becomes more aggressive, the layers increase in thickness and porosity, but as a rule the cuprite does not penetrate the substrate metal.

Corrosive attack proceeds differently if a bronze is buried. Soils contain mixtures of dissolved salts that function as weak or strong electrolytes, depending on their composition. Facilitated by these electrolytes, the oxygen present in aerated burial environments migrates very slowly through the superficial oxide layer into the metal, affecting first the areas most susceptible to deterioration, which are usually the grain boundaries. At the same time, metal cations move through the cuprite layer to produce secondary corrosion products on the surface. A layered structure develops, comprised of a porous, uneven deposit of secondary corrosion products overlying a coherent cuprite layer that penetrates the metal substrate along its grain boundaries. The intergranular corrosion formed by this process is typical of archaeological copper alloys (Figure 1).

The most common means of patinating bronze surfaces is the application of a reactive solution, but no known formulations have proven effective for producing convincing archaeological corrosion because they do not generate a significant cuprite layer (Figure 2). Recipes designed to produce green and blue corrosion products generally are solutions of copper salts, and in such cases the patina results from the deposition of these salts on the metal surface and their subsequent conversion to more stable, hydrated sulfate, nitrate, or chloride species. Cations from the substrate metal are minimally involved in these reactions, usually only for initial oxidation. Experimental work designed to produce convincing corrosion layers using repeated applications of such solutions followed by the reduction of the precipitated copper salts to cuprite is now in progress.

Another possibility for reproducing archaeological patinas involves treatment with solutions that do not contain copper ions and replicate conditions encountered in natural burial environments. To this end, bronze samples are exposed for extended periods to aerated environments containing different salts in various concentrations, and thus far these attempts have produced visually convincing blue and green corrosion crusts. The next phase of this project will concentrate on the characterization of these corrosion layers, and the comparison of their structure and composition with those observed on authentic archaeological bronzes.

Svetlana Burshneva is the L.W. Frohlich Fellow at the Sherman Fairchild Center. She received a diploma in archaeology from St. Petersburg University and in metal conservation at West Dean College. Since 1990, she has worked in the State Hermitage Museum as an archaeological metal conservator and she received her superior qualification in restoration in 1998. She has participated in excavations in Siberia, Central Asia, and Russia as archaeologist and field conservator, and has taught intensive metal conservation courses for conservators in provincial museums. Her research interest is the relationship between burial environments and the composition and structure of corrosion products on archaeological bronzes.