Electrochemical Age Determinations of Metallic Specimens-Utilization of the Corrosion Clock.

Abstract

Dating needs an age-dependent phenomenon (a "clock"), a procedure for monitoring the advance of time by measuring a physicochemical quantity, and, in the case of archeological artifacts, a sampling procedure that guarantees the representativity and integrity of the dated objects. Metal corrosion in an aerobic atmosphere is a phenomenon whose advance can in principle be used as a clock that depends on the environmental conditions. In spite of the limitation imposed by differences in local conditions of corrosion, a new approach for age determinations has been developed and applied as a feasible tool for age determinations of metallic specimens studied by archeologists and historians. These techniques allow the recording of specific electrochemical features characterizing the state of growth of corrosion patinas, i.e., they are based on corrosion clocks. The application of corrosion clocks for age determination is possible in favorable cases where the corrosion happened to proceed uniformly and continuously. The proposed methods for dating of lead, copper/bronze, leaded bronze, and gold are mainly based on the voltammetry of immobilized particles (VIMP). This technique is exceptionally useful in the archeological domain because it requires only submicrogram sample amounts and permits sampling of different locations on the object, thus yielding representative data collected essentially noninvasively. Reported methods for dating of metals include lead, copper/bronze, and gold, obviously in all cases assuming uniform conditions of corrosion in a moderately aggressive environment. In the case of lead, age markers are porous PbO and PbO2 formed in the secondary patina. In the case of copper/bronze, aging is accompanied by a rise in the tenorite-to-cuprite ratio in the secondary patina. These changes in the composition of the patina can be monitored electrochemically using VIMP. The case of gold is different, as no "true" corrosion patina is formed. Here the age marker is the increase in electrochemically active gold sites, which is ultimately related to the adsorption of oxygen species and its diffusion/interchange/spillover through the external layers of the metal surface. Conjointly considered, such methods provide a new research line intersecting electrochemistry and cultural heritage that can be expanded via improvements in calibration and analysis to become an operative tool in the archeological domain.