Abstract
Abstract Background Archaeological silver objects from burial sites are of a grey-lavender aspect. The formation of AgCl leads to an increase in the volume of the objects, which may undergo a complete transformation into corrosion products. This degradation process has been little studied to date. In this paper, eight horn silver objects were studied by SEM and XRD. Results Analyses showed a corrosion system composed of bi-layers: a compact inner layer and a porous outer layer. Corrosion products were mainly silver chlorides (AgCl), chlorargyrite. Some objects displayed copper inclusions both in the metallic core and in the inner layer. Highly mineralized objects contained other oxygen-rich phases (with Si, Ca) in the inner layer. Soil markers were detected in the outer layer. Conclusion Based on these results, we put forward a corrosion mechanism for horn silver objects. Silver chloride is formed by the disappearance of the initial silver oxide layer. This AgCl layer is not protective and supports ion transport. Thus an outer layer of silver chloride, incorporating soil markers, is formed.
Highlights
Silver was one of the first metals to be used by humans, and is found in many archaeological objects such as coins, jewellery and ornaments [1]
scanning electron microscope (SEM)-energy-dispersive X-ray spectroscopy (EDS) analyses show the presence of 3 phases in the metallic core: a copper-rich phase (85at% Cu, 10at% Zn, 5at% Ag), a silver-rich phase (85at% Ag, 12at% Cu) and an oxygen-rich phase (57at% O, 10at% Zn, 5at% Ag)
As sample S5 is an Ag/Cu alloy, it will not be used in the study of horn silver corrosion
Summary
Silver was one of the first metals to be used by humans, and is found in many archaeological objects such as coins, jewellery and ornaments [1]. We find pure silver in many precious objects, like jewellery, which are often stored indoors in locations such as museums or churches. The degradation of these objects depends on the presence of pollutants in the atmosphere, a phenomenon which has been widely studied. The formation of AgCl leads to an increase in the volume of the objects, which may undergo a complete transformation into corrosion products. This degradation process has been little studied to date. Eight horn silver objects were studied by SEM and XRD
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