Degradation of archaeological horn silver artefacts in burials

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.