Problems with coated silver: whisker formation and possible filiform corrosion

This paper discusses the corrosion observed on lacquered Tibetan silver after 26 years on display in sealed cases at the Field Museum in Chicago. The process used to identify the sources of corrosive agents and severity of the corrosion problem is described; the sources were mainly the rubber carpet backing and wool inside the cases. The predominant corrosion is black silver sulphide crystals (whiskers) growing perpendicular to the surface. The other, less obvious type of corrosion has the thread-like appearance of filiform corrosion which forms under a coating. Filiform corrosion has not been observed before on coated silver; a possible mechanism is suggested. The corrosion damage to the lacquered Tibetan silver discussed in this paper should serve as a warning to conservators considering the application of a coating to a metal surface.