Observation and analysis of Zwischgold on late medieval sculptures

Zwischgold is a gold-silver bi-layer foil that was popular in medieval European sculptures yet is poorly documented in both historical and modern literature. It is notoriously difficult to identify unambiguously and to study in detail due to its ultra-thinness and tendency to corrode. This article focuses on understanding the corrosion states and technological features of medieval Zwischgold. High-resolution scanning electron microscopy/energy-dispersive X-ray analysis was conducted on 72 Zwischgold samples taken from 47 objects, prepared with ultra-microtomy to investigate the foil structures, layer thickness, elemental composition and corrosion products. Supporting data was obtained through FTIR and Raman spectroscopy regarding the surrounding materials, especially the binding media and pigments. This article aims to fill the knowledge gap in the history of medieval gilding technology and to provide a scientific basis for future conservation and restoration of Zwischgold artefacts in strongly corroded states.