New insights into Zwischgold application from a multi-analytical survey of late medieval polychrome sculptures at the Swiss National Museum

Zwischgold, or part gold, is a bi-layered metal foil made from gold and silver, popular in European sculptures from the 14th century. It is notoriously difficult to unambiguously identify and to study in detail, due to its close appearance to gold leaf (when still well preserved) and complicated individual situations. Although Zwischgold is nowadays commonly viewed as an economical substitute for gold leaf, arguments regarding its function have circulated for decades. In order to understand the application and related functions of Zwischgold in late medieval sculptures, a survey of 162 objects was recently implemented at the Swiss National Museum Collection Centre, focusing on polychrome wooden sculptures mainly dated 1400–1550 and produced in southern Germany and Switzerland. Handheld X-ray fluorescence (XRF) measurements show a high rate of correlated silver and gold signals mainly in a proportion range of 1:5–3:1, indicating Zwischgold in extremely diverse areas such as hair, beard, crown, gown fold, bordure, dalmatic, undergarment and attribute of saint statues, as well as altar background, frame and framework. This non-invasive XRF analysis is a first step to efficiently locate Zwischgold applied areas and obtain basic statistics regarding the Zwischgold applied artefacts. It provides objective evidence showing that the reasons for medieval Zwischgold application are not limited to cost-saving, but include other factors such aesthetics, symbolism, workshop preference and workability. We discuss such factors with some illustrative case studies, in which visual light microscope (VLM) and scanning electron microscopy coupled with energy dispersive X-ray (SEM-EDX) measurements on samples exhibit certain technological features of Zwischgold of that epoch, including its foil structure, thickness and materials compositions