The identification of Zwischgold and other metal foils on historical sculptures by handheld XRF spectrometry

Zwischgold (also part-gold in English) is a two-layered metal foil made of gold and silver that was historically applied to altarpieces, sculptures and panel paintings. It tends to oxidize quickly in air and therefore was usually applied with a protective varnish layer. The purpose of applying Zwischgold is an interesting topic in the research of art history and art technologies. The identification of Zwischgold helps to define its application areas and thus the purpose of its application. However, when the Zwischgold surface is still well preserved, it can be challenging to differentiate it from surfaces applied with gold foils. In this paper, we present a handheld XRF analyser, "Niton XL3t GOLDD+", for a qualitative identification of Zwischgold surfaces. Its performance has been evaluated through measurements of Zwischgold models and also a 15-century altar figure applied with Zwischgold. The identification of Zwischgold by the "Niton XL3t GOLDD+" has proven to be practical and sufficiently reliable with confirming results from the micro-XRF analyser "Artax 800" and FIB-SEM with EDX on the same artefact. The convenience of a handheld device makes the "Niton XL3t GOLDD+" a very useful instrument for preliminary investigations where fast identification of surface materials on a great number of objects, especially large-scale sculptures and altarpieces, is required