WEB Investigations of Medieval Zwischgold Samples by means of Transmission Electron Microscopy
Ultrathin gold supported with a thin silver backing foil is called Zwischgold. Due to the grain-boundary diffusion and few-monolayer surface coverage of silver, the Zwischgold appears lighter than pure gold leaf.  Since the 13th century, Zwischgold has been used as a cheaper alternative of gold leafs in panel paintings and sculptures.
From the point-of-view of materials science, these samples, some of which are 800 years old, are of great interest because they can be used to investigate diffusion and corrosion processes over very long time periods, which cannot be reproduced under normal laboratory conditions.
The present work concentrates on the preparation of Zwischgold samples by means of focused ion beam microscopy (FIB) and subsequent examination with transmission electron microscopy (TEM) in cross-section geometry. For this purpose, various sample fragments of medieval statues were collected. As protection against environmental influences over the centuries, organic varnish layers were applied to the Zwischgold. This results in two challenges with regard to the cross-sectional preparation in the FIB. On the one hand, the Zwischgold layers are sometimes found several µm below the surface, making it difficult to perform standard lift-out. On the other hand, charging effects might occur which require a specific adjustment of the microscope parameters for precise cutting.
The production of Zwischgold was already a reliably consistent process in the Middle Ages. It is therefore surprising that the Zwischgold layer is sometimes undermined by larger gaps between it and the substrate, as could be proven by TEM investigations. In addition, chemical analyses by energy dispersive X-ray spectroscopy (EDXS) have shown that even after about 600 years stored at room temperature the silver and gold layers can still be clearly distinguished from each other despite their complete miscibility. The mixing amounts to less than 1 at.%.
Moreover, diffusion of silver occurs along the grain boundaries in the gold layer, such that even smaller silver deposits could be found on top of the gold surface, often in combination with oxygen or sulfur. Thus, Zwischgold is getting a darker tone over time. This phenomenon is explained by the corrosion of silver, in accordance with our EDXS measurement.
 Qing Wu et al., Investigation of the foil structure and corrosion mechanisms of modern Zwischgold using advanced analysis techniques, J. Cult. Herit. 31, 2018, 122-132