by The latest nanoscale 3D scanning techniques have been used to reveal some of the best-kept secrets of a medieval material known as Zwischgold (partial gold): an ultra-thin metal sheet consisting of a gold top layer and a silver base, used for gold sculptures .
Until now, only 2D cross-sections of the materials had been studied, but in a new study researchers were able to create 3D representations of Zwischgold for the first time, revealing how it was put together and why historians may face challenges in restoring medieval art.
The four 15th-century specimens studied included one from an altar originally housed in a mountain chapel at Alp Leiggern in Valais, Switzerland, and now on display in the Swiss National Museum (Landesmuseum Zürich).
“Although Zwischgold was often used in the Middle Ages, very little was known about this material until now,” says physicist Benjamin Watts, from the Paul Scherrer Institute in Switzerland.
“So we wanted to investigate the samples using 3D technology that can image extremely fine details.”
To do this, Watts and his colleagues used a sophisticated microscope imaging technique called pytogram tomography, which emits X-rays through a sample of material to create shadows of varying intensity called diffraction patterns.
By adapting the imaging technique and combining different diffraction patterns, it is possible to reveal details that may be only millionths of a millimeter in size. The researchers describe it as a “giant Sudoku puzzle”, where the entire image of an object is gradually revealed with each additional image.
The scans reveal a layer of gold about 30 nanometers in size, thinly and evenly spread over a silver base layer (some of the thinnest human hairs are about 50,000 nanometers). By comparison, an analysis of modern Zwischgold samples conducted in the same study measured thicknesses in the range of 48 to 82 nanometers.
Pure gold leaf produced in the Middle Ages without the silver would have measured around 140 nanometers, so Zwischgold was cheaper to produce.
It may also have been complex to create, possibly requiring special beating tools and cases containing different materials to insert the foils. The researchers suggest that the gold and silver would have been forged together before working as a single sheet.
Fortunately for sculptors and gilders, gold and silver maintain a uniform morphology when the crystals are pressed together.
It required the expertise of an expert – this was not going to be a job that anyone could do. And a job that has probably been kept secret.
There was also a hierarchy to be considered, in terms of which figures could be covered with gold leaf and which had to settle for Zwischgold.
“Many people had assumed that technology in the Middle Ages was not particularly advanced,” says art historian Qing Wu, of the University of Zurich in Switzerland.
“On the contrary: this was not the Dark Ages, but a period where metallurgy and gilding techniques were incredibly well developed.”
3D images produced as part of this study reveal one of the drawbacks of working with Zwischgold, despite its relative affordability: the silver in the mixture moves quickly, even at room temperature, and can coat the gold within days .
This in turn leads to corrosion as the silver comes into contact with water and sulfur in the air – the corrosion pulls more silver to the surface and over time the material ends up looking black. The solution is to use some kind of varnish and medieval craftsmen would use resin, glue or other similar material for the job.
However, the varnish loses its effectiveness over the centuries, and the researchers’ investigations also showed that over time corrosion had excavated a void beneath the metal layer in some specimens. The researchers hope that in the near future they could develop a special material to fill the void and restore the artworks.
“If we remove the unsightly corrosion products, the varnish layer will also fall off and we’ll lose everything,” says Wu.
“Using CT, we could test how well such a consolidation material would do its job.”
The research has been published in Nanoscale.
