The latest nanoscale 3D scanning techniques have been used to reveal some of the best kept secrets of a medieval material known as Zwischgold (part-gold): an ultra-thin metal sheet consisting of a top layer of gold and a silver base, which is used for gilding. sculptures
Until now, only 2D cross-sections of the materials had been studied, but in a new study researchers have been able to create 3D representations of Zwischgold for the first time, revealing how it was assembled and why historians might face challenges to restore it. medieval art
The four 15th-century samples studied included one from an altar that was originally in a mountain chapel at Alp Leiggern in Valais, Switzerland, and is now on display in the Swiss National Museum (Landesmuseum Zürich).
“Although Zwischgold was frequently used in the Middle Ages, until now very little was known about this material,” says physicist Benjamin Watts of the Paul Scherrer Institute in Switzerland.
“So we wanted to investigate the samples using 3D technology that can visualize extremely fine details.”
To do this, Watts and his colleagues used a sophisticated microscopy imaging technique called ptographic tomography, which shines X-rays through a sample of material to create shadows of varying intensity called diffraction patterns .
By adjusting the imaging technique and combining different diffraction patterns, it is possible to reveal details that may only be 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 gold layer of about 30 nanometers, thinly and evenly distributed over a silver base layer (some of the thinnest human hairs are about 50,000 nanometers). By comparison, an analysis of modern Zwischgold samples 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 about 140 nanometers, so Zwischgold was cheaper to produce.
It may also have been complicated to create, requiring special beating tools and bags containing different materials to insert the sheets into. The researchers suggest that the gold and silver would have been hammered together before being worked as a single sheet.
Fortunately for sculptors and gilders, gold and silver maintain a uniform morphology when the crystals are pressed together.
It was demanding the experience of a specialist; this would not have been a job that anyone could have done. And a job that in all probability has been kept secret.
There was also a hierarchy to consider, as to which figures could be covered in gold bread and which had to make do with 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: it was not the Dark Ages, but a period when the techniques of metallurgy and gilding were incredibly well developed.”
The 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 in few days.
This in turn leads to corrosion as the silver comes into contact with water and sulfur in the air – the corrosion brings 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 have used resin, glue or some 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 dug a void under the metal layer in some samples. The researchers hope that in the near future they will be able to develop a special material to fill the void and restore the works of art.
“If we remove the unsightly corrosion products, the varnish layer will also fall off and we lose everything,” says Wu.
“Using psychographic tomography, we could test how well this consolidation material would do its job.”
The research has been published in Nanoscale.