Nanoscale 3D scanning techniques were used to uncover the best secrets of Zwischgold (part–gold), a medieval material that includes a top layer of gold and a base of silver. This foil is used to create gilded sculptures.
Up to now, only 2D cross sections of the materials were studied. But, in a new study, researchers were able create 3D representations Zwischgold. This allows historians to see how the material was assembled and explains why they might have difficulties in restoring medieval art.
The four 15th-century samples studied included one from an altar originally housed in a mountain chapel on Alp Leiggern in Valais, Switzerland, and now on show in the Swiss National Museum (Landesmuseum Zürich).
“Zwischgold was used frequently in the Middle Ages but very little is known about it up to now.” says physicist Benjamin WattsFrom the Paul Scherrer Institute, Switzerland.
“So we wanted 3D technology to inspect the samples, which can see extremely fine details.”
Watts and his collaborators used a sophisticated technique for microscopy imaging called Ptychographic tomographyThis uses X-rays to shine through samples of material and create shadows with varying intensities called diffraction patterns.
It is possible to see details as small as a millionth of a millimeter by altering the imaging technique or combining different difffraction patterns. Researchers describe it as a “smart” way to see details.Sudoku puzzle of giant size“Where the whole picture of an object is slowly revealed with each additional photo.
The scans revealed a gold layer that measured around 30 nanometers thick, evenly distributed over a silver base layer. (Some of the thinnest human hairs are only around 50,000 nanometers). An analysis of Zwischgold samples from modern times was done in the same study and measured thicknesses ranging from 48 to 82 nanometers.
Pure gold leaf without silver produced in the Middle Ages would measure around 140 nanometers. Therefore, Zwischgold was less expensive to produce.
It could have also been difficult to make, possibly requiring special beat tools or pouches with different materials to insert foils into. Researchers suggest that the silver and gold would have been hammered together before being made into a single foil.
The good news for sculptors is that silver and gold retain a uniform morphology once they’re pressed together.
It was demanding the expertise of a specialist – this wouldn’t have been a job that just anyone could’ve done. It was a job that most likely would have been kept secret.
A hierarchy was also to be considered, with regard to the figures that could have gold leaf applied and those that would settle for Zwischgold.
“Many assumed that technology during the Middle Ages wasn’t very advanced.” Qing Wu, an art historianThe University of Zurich in Switzerland.
“The contrary is true: it was not the Dark Ages. This was a period when metallurgy & gilding techniques were extraordinarily well developed.”
This study revealed one of the downsides to Zwischgold despite its affordability. The silver in the mixture can coat the gold in days and move quickly at room temperature.
That in turn leads to corrosion as the silver comes into contact with water and sulfur in the air – the corrosion draws more silver to the surface, and over time the material ends up looking black. You can fix this by using varnish. The medieval artisans used glue, resin or other similar materials.
But varnish is less effective over time. The researchers also discovered that the corrosion had created a crack beneath some samples’ metal layers. Researchers hope to develop a special material in the near future that will fill the gap and restore artworks.
“If we remove the unsightly corrosion products, the varnish will also fall off and we will be without everything.” Wu.
“Using ptychographic imaging, we could verify how well this consolidation material would perform its job.”
The research was published in Nanoscale.
