The latest in nanoscale 3D scanning techniques have been used to reveal some of the finest secrets of a medieval material known as Zwischgold (part-gold): an ultra-thin metal foil consisting of a gold top layer and a silver base, used to gild sculptures.
Up 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 put together and why historians might face challenges 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).
"Although Zwischgold was frequently used in the Middle Ages, very little was known about this material up to now," says physicist Benjamin Watts, from the Paul Scherrer Institute in Switzerland.
"So we wanted to investigate the samples using 3D technology which can visualize extremely fine details."
To do that, Watts and his colleagues used a sophisticated microscopy imaging technique called ptychographic tomography, which shines X-rays through a sample of material to create shadows of varying intensity called diffraction patterns.
By tweaking the imaging technique and combining different diffraction patterns, it's possible to reveal details that may only be millionths of a millimeter in size. The researchers describe it as being like a "giant Sudoku puzzle" where the entire picture of an object is gradually revealed with each additional image.
The scans reveal a gold layer measuring around 30 nanometers, thinly and evenly spread over a silver base layer (some of the thinnest human hairs are around 50,000 nanometers). By comparison, an analysis on modern samples of Zwischgold 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 the Zwischgold was cheaper to produce.
It may also have been complicated to create, potentially requiring special beating tools and pouches containing different materials to insert the foils into. The researchers suggest the gold and silver would've been hammered together before being worked on as a single foil.
Fortunately for the sculptors and gilders, gold and silver maintain a uniform morphology when they're crystals are pressed together.
It was demanding the expertise of a specialist – this wouldn't have been a job that just anyone could've done. And a job that in all likelihood have been kept a secret.
There was also a hierarchy to consider, in terms of the figures that could be covered in gold leaf and those that had to settle for Zwischgold.
"Many people had assumed that technology in the Middle Ages was not particularly advanced," says art historian Qing Wu, from the University of Zurich in Switzerland.
"On the contrary: this was not the Dark Ages, but a period when metallurgy and gilding techniques 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 rapidly, even at room temperature, and can coat the gold within days.
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. The fix is to use some kind of varnish, and the medieval artisans would've used resin, glue or another similar material for the job.
However, varnish loses its effectiveness over the centuries, and the researchers' investigations also showed that over time the corrosion had excavated a gap beneath the metal layer in some samples. The researchers hope that in the near future they could develop a special material to fill the gap and restore the artworks.
"If we remove the unsightly products of corrosion, the varnish layer will also fall away and we will lose everything," says Wu.
"Using ptychographic tomography, we could check how well such a consolidation material would perform its task."
The research has been published in Nanoscale.