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In the late Middle Ages, artists gilded sculptures using ultra-thin gold foil reinforced by a silver base layer. For the first time, researchers at the Paul Scherrer Institute (PSI) have successfully produced nanoscale three-dimensional (3D) images of this material, called Zwischgold. 
The altar examined is thought to have been made around 1420 in Southern Germany and for a long time stood in a mountain chapel on Alp Leiggern in the Swiss canton of Valais. Today it is on display at the Swiss National Museum (Landesmuseum Zürich). Image Credit: Swiss National Museum, Landesmuseum Zürich
This was an extremely state-of-the-art medieval production method and demonstrates why restoring such valuable gilded artifacts is very challenging.
The samples studied at the Swiss Light Source (SLS) using one of the most cutting-edge microscopy techniques were uncommon even for the extremely experienced PSI researchers: tiny samples of materials procured from an altar and wooden statues originating from the 15th century.
The altar is believed to have been built around 1420 in Southern Germany and was located in a mountain chapel on Alp Leiggern in the Swiss canton of Valais for a long time.
Recently, it was on display at the Swiss National Museum (Landesmuseum Zürich). In the middle, one can see Mary cradling Baby Jesus. The material sample was procured from a fold in the Virgin Mary’s robe. Basel Historical Museum provided minute samples from the other two ancient structures.
The material used to gild the sacred figures was a two-sided foil of gold and silver, where the gold can be ultra-thin as the silver base reinforces it. This material, called Zwischgold (part-gold) was considerably more economical than pure gold leaf.
Although Zwischgold was frequently used in the Middle Ages, very little was known about this material up to now. So we wanted to investigate the samples using 3D technology which can visualize extremely fine details.
Benjamin Watts, Physicist, Paul Scherrer Institute
Although other microscopy methods had been used earlier to analyze Zwischgold, they only delivered a two-dimensional (2D) cross-section through the material. It was only possible to observe the surface of the cut portion, rather than viewing inside the material.
The researchers were also concerned that cutting through it may have altered the sample’s structure. For the first time, ptychographic tomography, the cutting-edge microscopy imaging technique used nowadays, offers a 3D image of Zwischgold’s precise composition.
The PSI researchers carried out their research using X-rays made by the Swiss Light Source (SLS). These tomographs display particulars in the nanoscale range, in other words, millionths of a millimeter.
“Ptychography is a fairly sophisticated method, as there is no objective lens that forms an image directly on the detector,” Watts explains.
Ptychography creates a diffraction pattern of the irradiated area, in other words, an image with points of different intensities. By controlling the sample precisely, it is possible to produce hundreds of overlapping diffraction patterns.
“We can then combine these diffraction patterns like a sort of giant Sudoku puzzle and work out what the original image looked like,” says the physicist. A set of ptychographic images captured from various directions can be integrated to form a 3D tomogram. The benefit of this technique is its very high resolution.
We knew the thickness of the Zwischgold sample taken from Mary was of the order of hundreds of nanometers. So we had to be able to reveal even tinier details.
Benjamin Watts, Physicist, Paul Scherrer Institute
The researchers accomplished this using ptychographic tomography, as demonstrated in their recent Nanoscale journal article.
“The 3D images clearly show how thinly and evenly the gold layer is over the silver base layer,” says Qing Wu, lead author of the publication.
Qing Wu is an art historian and conservation scientist who completed her PhD at the University of Zurich, in partnership with PSI and the Swiss National Museum.
Many people had assumed that technology in the Middle Ages was not particularly advanced. On the contrary: this was not the Dark Ages, but a period when metallurgy and gilding techniques were incredibly well developed.
Qing Wu, Study Lead Author, Art Historian, and Conservation Scientist, University of Zurich
Regrettably, there are no accounts of how Zwischgold was created back in the day. “We reckon the artisans kept their recipe secret,” says Wu.
Based on documents and nanoscale images from later eras, however, the art historian currently knows the technique used in the 15th century: first, the gold and the silver were pounded individually to create thin foils, whereby the gold film had to be a lot thinner than the silver. Then, they were worked on together.
Wu illustrates the process: “This required special beating tools and pouches with various inserts made of different materials into which the foils were inserted,” Wu explains. This was quite a complicated process that needed extremely skilled experts.
“Our investigations of Zwischgold samples showed the average thickness of the gold layer to be around 30 nanometres, while gold leaf produced in the same period and region was approximately 140 nanometres thick. This method saved on gold, which was much more expensive.”
There was also an extremely strict hierarchy of materials: gold leaf was used to create the halo of one statue, for instance, while Zwischgold was utilized for the robe. Since this material has less shine, the artists frequently used it to color the beards or hair of their statues.
“It is incredible how someone with only hand tools was able to craft such nanoscale material,” Watts says.
Medieval artisans also gained from an exclusive feature of gold and silver crystals combined: their morphology is well-kept throughout the whole metal film. “A lucky coincidence of nature that ensures this technique works,” says the physicist.
The 3D images do highlight one disadvantage of using Zwischgold: the silver can come through the gold layer and conceal it. The silver moves astonishingly quickly—even at ambient temperature. In a matter of days, a thin silver coating conceals the gold.
The silver is exposed to water and sulfur in the air at the surface and corrodes.
“This makes the gold surface of the Zwischgold turn black over time,” Watts explains. “The only thing you can do about this is to seal the surface with a varnish so the sulfur does not attack the silver and form silver sulfide.”
The artisans using Zwischgold were conscious of this issue from the beginning. They used glue, resin, or other organic materials as a varnish.
“But over hundreds of years this protective layer has decomposed, allowing corrosion to continue,” Wu explains.
The corrosion also boosts more and more silver to move to the surface, forming a gap below the Zwischgold. “We were surprised how clearly this gap under the metal layer could be seen,” says Watts. The Zwischgold had distinctly moved away from the base layer, particularly in the sample procured from Mary's robe.
“This gap can cause mechanical instability, and we expect that in some cases it is only the protective coating over the Zwischgold that is holding the metal foil in place,” Wu warns.
This is a huge issue for renovating historical artifacts, as the silver sulfide has become entrenched in the varnish layer or even further down.
“If we remove the unsightly products of corrosion, the varnish layer will also fall away and we will lose everything,” explains Wu.
She believes it will be feasible in the future to create a special material that can be used to fill the openings and ensure the Zwischgold stays attached.
“Using ptychographic tomography, we could check how well such a consolidation material would perform its task,” states the art historian.
Wu, Q., et al. (2022) A modern look at a medieval bilayer metal leaf: nanotomography of Zwischgold. Nanoscale. doi.org/10.1039/D2NR03367D.
Source: https://www.psi.ch/en
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