An ultra-thin metal foil consisting of a gold top layer and a silver base is used to gild sculptures.
Until now, only 2D cross-sections of the materials had been studied, but researchers have been able to create 3D representations of Zwischgold for the first time.
The altar from the mountain chapel on Alp Leiggern in Valais is on display in the Swiss National Museum.
Benjamin Watts is a physicist from the Paul Scherrer Institute in Switzerland.
We wanted to use 3D technology to look at the samples.
Watts and his colleagues used a method called ptychographic tomography, which shines X-rays through a sample of material, to create shadows of varying intensity.
It is possible to reveal details that are millionths of a millimeter in size by tweaking the technique. The picture of an object is gradually revealed with each additional image, according to the researchers.
The scans show a gold layer spread over a silver base layer that is around 30 nanometers thick. The thicknesses of modern samples were measured in the range of 48 to 80 nanometers.
The gold leaf would have measured around 140 nanometers without the silver.
It may have been difficult to create because of the different materials used to make the foils. The gold and silver would have been hammered together before being foiled.
Fortunately for the sculptors and gilders, gold and silver have the same shape.
This wouldn't have been a job that just anyone could do. It's likely that the job has been kept a secret.
The figures that could be covered in gold leaf and the ones that had to settle for Zwischgold were considered.
Many people thought technology in the Middle Ages was not very advanced.
This was not the Dark Ages but a time when metallurgy and gilding techniques were well developed.
One of the drawbacks of working with Zwischgold, despite its relative affordability, is that the silver in the mixture can coat the gold within days.
As the silver comes into contact with water and sulfur in the air, it draws more silver to the surface, causing it to look black over time. The medieval artisans would've used a variety of materials for the fix, including varnish, glue, or something similar.
The researchers' investigations showed that the gap beneath the metal layer in some samples was caused by the corrosive nature of varnish. In the near future, the researchers hope to develop a special material that can be used to fill the gap.
The varnish layer will fall away and we will lose everything if we remove the ugly products of the corrosive environment.
We could use ptychographic tomography to see how well a consolidation material would do.
The research has appeared in a journal.
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