Researchers in Switzerland have discovered more details about the medieval material Zwischgold, which can help in the restoration process of 15th century artworks.
For modern day art collectors and curators it can be difficult to know how to repair centuries old work if it was made using techniques and materials that have fallen out of fashion. One such material that was particularly popular in the middle ages was Zwischgold.
Zwischgold, or part-gold, was a more affordable alternative to gold that could also be used to create a gilded appearance on statues, sculptures or other decorative pieces. It consists of a layer of gold on top of a layer of silver.
“Although Zwischgold was frequently used in the Middle Ages, very little was known about this material up to now,” Benjamin Watts told the Paul Scherrer Institute in Switzerland, where he led a study into the composition of Zwischgold.
Last month, Watts and colleagues published an article in the journal Nanoscale in which they shared how they have managed to create a 3D image revealing the details of Zwischgold’s composition in several historic artworks.
One of the Zwischgold samples came from a 15th century altar depicting Mary and baby Jesus. The altar spent a long time in a mountain chapel in the Alps but is currently on display at the Swiss National Museum.
“We knew the thickness of the Zwischgold sample taken from Mary was of the order of hundreds of nanometres,” said Watts. Because it was so small, they used a technique known as ptychographic tomography which allowed them to zoom in on the material.
“The 3D images clearly show how thinly and evenly the gold layer is over the silver base layer,” the study’s lead author, Qing Wu, told the PSI. Wu is an art historian and conservation scientist and this research has allowed her to understand in more detail how medieval artists created Zwischgold. “It is incredible how someone with only hand tools was able to craft such nanoscale material,” she says.
One of the problems of Zwischgold is that the silver eventually comes through the extremely thin gold layer and causes the material to turn black when it corrodes. At the same time, this creates a small gap between the Zwischgold and the sculpture below. “We were surprised how clearly this gap under the metal layer could be seen,” says Watts.
There are still a lot of unanswered questions about Zwischgold, but understanding more about how it was created and what its weaknesses are will be very helpful for anyone tasked with the restoration of these old artifacts.