Nanomaterial from the Middle Ages

(ORDO NEWS) — “It is generally accepted that technology in the Middle Ages was not particularly developed. In fact, it was quite the opposite – these were not dark ages, but a period when metallurgy and gilding techniques were incredibly well developed.

For gilding sculptures in the late Middle Ages, artists often used ultra-thin gold foil deposited on an equally thin layer of silver.

For the first time, scientists from the Paul Scherrer Institute PSI have been able to obtain nanoscale 3D images of this material, known as Zwischgold.

The photographs show that this was a very complex medieval production technique and demonstrate why the restoration of such precious gilded artifacts is so difficult.

The specimens examined at the Swiss Light Source SLS using one of the most advanced microscopy methods were unusual even for the very experienced PSI team: tiny samples of materials taken from the altar and wooden statues from the fifteenth century.

It is believed that the altar was made around 1420 in southern Germany and for a long time stood in a mountain chapel on the Alp-Leigern in the Swiss canton of Valais.

Today it is on display at the Swiss National Museum (Landesmuseum Zürich). In the center you can see Mary cradling the Baby Jesus.

The material sample was taken from the fold of the Virgin Mary’s robe. Tiny samples from two other medieval statues were provided by the Basel Historical Museum.

This material was used to gild sacred figures. This is not actually gold leaf, but a special gold and silver double sided foil where the gold can be ultra thin as it is supported by a silver backing. This material, known as Zwischgold (partial gold), was significantly cheaper than using pure gold leaf.

“While Zwischgold was often used in the Middle Ages, very little was known about this material until now,” says PSI physicist Benjamin Watts. “Therefore, we wanted to examine the samples using 3D technology, which allows us to visualize extremely small details.”

Scientists have achieved this with ptychographic tomography, as they report in their latest paper in the journal Nanoscale. “The 3D images clearly show how thin and uniform the gold layer is on top of the silver base layer,” says Qing Wu, lead author of the publication.

“Many people assumed that technology in the Middle Ages was not particularly advanced,” Wu comments. “On the contrary: these were not dark ages, but a period when metallurgy and gilding techniques were incredibly well developed.”

Unfortunately, there is no record of how Zwischgold was made at that time. “We believe the artisans kept their recipe a secret,” Wu says.

However, based on nanoscale images and documents from later eras, the art historian now knows the method used in the 15th century: first, gold and silver were minted separately to produce thin foil, with the gold layer having to be much thinner than the silver layer. The two metal foils were then bonded together.

“Our studies of Zwischgold samples showed that the average thickness of the gold layer is about 30 nanometers, while gold leaf produced in the same period and in the same region was about 140 nanometers thick,” Wu explains.

“This method saved on gold, which was much more expensive.” At the same time, there was also a very strict hierarchy of materials: gold leaf was used, for example, to make a halo of one figure, and Zwischgold was used for a mantle.

Because this material has less sheen, artists often used it to dye the hair or beards of their statues. “It’s incredible how someone with only hand tools could create such a nanoscale material,” says Watts.

“Hand tools” of course… even having established the fact that nanotechnologies were used in the Middle Ages, scientists habitually attribute everything to “manual labor”.

By the way, scientists reported that although they understood what this nano-gilding was made of, this will not help restorers, simply because now there are no technologies that can repeat what was done in the Middle Ages.

Probably scientists do not have enough “hand tools of the Middle Ages” to create an ancestral nano-foil….

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