In the secrets of Zwischgold, the... nanomaterial of the Middle Ages

From the scholars of the Paul Scherrer Institute first 3D visions of the thin double-sided gold and silver plates of ancient Swiss sacred icons

The Fifteenth-Century German Altar Examined by PSI
It is believed that the altar examined by the Paul Scherrer Institute was built around 1420 in southern Germany and that for a long time it was located in a mountain chapel on Alp Leiggern in the Swiss canton of Valais: today it is exhibited in the Museum Swiss National Museum, headquarters of the Landesmuseum Zürich(Photo: Landesmuseum Zürich/Swiss National Museum)

To gild sculptures in the late Middle Ages, artists often applied a very thin sheet of gold, “supported” by a layer of silver as a base.

For the first time, scientists at the Paul Scherrer Institute in Switzerland have succeeded in producing nanoscale 3D images of this material, known as Zwischgold.

The images show that this was a very sophisticated medieval production technique and demonstrate why the restoration of such precious gilded artefacts is tremendously difficult.

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Bird's eye view of the Paul Scherrer Institute in the canton of Aargau
Bird's eye view of the Paul Scherrer Institute in the canton of Aargau

Samples from a fold of the Virgin's robe in an altar from 1420

The samples examined at the "Swiss Light Source" (SLS), with one of the most advanced microscopy methods in the world, were unusual even for the PSI team, notoriously very expert: tiny samples of material taken from an altar and statues of wood dating back to the XNUMXth century.

It is thought that the altar in question was built around 1420 in southern Germany and that it remained for a long time in a mountain chapel on Alp Leiggern in the Swiss canton of Valais.

Today it is exhibited at the Swiss National Museum, at the headquarters of the Landesmuseum Zürich. In the center we see Mary cradling the Infant Jesus.

The material sample was taken from a fold of the Virgin's robe. The small elements of the other two medieval structures were supplied by the Basel History Museum.

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Comparison of the histograms of the δ PXCT value of the PSI studies on the Zwischgold
Comparison of histograms of the δPXCT value of recent samples, at 10 and 35 years, and a pure gold reference, as part of the study by the Paul Scherrer Institute on the Zwischgold of a German altar from 1420 and two Basel bishop statuettes . histograms were scaled to match the mean number of total voxels with a δ value greater than 3 × 10 -6 in order to normalize against different sample sizes. the color bar along the bottom edge indicates the segmentation for PXCT tomogram presentations in other statistics

Benjamin Watts: “Until now very little was known about this element…”

The material was used to gild the sacred figures. It is not a real gold leaf, but a special double-sided gold and silver foil in which the gold can be very thin because it is supported by the silver base.

This material, known as Zwischgold ("partial gold"), was naturally much cheaper than using pure gold leaf.

“Although 'Zwisch gold' was often used in the Middle Ages, especially in the Germanic area, so far very little was known about this material”, explains Benjamin Watts, a physicist at the PSI in Villigen in the canton of Aargau.

“We therefore wanted to study the samples using a 3D technology capable of displaying extremely fine details”.

While other microscopy techniques had previously been used to examine Zwischgold, they only provided a 2D cross section of the material.

In other words, it was possible to view only the surface of the cut segment, rather than looking inside the material.

Scientists also feared that the cutting operation might have changed the structure of the sample.

The advanced microscope imaging method used today, Ptychographic tomography, provides for the first time a 3D image of the exact composition of the Zwischgold.

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Niels Schröter, left, and Vladimir Strocov, right, in one of the experimentation stations of the Swiss Light Source SLS at the Paul Scherrer Institute: here the researchers used “soft” X-ray photoelectron spectroscopy, with angular resolution, to measure the distribution of electrons under the oxide layer of indium arsenide and indium antimonide (Photo: Mahir Dzambegovic/Paul Scherrer Institute)
Niels Schröter, left, and Vladimir Strocov, right, in one of the test stations of the Swiss Light Source (SLS) at the Paul Scherrer Institute in Villigen in the canton of Aargau
(Photo: Mahir Dzambegovic/Paul Scherrer Institute)

X-rays generate a diffraction pattern of millionths of a millimeter

Scientists from the Paul Scherrer Institute conducted their research using X-rays produced by the instrumentation of the “Swiss Light Source” (SLS).

These produce CT scans that display nanoscale detail—millionths of a millimeter, in other words.

“Pictography is a rather sophisticated method, as there is no objective lens generating an image directly on the detector”Watts explains.

Pictography actually produces a diffraction pattern of the illuminated area: in other words, it is an image with dots of different intensities.

By manipulating the sample precisely, hundreds of superimposed diffraction patterns can be generated.

“We can then combine these diffraction patterns like a giant Sudoku puzzle and figure out what the original image looked like”, explains the physicist.

A series of pictographic images taken from different directions can be combined to create a 3D tomogram.

The advantage of this method is the very high resolution.

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“We knew that the thickness of the Zwischgold sample taken from the painted robe of the Virgin Mary was in the order of hundreds of nanometres”adds Watts.

“So we had to be able to reveal even smaller details”.

Swiss scientists achieved this using ptychographic tomography, as reported in the latest article published in the journal "Nanoscale".

“The 3D images clearly show how thin and uniform the gold layer is over the silver base layer”, says Qing Wu, lead author of the publication.

The art historian and conservation scientist completed her PhD at the University of Zurich in cooperation with PSI and the Swiss National Museum.

“Many thought that technology in the Middle Ages was not particularly advanced”comments Wu.

“On the contrary: it was not the middle age in the strict sense, but a period in which metallurgy and gilding techniques were incredibly developed”.

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A card game from Württemberg dated 1239 and covered with Zwischgold
A card game from 1239 covered with Zwischgold and kept in the Kunstkammer of the Württemberg State Museum in Stuttgart

Gold and silver were hammered separately, then worked together

Unfortunately there are no documents describing how Zwischgold was produced at the time. “We think that the craftsmen secretly guarded their technique…”Wu says.

Based on nanoscale images and documents from later periods, however, the art historian is now able to learn about the method used in the XNUMXth century.

Previously, gold and silver were hammered separately to produce thin sheets, so the gold film had to be much thinner than the silver film. Then, the two metal plates were worked together.

Wu describes the process: “This required special typing tools and envelopes with various inserts of different materials into which the foils were inserted”Wu argues.

This was a rather complicated procedure, requiring highly qualified specialists.

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Comparison of the δ-value histograms from the PSI studies on the Zwischgold
Comparison of histograms of δ-values ​​of samples Nicolaus Border, Nicolaus, Bishop and Mary, together with the Recent histogram for comparison with modern Zwischgold, as part of the Paul Scherrer Institute study of a German altar from 1420 and two Basel statuettes of bishops; the data were filtered by the median to reduce the amplitude of the peaks and the histograms were scaled to normalize them with respect to the different sample sizes; the color bar along the bottom edge indicates the segmentation for PXCT 3D model presentations in other statistics

30 nanometers for “Zwisch gold” against a good 140 for pure foil

“Our investigations of the Zwischgold samples showed that the average thickness of the gold layer was about 30 nanometers, while gold foils produced in the same period and in the same region had a thickness of about 140 nanometers”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 to make a figure's halo, for example, while “Zwisch gold” was used for the robe.

Because this material has a lower sheen, artists often used it to color the hair or beards of their statues.

“It's amazing how someone with only hand tools was able to create a nano-sized material”Watts says.

A unique property of gold and silver crystals, when pressed together, facilitated the work of medieval craftsmen: the morphology of the two noble elements is preserved throughout the metal film.

“A lucky coincidence of nature that guarantees the functioning of this technique”, adds the scientist.

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The XNUMXth century German altarpiece and the Basel statues of bishops examined by the PSI
The Paul Scherrer Institute has examined some statuettes of bishops from Basel and a former mountain altar on Alp Leiggern now exhibited at the Landesmuseum Zürich: they are cult objects covered with Zwischgold, a gold-silver mixture
(Photo: Landesmuseum Zürich/Swiss National Museum)

The surface is black: silver corrodes in contact with water and sulphur

However, the 3D images highlight a drawback of the use of “Zwisch gold”: the silver can pass through the gold layer and cover it. The latter moves surprisingly quickly, even at room temperature.

Within days, a thin coating of silver completely covers the gold. On the surface, silver comes into contact with water and sulfur in the air and corrodes.

“This causes the gold surface of the Zwischgold to turn black over time”Watts explains.

“The only thing that can be done is to seal the surface with a varnish, so that the sulfur does not attack the silver forming silver sulphide”.

Craftsmen using Zwischgold were aware of this problem from the start. As varnish they used resin, glue or other organic substances.

“But over hundreds of years this protective layer decomposed, allowing the corrosion to continue”Wu explains.

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STEM-EDX analysis on a sample of Zwischgold in PSI studies
STEM-EDX analysis of a sample taken in a region adjacent to the Virgin Mary sample from a German altar of 1420, covered with Zwischgold and studied by the Paul Scherrer Institute: (a and b) EDX elemental maps for Au (gold) and Ag (silver ) and EDX spectra of areas 1 and 2; (ced) EDX elemental maps for Au (gold) and Ag (silver) and EDX spectra of areas 3 and 4; (eg) EDX maps for Au (gold) and Ag (silver) and high angle circle darkfield (HAADF) image of the boundary of a gold plate filled with silver atoms; (h) EDX line profile across a grain boundary of the gold layer (along the green arrow); subpanels (eh) reproduced from reference 38 of the study published by the journal “Nanoscale”

From corrosion through the centuries the formation of very dangerous voids

Corrosion also favors the migration of more and more silver towards the surface, creating a void under the "gold of Zwisch".

“We were surprised how clearly this gap is visible under the metal layer”Watts explains.

Especially in the sample taken from Maria's robe, the Zwischgold had clearly detached from the base layer.

“This gap can cause mechanical instability and we expect that in some cases it is only the protective coating over the 'Zwisch gold' that holds the metal foil in place”warns Wu.

This is a huge problem for the restoration of historic artifacts, as the silver sulphide has become embedded in the paint layer or even below it.

“If we remove the unsightly corrosion products, the paint layer will also fall off and we will lose everything”says Wu.

The author hopes that in the future it will be possible to develop a special material that can be used to fill the space and keep Zwischgold attached to the artwork.

“Using pictographic tomography, we could verify the ability of this consolidation material to do its job”, explains the art historian.

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Researchers Qing Wu and Benjamin Watts from PSI working on Zwischgold
Qing Wu and Benjamin Watts, researchers from the Paul Scherrer Institute, next to the instrumentation of the cSAXS light beam, with which they conducted their investigations on Zwischgold, a very rare nanomaterial in history: the first of the two scientists holds a plate in his hand contains, from top to bottom, three different segments: pure gold foils, so-called “Zwisch gold” foils and silver foils, where the dark spots are formed by oxide (Photo: Mahir Dzambegovic/Paul Scherrer Institute)