Tag Archives: Barbara Vonarburg

Gilding medieaval statues with nanoscale gold sheets

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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). (Photo: Swiss National Museum, Landesmuseum Zürich) [ddownloaded from https://www.psi.ch/en/media/our-research/nanomaterial-from-the-middle-ages]

As amazing as the altar appears, it was hiding some even more amazing secrets. From an October 10, 2022 Paul Scherrer Institute (PSI) press release (also on EurekAlert but published October 11, 2022) by Barbara Vonarburg,

To gild sculptures in the late Middle Ages, artists often applied ultra-thin gold foil supported by a silver base layer. For the first time, scientists at the Paul Scherrer Institute [PSI] have managed to produce nanoscale 3D images of this material, known as Zwischgold. The pictures show this was a highly sophisticated mediaeval production technique and demonstrate why restoring such precious gilded artefacts is so difficult.

The samples examined at the Swiss Light Source SLS using one of the most advanced microscopy methods were unusual even for the highly experienced PSI team: minute samples of materials taken from an altar and wooden statues originating from the fifteenth century. The altar is thought to have been made around 1420 in Southern Germany and stood for a long time 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). In the middle you can see Mary cradling Baby Jesus. The material sample was taken from a fold in the Virgin Mary’s robe. The tiny samples from the other two mediaeval structures were supplied by Basel Historical Museum.

The material was used to gild the sacred figures. It is not actually gold leaf, but a special double-sided foil of gold and silver where the gold can be ultra-thin because it is supported by the silver base. This material, known as Zwischgold (part-gold) was significantly cheaper than using pure gold leaf. “Although Zwischgold was frequently used in the Middle Ages, very little was known about this material up to now,” says PSI physicist Benjamin Watts: “So we wanted to investigate the samples using 3D technology which can visualise extremely fine details.” Although other microscopy techniques had been used previously to examine Zwischgold, they only provided a 2D cross-section through the material. In other words, it was only possible to view the surface of the cut segment, rather than looking inside the material.  The scientists were also worried that cutting through it may have changed the structure of the sample. The advanced microscopy imaging method used today, ptychographic tomography, provides a 3D image of Zwischgold’s exact composition for the first time.

X-rays generate a diffraction pattern

The PSI scientists conducted their research using X-rays produced by the Swiss Light Source SLS. These produce tomographs displaying details in the nanoscale range – millionths of a millimetre, in other words. “Ptychography is a fairly sophisticated method, as there is no objective lens that forms an image directly on the detector,” Watts explains. Ptychography actually produces a diffraction pattern of the illuminated area, in other words an image with points of differing intensity. By manipulating the sample in a precisely defined manner, it is possible to generate 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 taken from different directions can be combined to create a 3D tomogram.

The advantage of this method is its extremely high resolution. “We knew the thickness of the Zwischgold sample taken from Mary was of the order of hundreds of nanometres,” Watts explains. “So we had to be able to reveal even tinier details.” The scientists achieved this using ptychographic tomography, as they report in their latest article in the journal Nanoscale. “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. The art historian and conservation scientist completed her PhD at the University of Zurich, in collaboration with PSI and the Swiss National Museum. “Many people had assumed that technology in the Middle Ages was not particularly advanced,” Wu comments. “On the contrary: this was not the Dark Ages, but a period when metallurgy and gilding techniques were incredibly well developed.”

Secret recipe revealed

Unfortunately there are no records of how Zwischgold was produced at the time. “We reckon the artisans kept their recipe secret,” says Wu. Based on nanoscale images and documents from later epochs, however, the art historian now knows the method used in the 15th century: first the gold and the silver were hammered separately to produce thin foils, whereby the gold film had to be much thinner than the silver. Then the two metal foils were worked on together. Wu describes 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 a fairly complicated procedure that required highly skilled specialists.

“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,” 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 the halo of one figure, for example, while Zwischgold was used for the robe. Because this material has less of a sheen, the artists often used it to colour the hair or beards of their statues. “It is incredible how someone with only hand tools was able to craft such nanoscale material,” Watts says. Mediaeval artisans also benefited from a unique property of gold and silver crystals when pressed together: their morphology is preserved across the entire metal film. “A lucky coincidence of nature that ensures this technique works,” says the physicist.

Golden surface turns black

The 3D images do bring to light one drawback of using Zwischgold, however: the silver can push through the gold layer and cover it. The silver moves surprisingly quickly – even at room temperature. Within days, a thin silver coating covers the gold completely. At the surface the silver comes into contact with water and sulphur in the air, 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 sulphur does not attack the silver and form silver sulphide.” The artisans using Zwischgold were aware of this problem from the start. They used resin, glue or other organic substances as a varnish. “But over hundreds of years this protective layer has decomposed, allowing corrosion to continue,” Wu explains.

The corrosion also encourages more and more silver to migrate to the surface, creating a gap below the Zwischgold. “We were surprised how clearly this gap under the metal layer could be seen,” says Watts. Especially in the sample taken from Mary’s robe, the Zwischgold had clearly come away 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 Zwischgold that is holding the metal foil in place,” Wu warns. This is a massive problem for the restoration of historical artefacts, as the silver sulphide has become embedded 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,” says Wu. She hopes it will be possible in future to develop a special material that can be used to fill the gap and keep the Zwischgold attached. “Using ptychographic tomography, we could check how well such a consolidation material would perform its task,” says the art historian.

About PSI

The Paul Scherrer Institute PSI develops, builds and operates large, complex research facilities and makes them available to the national and international research community. The institute’s own key research priorities are in the fields of matter and materials, energy and environment and human health. PSI is committed to the training of future generations. Therefore about one quarter of our staff are post-docs, post-graduates or apprentices. Altogether PSI employs 2100 people, thus being the largest research institute in Switzerland. The annual budget amounts to approximately CHF 400 million. PSI is part of the ETH Domain, with the other members being the two Swiss Federal Institutes of Technology, ETH Zurich and EPFL Lausanne, as well as Eawag (Swiss Federal Institute of Aquatic Science and Technology), Empa (Swiss Federal Laboratories for Materials Science and Technology) and WSL (Swiss Federal Institute for Forest, Snow and Landscape Research). Insight into the exciting research of the PSI with changing focal points is provided 3 times a year in the publication 5232 – The Magazine of the Paul Scherrer Institute.

Here’s a link to and a citation for the paper,

A modern look at a medieval bilayer metal leaf: nanotomography of Zwischgold by
Qing Wu, Karolina Soppa, Elisabeth Müller, Julian Müller, Michal Odstrcil, Esther Hsiao Rho Tsai, Andreas Späth, Mirko Holler, Manuel Guizar-Sicairos, Benjamin Butz, Rainer H. Fink, and Benjamin Watts. Nanoscale DOI: https://doi.org/10.1039/D2NR03367D First published: 10 Oct 2022

This paper is open access.

How do you know that’s extra virgin olive oil?

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Who guarantees that expensive olive oil isn’t counterfeit or adulterated? An invisible label, developed by ETH researchers, could perform this task. The tag consists of tiny magnetic DNA particles encapsulated in a silica casing and mixed with the oil.

So starts Barbara Vonarburg’s April 24, 2014 ETH Zurich (Swiss Federal Institute of Technology or Eidgenössische Technische Hochschule Zürich) news release (also on EurekAlert). She goes on to describe the scope of the situation regarding counterfeit foods,

The worldwide need for anti-counterfeiting labels for food is substantial. In a joint operation in December 2013 and January 2014, Interpol and Europol confiscated more than 1,200 tonnes of counterfeit or substandard food and almost 430,000 litres of counterfeit beverages. The illegal trade is run by organised criminal groups that generate millions in profits, say the authorities. The confiscated goods also included more than 131,000 litres of oil and vinegar.

Jon Henley’s Jan. 4, 2012 article for the UK’s Guardian provides more insight into the specifics of counterfeit olive oil (Note: A link has been removed),

Last month [December 2011], the Olive Oil Times reported that two Spanish businessmen had been sentenced to two years in prison in Cordoba for selling hundreds of thousands of litres of supposedly extra virgin olive oil that was, in fact, a mixture of 70-80% sunflower oil and 20-30% olive.

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… So with a litre of supermarket extra virgin costing up to £4, and connoisseurs willing to pay 10 times that sum for a far smaller bottle of seasonal, first cold stone pressed, single estate, artisan-milled oil from Italy or Greece, can we be sure of getting what we’re paying for?

The answer, according to Tom Mueller in a book out this month [January 2012], is very often not. In Extra Virginity: the Sublime and Scandalous World of Olive Oil, Mueller, an American who lives in Italy, lays bare the workings of an industry prey, he argues, to hi-tech, industrial-scale fraud. The problem, he says, is that good olive oil is difficult, time-consuming and expensive to make, but easy, quick and cheap to doctor.

Most commonly, it seems, extra virgin oil is mixed with a lower grade olive oil, often not from the same country. Sometimes, another vegetable oil such as colza or canola is used. The resulting blend is then chemically coloured, flavoured and deodorised, and sold as extra-virgin to a producer. Almost any brand can, in theory, be susceptible: major names such as Bertolli (then owned by Unilever [see Henley’s article for details about the 2008 Italian olive oil scandal]) have found themselves in court having to argue, successfully in this instance, that they had themselves been defrauded by their supplier.

Meanwhile, the chemical tests that should by law be performed by exporters of extra virgin oil before it can be labelled and sold as such can often fail to detect adulterated oil, particularly when it has been mixed with products such as deodorised, lower-grade olive oil in a sophisticated modern refinery.

Given the benefits claimed for olive oil, I imagine lower grade olive oil which is more highly processed or, worse yet, a completely different kind of oil would diminish or, possibly, eliminate any potential health benefit.

Getting back to the ETH Zurich news release, here’s more about the anti-counterfeiting ‘label’,

Just a few grams of the new substance are enough to tag [label] the entire olive oil production of Italy. If counterfeiting were suspected, the particles added at the place of origin could be extracted from the oil and analysed, enabling a definitive identification of the producer. “The method is equivalent to a label that cannot be removed,” says Robert Grass, lecturer in the Department of Chemistry and Applied Biosciences at ETH Zurich.

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A forgery-proof label should not only be invisible but also safe, robust, cheap and easy to detect. To fulfil these criteria ETH researchers used nanotechnology and nature’s information storehouse, DNA. A piece of artificial genetic material is the heart of the mini-label. “With DNA, there are millions of options that can be used as codes,” says Grass. Moreover, the material has an extremely low detection limit, so tiny amounts are sufficient for labelling purposes.

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However, DNA also has some disadvantages. If the material is used as an information carrier outside a living organism, it cannot repair itself and is susceptible to light, temperature fluctuations and chemicals. Thus, the researchers used a silica coating to protect the DNA, creating a kind of synthetic fossil. The casing represents a physical barrier that protects the DNA against chemical attacks and completely isolates it from the external environment – a situation that mimics that of natural fossils, write the researchers in their paper, which has been published in the journal ACS Nano. To ensure that the particles can be fished out of the oil as quickly and simply as possible, Grass and his team employed another trick: they magnetised the tag by attaching iron oxide nanoparticles.

Experiments in the lab showed that the tiny tags dispersed well in the oil and did not result in any visual changes. They also remained stable when heated and weathered an ageing trial unscathed. The magnetic iron oxide, meanwhile, made it easy to extract the particles from the oil. The DNA was recovered using a fluoride-based solution and analysed by PCR, a standard method that can be carried out today by any medical lab at minimal expense. “Unbelievably small quantities of particles down to a millionth of a gram per litre and a tiny volume of a thousandth of a litre were enough to carry out the authenticity tests for the oil products,” write the researchers. The method also made it possible to detect adulteration: if the concentration of nanoparticles does not match the original value, other oil – presumably substandard – must have been added. The cost of label manufacture should be approximately 0.02 cents per litre.

The researchers have plans for other products that could benefit from this technology and answers to questions about whether or not people would be willing to ingest a label/tag along with their olive oil,

Petrol could also be tagged using this method and the technology could be used in the cosmetics industry as well. In trials the researchers also successfully tagged expensive Bergamot essential oil, which is used as a raw material in perfumes. Nevertheless, Grass sees the greatest potential for the use of invisible labels in the food industry. But will consumers buy expensive ‘extra-virgin’ olive oil when synthetic DNA nanoparticles are floating around in it? “These are things that we already ingest today,” says Grass. Silica particles are present in ketchup and orange juice, among other products, and iron oxide is permitted as a food additive E172.

To promote acceptance, natural genetic material could be used in place of synthetic DNA; for instance, from exotic tomatoes or pineapples, of which there are a great variety – but also from any other fruit or vegetable that is a part of our diet. Of course, the new technology must yield benefits that far outweigh any risks, says Grass. He concedes that as the inventor of the method, he might not be entirely impartial. “But I need to know where food comes from and how pure it is.” In the case of adulterated goods, there is no way of knowing what’s inside. “So I prefer to know which particles have been intentionally added.”

Here’s a link to and a citation for the researchers’ paper,

Magnetically Recoverable, Thermostable, Hydrophobic DNA/Silica Encapsulates and Their Application as Invisible Oil Tags by Michela Puddu , Daniela Paunescu , Wendelin J. Stark , and Robert N. Grass. ACS Nano, 2014, 8 (3), pp 2677–2685 DOI: 10.1021/nn4063853 Publication Date (Web): February 25, 2014

Copyright © 2014 American Chemical Society

This article is behind a paywall.

The Swiss aren’t the only ones interested in tagging petrol (gas), they’re already tagging petrol with nanoparticles in Malaysia with as per my Oct. 7, 2011 posting on the topic.