State Of The Art: How XRD/XRF Reveals A Piece's Hidden Composition
By Abby Proch, former editor
On my flight home from SPIE Defense + Commercial Sensing, the in-flight entertainment featured a buzzy new documentary called “The Lost Leonardo.” This one centered on a newly-resurfaced painting of Jesus that art historians, critics, and auctioneers just sort of agreed was created by Leonardo da Vinci. Until they didn’t. The mystique of the “Salvator Mundi” built so rapidly — as did the purported value — that naysayers quickly multiplied and questioned its verification process.
It reminded me that I once had a conversation with Mark Donaghy of Raptor Photonics about how the company’s CCD cameras have found niche use with a client specializing in cultural preservation, namely art inspection. A few weeks later, Mark made the introduction between me and Philippe Sarrazin of eXaminart, headquartered in Mountain View, California. Though Sarrazin has no stake in the validity of the Lost Leonardo, his non-destructive, portable X-ray diffraction (XRD) and X-ray fluorescence (XRF) instrument called the Duetto has the potential to digitally peel back the layers of paint, so to speak, and see what is normally unseen — and do it just about anywhere.
When held up to a painting, for example, the handheld Duetto employs XRD to identify a material’s crystalline structure, while XRF identifies its chemical composition. And the fact that the Duetto conducts both processes at the same time and on the same sample is unique. The Duetto can help conservation scientists understand an artists’ pigment choices and application techniques, discern whether a piece of art came from a particular workshop or school of artists, or even determine whether a piece of art had undergone restoration or suffered degradation.
Consider, for instance, Egyptian Blue, the first synthetic pigment developed by man. It appears across a swath of 200 AD Egyptian mummy portraits at the Phoebe A. Hearst Museum of Anthropology in California. Its presence can be affirmed by NIR luminescence, but not definitely, because other elements also illuminate under the same wavelength. Enter the Duetto.
Duetto confirmed the presence of Egyptian blue in some unexpected places – the toning of the gray background and under-drawing pigment that outlines a face, among others. Its locations led researchers to believe that, since it was used in the early stages of the painting and as a mixer, the pigment was actually more plentiful than assumed and had, perhaps, even started to fall out of favor.
What’s more, the Duetto is lightweight and portable, capable of being toted nearly anywhere around the world. Sarrazin says Duetto was the first mobile instrument of its kind, giving conservationists the option to test in situ and avoid transporting art to a lab.
“You have to fly back to Chicago, London who knows where, where your instrumentation is. That's very often impossible, just plain impossible because the country does not want to see their cultural wealth leave its borders,” said Sarrazin.
It also don’t want it damaged, which is what happens during traditional XRD and XRF techniques wherein a physical sample is removed, crushed, and tested inside an instrument. Duetto doesn’t even have to make contact.
“That’s a big deal for conservation. You're trying to preserve the works of art without changing them, without altering them, by stopping corrosion and things like that,” said Sarrazin.
And there’s a dozen more examples, most of which were published by Sarrazin’s now-colleague and former Getty Conservation Institute Chief Scientist Giacomo Chiari. Chiari and others have used Duetto to distinguish clad daguerreotypes (before 1860) from electroplated daguerreotypes (1860 and later), determine whether a gold finish has been laid with ancient or modern gilding methods, and distinguish encaustic beeswax from emulsion (modified wax) and how they were applied.
Sarrazin calls Chiari his “best salesman.”
“It's clear that without him this product would never have come out other woods, never. And even today, he’s still involved, and he's retired twice — retired from his professor position and then retired from the Getty. But he still works on with Duetto.”
Roughly 15 years ago, Sarrazin originally developed an XRD/XRF tool for the Mars Curiosity rover. Getting wind of the instrument, Chiari surmised that if he could make one for another planet, surely, he could make one for here on Earth, recalls Sarrazin.
Soon, a partnership developed in which Chiari asked Sarrazin to develop what would become the Duetto. It wasn’t a commercial success and in fact nearly died off completely when Sarrazin’s company was bought and its new owner nixed the instrument after “making decisions with Excel spreadsheets and not the heart,” said Sarrazin.
But Sarrazin revived the design years later and, with the help of a few folks at Raptor Photonics, began working on Duetto 2. Raptor provided a custom deep-cooled, deep-depleted CCD camera, and rather than house it within the instrument, protruded it from body to get within 2mm of a sample — a crucial step to achieving contactless sampling.
“I think where we differentiate as Raptor is our ability to custom design solutions to meet the needs of our customers. [Sarrazin] had some specific requests in terms of custom design. While others were prepared to take that on, we embraced it,” said Donaghy.
Duetto 2 is now in production. Readout time has improved by a factor of 10, power consumption has dropped from 60W to 30W, and battery life has expanded from four to eight hours. And with its more user-friendly operations and data analysis functionality, Sarrazin thinks it’s ready for more widespread use.
Roughly 10 years ago, Sarrazin got a request from an oil and gas representative to develop an instrument that would help him determine the tampering treatment used to make alloy transmission pipes. The pipes reportedly had the right chemical composition but not the right structure and so became more susceptible to corrosion, he said. Sarrazin needed six months for the build, but the rep could only afford six weeks. But it always comes back to art authentication.
“It's definitely growing, and I can see a market here. …There’re so many high value transactions happening in the art world that if only 1% of it was actually now backed by scientific evaluation, I think it would be a nice market for us,” said Sarrazin.
In 2017, British auction house Christie’s sold the Salvator Mundi — which it did attribute to da Vinci — for $450 million to a person reportedly representing Saudi Arabian Crown Prince Mohammed bin Salman. That painting was reportedly destined for the Louvre Abu Dhabi. But since its sale, the painting has not been seen. Some have speculated it’s being kept safe in Geneva or even stowed aboard the Prince’s superyacht. If the latter is true, maybe Sarrazin can meet him at the docks if the ship ever drops anchor in the Bay Area.
Interested in adding another doc to your queue? You can watch “The Lost Leonardo” on a number of streaming services. If good old-fashioned case studies are your preference, explore the findings from Chiaro, Sarrazin, and others using eXaminart’s Duetto and Duetto 2. Camera details can be found in this Raptor Photonics application note.