Every field has its raging debates among impassioned experts, and the art world is no exception. Case in point: some art historians long suspected that master painter Pablo Picasso used common house paint rather than the oil paints traditionally used in his era, which would make him the first known artist to do so. But how to prove it? If you're Francesca Casadio, conservation scientist for the Art Institute of Chicago, you take advantage of the close proximity of top-notch physics laboratories in the area.

In this case, she partnered with Volker Rose, a physicist at Argonne National Laboratory who uses high-energy x-ray beams to study materials at the molecular level. Rose was on hand at the APS March Meeting in San Antonio early this month to talk about that collaboration, and the intriguing results that emerged from studying Picasso at the nanoscale.

Let's rewind a bit to 2006, when preparations for an upcoming Picasso exhibition prompted museum administrators to challenge Casadio to settle the house paint conundrum once before. She doggedly traipsed through Europe, checking as many Picasso canvases as she could with a handheld fluorescence spectrometer, a device that reveals inorganic elements in things. ("Airport security's not always happy to see it," Casadio admitted to the Chicago Tribune in 2012. "But I tell them it's a fancy hair dryer to do paint testing.")

Credit: Art Institute of Chicago.

And she tracked down some old paint manufactured by the Ripolin company circa 1912, via the French equivalent of eBay for comparison purposes.

It was a noble effort, but she didn't have instruments capable of sufficiently high resolution to make a definitive diagnosis. So the quest fell by the wayside again until 2010, when one of Casadio's colleagues was dating a scientist at Argonne.

He suggested the lab's Advanced Photon Source -- a source of high-energy x-rays -- would be ideal for the kind of fine resolution analysis needed to solve the mystery, And thus did Casadio team up with Volker, analyzing chips of paint from several Picasso canvases (removed by microplaning samples off errant drips, in case you're worried about damaging priceless artworks) as well as the old paint Casadio had picked up back in 2006.

It's the higher frequency of x-rays that make them so useful for analyzing art. They pass right through paintings without harming them. Since denser material collects more x-ray photons, particularly dense pigments (zinc oxide, red lead) show up as bright white spots in the x-ray image. X-ray imaging can also reveal anything that has been painted over a canvas, or where the artist may have altered his (or her) original vision.

Synchrotron radiation is a bit different from conventional x-rays discovered by William Roentgen back in 1895; it’s a thin beam of very high-intensity x-rays generated within a particle accelerator. You fire electrons into a linear accelerator (linac), boost their speeds in a small synchrotron and inject them into a storage ring, where they zoom through at near-light-speed. A series of magnets bend and focus the electrons, and in the process, they give off x-rays, which can then be focused down beamlines. This is useful for imaging purposes, and for analyzing structure, because in general, the shorter the wavelength used (and the higher the energy of the light), the finer the details one can image and/or analyze.

So Casadio and Rose chose a very good technique for their Picasso project. The upshot: the historian' suspicions were correct. Picasso did indeed use common house paint. The smoking gun was certain impurities in the zinc oxide found in the white paint used by the master. That finding led to a 2013 paper in Applied Physics A and formed the basis for Volker's presentation at the APS meeting.

"The Old Guitarist," Pablo Picasso (1903). Via http://www.pablopicasso.org

Oh, and the analysis also revealed another painting underneath Picasso's "Old Guitarist" (left). X-rays revealed the image of woman and child, plus a few animals, lurking behind the top painting.

Via Before the Art, Jen-Luc Piquant learned about an infamous case in the 1920s involving a Kansas City socialite who claimed to own the authentic version of a famous 16th century painting sometimes attributed to Leonardo da Vinci. La Belle Ferroniere belongs to the Louvre, but in 1929 Mrs. Andre Hahn sued an art dealer named Joseph Duveen for refuting her claim that the Louvre had a copy, and hers was the original.

It was x-ray radiographs that helped put the debate to rest and the news was not good for Mrs. Hahn. Images of the Louvre canvas clearly showed the changes the artist had made during the painting's composition; Mrs. Hahn's canvas didn't show those changes, evidence that it was a copy. The jury couldn't reach a verdict, but at least x-rays emerged triumphant as a good way to authenticate art.

I have previously written about this fascinating intersection between physics and art. For instance, back in 2008, a team of European scientists who used synchrotron radiation to reconstruct the portrait of a peasant woman painted by Vincent van Gogh that the artist had then painted over when he created 1887′s “Patch of Grass.” It lay there, dormant, for 121 years until we finally had the technology to nondestructively analyze the painting and reproduce the hidden image, courtesy of the Deutches Elektronen-Synchrotron (mercifully known by the acronym DESY) in Hamburg, Germany.

"Wheat Stack Under a Cloudy Sky," Vincent van Gogh

Most recently the same folks at DESY used x-ray diffraction mapping combined with tomography to study Van Gogh's "Wheat Stack Under a Cloudy Sky." They were interested in better understanding how red lead oxide pigment discolors as it ages. Sometimes it turns into plattnerite or galena, blackening the color, and other times it might convert to red lead or lead sulfite, bleaching it out.

The DESY team collaborated with researchers at the University of Antwerp in the analysis, and discovered a very rare mineral called plumbonacrite in the van Gogh canvas that seems to play a role in the bleaching process of the pigment. Over time, light triggers CO2 absorption, which causes plumbonacrite to form, eventually turning into white-hued lead carbonate.

There are lots of variations on x-ray based techniques, such as one called scanning macro x-ray fluorescence spectroscopy developed by researchers at the University of Antwerp and the Delft University of Technology. It's particularly good at highlighting pigments concealed beneath the surface of a painting. At the behest of the Netherlands’ Rijksmuseum, they used it to analyze Goya’s “Portrait of Don Ramon Satue,” an 1823 canvas depicting a man who was then a judge in Madrid — and a pal of the artist. And lo and behold, underneath the portrait is another, half-finished portrait of a French general. For some reason, Goya decided to paint over it.

The technique was also used in 2011 to authenticate a small panel ("Old Man with a Beard") believed to be by Rembrandt. The researchers discovered an unfinished self-portrait by the Dutch master underneath the surface.

Nor are X-rays aren’t the only physics tool being deployed to illuminate the secrets of art: a group of Italian and German scientists have used nuclear magnetic resonance — the underlying physics behind MRI machines — to non-invasively map out the layers of historical fine paintings. This is known as “stratigraphy.” That includes any preparatory layers, under-drawings, the actual layers of paint, and in many cases, a layer of varnish.

The new technique does much the same thing for paintings as it does for the human body, except instead of using x-rays, detectors, and cutting-edge computers to provide information about soft tissue and the possible presence of tumors, it provides information about the binding agents used in the painted layers. Those agents were often made of things like egg yolk or oil.

Keen to learn more about this fascinating area? It just so happens there is an entire documentary film -- narrated by Donald Sutherland, no less -- that centers on those intrepid scholars who roam the earth to examine paintings once believed to be mere copies, since authenticated as lost Rembrandts. It's called Out of the Shadows. Can a documentary about the science and art connecting Picassos, van Goghs and Goyas be far behind?


Casadio, Francesca and Rose, Volker. (2013) "High-resolution fluorescencemapping of impurities in historical zinc oxide pigments: hard x-ray nanoprobe applications to the paints of Pablo Picasso," Applied Physics A 111: 1.

Joris, Dik et al. (2008) "Visualization of a lost painting by Vincent van Gogh using synchrotron radiation based X-ray fluorescence elemental mapping," Analytical Chemistry 0003-2700-80, p. 6436-6442

Koen, Janssens et al. (2013) "The use of synchrotron radiation for the characterization of artists' pigments and paintings," Annual Review of Analytical Chemistry 1936-1327-6, p. 399-425

Matthias, Alfeld et al. (2011) "Optimization of mobile scanning macro-XRF systems for the in situ investigation of historical paintings," Journal of Analytical Atomic Spectrometry 0267-9477-26, p. 899-909

Presciutti, Federica et al. (2008) "Noninvasive nuclear magnetic resonance profiling of painting layers," Applied Physics Letters 93: 033505.

Vanmeert, Frederick; Van der Snickt, Geert; and Janssens, Koen (2015) "Plumbonacrite Identified by X-ray Powder Diffraction Tomography as a Missing Link during Degradation of Red Lead in a Van Gogh Painting,“ Angewandte Chemie 127(12): 3678-3681.