Harvard Art Museumsã¢â⢠Straus Center for Conservation and Technical Studies

Uncovering a hidden composition behind Gauguin'southward "Poèmes Barbares''

Known for his utilize of symbolism and his syphilis, the post-impressionist creative person Paul Gauguin is perhaps virtually famous for his work made in  French Polynesia. Gauguin went to Tahiti in two trips, first in 1891, returning to French republic in 1893, then again in 1895. He stayed until he died in the Marquesas Islands in 1903.

Kate Smith is a conservator of paintings and the caput of the paintings lab at the Straus Center for Conservation and Technical Studies at the Harvard Fine art Museums. She says that there's a clear distinction betwixt Gauguin'due south piece of work on his first and 2nd trip. When he first travels to Tahiti, Smith says his work was more documentary in nature. 

"He painted the people riding their horses, he painted the bounding main, he painted people fishing, he painted all the different plants that he'd never heard of," Smith says.

Merely when he returns to Tahiti in 1895, Smith says his work focuses more on Tahitian spiritual practices.

"He's starting to pigment... philosophical and spiritual pictures of his interpretation of the civilisation and the religion," Smith says.

The Harvard Art Museums has a painting that Smith says really encapsulates Gauguin's fourth dimension in Tahiti. The painting in question is "Poèmes Barbares'' by Paul Gauguin. Painted in 1896 on his final trip to Tahiti, the piece depicts an affections and an animal god, combining imagery from Tahitian, Christain and Buddhist traditions. The colors are warm, dark and saturated. 

Merely below the surface of "Poèmes Barbares" lies a second, hidden landscape limerick with riders on horseback that a team at the Straus Middle for Conservation and Technical Studies at the Harvard Fine art Museums was able to uncover with the help of the Center for Scientific Studies in the Arts (NU-ACCESS). NU-ACCESS published a May 2021 paper in Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy on the apply of Uniform Manifold Approximation and Project (UMAP) in the spectral imaging of  "Poèmes Barbares."

Both the lower and upper compositions of "Poèmes Barbares" were painted on Gauguin'southward second trip. Nonetheless, the documentary nature of the landscape with the riders on horseback in the lower composition seems similar to Gauguin'south other piece of work on his first trip. The spiritual imagery of the upper portrait is feature of his second trip. Smith says both compositions together in a way sum upwardly the entirety of Gauguin'southward work and time in Tahiti.

"It'south sort of similar the painting contains his whole trip," Smith says.

The story of the discovery of the painting'south subconscious limerick begins in 1950 when it was bequeathed to Harvard by Maurice Wertheim. "Poèmes Barbares" is one of 26 paintings in Wertheim'due south collection at the Harvard Art Museums. He stipulated that the pieces must ever be on display and shown together in a single gallery.

Pieces can just exist removed from exhibition for no longer than 90 days, which Smith says made it hard to report them in-depth. An opportunity for a "loophole" came when the museum buildings were closed for renovation and expansion from 2008 to 2014.

"All of a sudden, we had all these paintings in storage, and we had access to them," Smith says.

According to Smith, each painting in the Wertheim collection was imaged using a technique called X-radiography to aid reveal its inner structure (similar equipment as used to image bones in the human torso), infrared reflectography helped the conservators to  await for underdrawings or preparatory work made by the artist, and a microscopic exam of the surface to understand the paint layers and buildup.

From visual inspection of the Wertheim collection, the team had e'er  suspected that some of the paintings had interesting features below the visible compositions that scientific analyses might help reveal, only "Poèmes Barbares" had non been one of them.

"The Gauguin ["Poèmes Barbares"] doesn't nowadays as having annihilation unusual going on. The surface texture corresponds with the composition on the surface," Smith says. "So we thought, 'We'll only get an Ten-ray taken of the canvas weave and motion on.'"

Merely in 2012, after looking at the X-ray, they realized something was indeed unusual every bit the prototype did not correspond at all to the surface limerick. Co-ordinate to Smith, just like an x-ray of your body, an x-ray of a painting is mapping density. Your basic appear white on an x-ray considering they are denser than your muscles. An x-ray of a painting similarly shows the density of the pigments used.

Pigments equanimous of  heavy elements like lead in the pigment lead-white (PbCO₃) or mercury in vermillion (HgS) volition appear as a lighter hue on the X-ray because they are denser and are lower on the periodic table. Organic pigments or pigments composed of lighter elements will announced darker in the X-ray image.

"So you expect it to look like a black and white rendition, to some degree, of the composition," Smith says.

The squad expected  the orange and red background of  "Poèmes Barbares" to be composed of  the pigment vermillion and thus information technology would appear lighter on the X-ray due to the relatively loftier density of mercury, just instead, the image contrast bore no resemblance to the surface limerick. After a week of puzzling over the 10-ray, Smith flipped it sideways and suddenly a landscape appeared.

"Your brain is then interesting," Smith says. "We just flipped it 90 degrees, and it started to feel more like a landscape."

Smith says the x-ray reveals what looks like trees or clouds in the groundwork of the subconscious, lower composition, and her colleague, Teri Hensick, noticed what looked like two riders on horseback.

To probe more beneath the surface of the painting the team turned to Ten-ray fluorescence (XRF) assay. XRF analysis involves shining an Ten-ray axle at an object and collecting the emitted fluorescence . The different wavelengths of the Ten-ray low-cal collected can exist used to characterize the elements present in the underlying materials.

XRF tin clarify non just the visible exterior, but has deep penetration into the painting from front end to back, so this technique was particularly useful in identifying the elements, and therefore the pigments, inside the lower composition.

"In paintings with multiple layers, each of which can contain several pigments with dissimilar elemental compositions, the XRF spectrum has numerous x-ray peaks. Once the elements are identified in the spectrum, the pigments present can be suggested from the elemental patterns," says Katherine Eremin, the Patricia Cornwall senior conservation scientist at the Straus Center who was brought in to exercise the point XRF analysis.

The Harvard Art Museums has an XRF unit that tin but wait at specific points on a painting. According to Eremin, they focused on points of the surface close to the edges of the painting where yous could run across the pigment layers of the lower compositions. However, they soon institute out the information didn't elucidate very much.

"Information technology became obvious that yous could get a certain amount from the point [XRF] analysis, but it was very express," Eremin says. "But we're never really going to be able to get an idea of what the sort of structure underneath was, what the underlying painting looked like."

But at that place was a technique that would permit them to carry XRF analysis across the canvas called macro X-ray fluorescence (MA-XRF) scanning. At the time, Smith says MA-XRF scanning was just coming into mode, and the Harvard Art Museums did not take a MA-XRF scanner. The team thought that if they could become their hands on a MA-XRF scanner it could better indicate the pigments used across the canvas, Smith says.

But at this point,  the newly renovated Harvard Art Museums were open and the stipulations of the Wertheim collection at present practical. Co-ordinate to Smith, the 90-solar day window where the painting could be removed from display did not give very much time to pack and send the painting to a place with a MA-XRF scanner.

That is why the squad turned to NU-ACCESS for help in 2016. NU-Admission has a mobile MA-XRF scanner and hyperspectral imaging (HSI) unit for the purpose of partnering with institutions that don't normally have those tools and techniques at their disposal. NU-ACCESS brought both to the Harvard Art Museums, and Smith says the MA-XRF scanner ran 24 hours a day, for three days to scan the entire canvas, salve a minor strip at the bottom.

The MA-XRF scanner was able to yield elemental maps of the sheet that could indicate to the location of certain pigments. For example, according to Smith, the mercury map corresponded with the squad'due south suspected utilise of the paint vermillion in the background and the affections'due south lips. But in that location were also small shapes showing concentrations of mercury that didn't represent to where vermillion was visibly present in the surface limerick. This could point to the employ of a pigment containing mercury in the lower composition.

"And so we simply started noting those kinds of differences. 'What's hither that doesn't correspond to what we can meet with our centre?'" Smith says

But information technology was the MA-XRF map of tin that presented one of the biggest mysteries of the project. Co-ordinate to Smith, the map showed a strong tin response in the area of the affections's blue sarong, but the only oil paint pigment in which tin is present is cerulean blue fabricated from cobalt stannate. However, according to the MA-XRF assay, in that location was no indication of the presence of cobalt in the sarong, which meant the pigment used was most likely not cerulean blue.

"Only the sarong itself is blue. So why is information technology blue?" Smith says. "There are no other blues that incorporate tin."

The strong presence of lead in the sarong indicated the utilise of lead white mixed with another pigment, but that pigment remained unknown. The squad searched the other element maps for some indication of the blue mystery pigment.

"All the maps were ruling out every bluish known to man," Smith says.

One answer to the tin puzzler came in the class of carmine, a cherry-red lake pigment made from a dye extracted from the cochineal insect. In the belatedly 19th century, Smith says there was a particular cherry-red pigment that was cast using tin as a base, then that explained the strong presence of tin can but inspired new questions since there was no visible red in the upper limerick.

"There's a blue colour with no blue and a red consequence with no carmine," Smith says.

As the team struggled to reply questions with non-invasive techniques such as MA-XRF, they turned to analyzing cross sections of the painting which involves flaking off pieces of pigment no bigger than a grain of sand. But they did not do this lightly.

"Taking samples from an object is a really large deal," says Georgina Rayner, the associate conservation scientist at the Straus Middle. Sampling the painting required extensive word and approval of the Harvard Art Museums curators, Rayner says.

Even afterwards securing approval, removing cantankerous sections does non mean slicing into a painting wherever you please.

"To take a sample we're using a surgical scalpel with a sharp bespeak and gently prying a little bit of paint away from the surface," says Rayner. "So areas of harm are good because there's by and large cracks...so you can control how much of the sample will break away."

Ultimately, Rayner and the squad took xiv microscopic samples from the flaking edges of the sail. These samples helped reveal the stratigraphy of the dissimilar pigment layers and differentiate betwixt the pigments in the upper and lower compositions.

"Y'all could interrogate all the dissever layers," Eremin says.

Through the analysis of the cross department, they plant the presence of atomic number 26 in the sarong which would point the pigment Prussian bluish. The fe had not shown up in the MA-XRF scan of the sarong due to a limitation of the technique.

Eremin says that while the MA-XRF can ordinarily penetrate all the layers of paint, it can be difficult for elemental data near the lower layers to exist relayed back if the upper layers are composed of pigments with heavy metals, similar lead or mercury, and the lower layers are equanimous of organic pigments or pigments with lighter elements, such equally fe or chromium.

Because the blue pigment was mixed with a lead white, the lead was preventing the MA-XRF from detecting the iron in the Prussian blue.

"The pb quenches the betoken of the iron. So all you go is a lead response and no iron response even though in that location is fe present," Smith says.

So the original colour of the sarong was the upshot of mixing lead white, Prussian blue and carmine using a tin base of operations.

"So it would have been sort of a purpley colour. Now it's this sort of greenish blue considering all the red has completely faded away," Smith says.

Smith says this non merely answered a "forensic puzzle," but it also gave insights into Gauguin's original color palette.

"Right now it's got this green-blue...quality in the sarong in this cooler cobalt and ultramarine bluish in the wings that kind of kind of sour each other a footling bit," Smith says. "But when yous put a red cast over this [to make the sarong purple], the whole thing falls into color harmony once again."

Using the shapes shown from the original X-radiograph, the cross section assay and the elemental mapping from the MA-XRF assay, Smith put together a rough sketch of hidden, lower composition with its suspected colors. Smith says the mercury in the vermillion background of the upper composition made it hard for the MA-XRF to detect how far the yellow paint extended into the canvas.

Meanwhile, members of NU-ACCESS accept been combing through the HSI data using statistical assay to yield more than data about the painting.

Due to the COVID-nineteen pandemic, the Harvard Art Museums are closed, which means Smith and the team have had more time to clarify the painting than the usual 90-24-hour interval window would permit. Currently, she has been removing the painting's varnish, which Smith says Gauguin would not have applied. Eremin says the varnish is disfiguring the painting, and a byproduct of its removal is the uncovering of some cracks away from the edges of the painting.

"All of a sudden, we have access to the middle of the painting. In that location are old losses and cracks and things that we can take samples from." So Smith hopes to do ane more than round of samples then publish in a yr what the results of all the analyses mean art historically.

From an art-historical perspective, Smith is excited about what the scientific analyses could reveal about Gauguin's process and frame of heed.

"What really fires me up is that this kind of investigation tin reveal decision-making," Smith says. "Fifty-fifty looking microscopically at a cross section getting that swirling expect, looking at him working quickly and fast, and so working deadening and methodical."

From a scientific point of view, Eremin says the project shows how one blazon of analysis can't tell you everything.

"Yous need lots and lots of different techniques and this kind of multidisciplinary collaboration," Eremin says. "You can't but utilise one scientific technique and hope to accept all the answers."

Eremin stresses how vital NU-ACCESS and its MA-XRF scanner were in this project.

"It was such a fantastic opportunity to find out more with equipment that we didn't take," Eremin says.

According to Smith, it has been peachy to work with NU-ACCESS to mull things over together.

"That kind of multidisciplinary collaboration is where information technology gets really rich and interesting," she says.

Overall, Smith believes that forensic investigation of art can be a not-intimidating style into art that may frequently seem unapproachable or fifty-fifty elitist.

"You beginning to think of paintings every bit things made of stuff by people, instead of a theoretical construct that you take to sympathize by some magical beam of whatever," Smith says. "There's a potential there to humanize fine art by reminding ourselves that information technology's fabricated up stuff and that information technology changes over time."

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Source: https://scienceforart.northwestern.edu/field-notes-events/articles/2021/uncovering-a-hidden-composition-behind-gauguins-po%C3%A8mes-barbares.html

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