One of Jackson Pollock's most famous 'action' paintings features a striking turquoise hue that, for 77 years, has remained a mystery.
Now, a team led by chemist Alexander Heyer from Stanford University has established that the light absorbed and emitted by this impressive blue paint matches that of a pigment that has since been banned due to toxicity concerns.
With black and white paint spattered across 2.7 meters (8.7 feet) of canvas, interspersed with bright dribbles and squirts of primary color, the painting, titled Number 1A, 1948, is a classic example of Pollock's anarchic, expressive style.
It is among the first works where, departing from the easel, he notoriously laid his canvas flat on the floor to drip paint from above. This achieved a primordial, expressive, yet complex effect that directly reflects Pollock's physical engagement with – and rebellion against – the act of painting.
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At this time, Pollock was breaking all the established rules of painting: Number 1A, 1948 includes artist-quality oil paints alongside industrially produced enamel house paints. He used a brush in some areas, but other marks are made by hand; some paint is squeezed from the tube, and some is poured from a tin.
This chaotic mix of materials, methods, and a 1958 fire in a MoMA gallery near where the painting was stored, gave the researchers a right riddle in terms of figuring out what exactly he had used for that dazzling blue.
"While past work has identified the red and yellow pigments that form part of his core palette, the vibrant blue in the painting has remained unassigned," Heyer and team write.
Careful scrapings from the painting's blue streaks were taken to the lab for analysis. Earlier efforts to identify the pigment using Raman spectroscopy had been unsuccessful. Each molecule has a unique way of scattering light, so by pointing a laser at a material, chemists can deduce its identity based on the way photons vibrate after bouncing off the material's surface.
They suspected the blue in Number 1A, 1948 to be the now-banned manganese blue (barium manganate sulfate, or PB33). This synthetic pigment was developed in 1907, but it only went to market for artists' use in the 1930s, so it would still have been an exciting new shade when Pollock created his painting.
Previous, non-invasive tests to find out if Pollock had indeed used manganese blue were collected at a high-energy laser line of 532 nm, but the fluorescence of the paint's oil-based binding medium, used to turn pure pigment into a workable art material, created inconclusive results.
This time around, the scientists were able to detect a match by comparing the scrapings with the known Raman spectra for manganese blue paint, collected at a lower energy line of 785 nm.
They found that two distinct bands of electronic transitions give this pigment its unique ability to filter non-blue light on either side of the spectrum: It's the gap between these bands that then reflects such a pure color back to the viewer.
"Manganese blue accomplishes a difficult task: creating clean hues from colors in the center of the visible spectrum," the authors write.
"While blue pigments including ultramarine and phthalocyanine blue, and to a lesser extent cerulean blue, cobalt blue, and Prussian blue, have been identified in Pollock's oeuvre, now this palette includes manganese blue."
Manganese blue's powerful effect is no longer available at art supply stores due to concerns for the health of artists and the environment, but chemists have been exploring alternatives that provide a similar vibrance without the toxicity.
In 2009, a chemist discovered the first 'new' blue in 200 years, known as YInMn blue, which has been embraced by artists as a stand-in. This new analysis of the Raman spectrum features of real manganese blue could help chemists create even more stable, safe alternatives to the now-forbidden hue.
"It's really interesting to understand where some striking color comes from on a molecular level," Stanford chemist Edward Solomon told Adithi Ramakrishnan at The Associated Press.
This research was published in PNAS.