Pigment darkening as case study of In-Air Plasma-Induced Luminescence

Abstract

We introduce the use of an In-Air Plasma-Induced Luminescence (In-Air-PIL) spectroscopy as an alternative to classical chemical and crystallographic methods used in materials science. The In-Air-PIL is evaluated on a case study investigating the effect of light aging on the darkening of five pristine yellow pigments commonly used in artworks. We show that the darkening is not associated to changes in the chemical composition, but to a loss in crystallinity, indicating an amorphization process of the pigments induced and catalyzed by the light irradiation. This favors the interaction of the pigment molecules with oxygen and carbon adsorbed from the environment or solved in the binding agent, subsequently leading to the formation of oxalates and carbonates as observed in other works. We demonstrate that the In-Air-PIL results are in perfect agreement with more complex classical materials science analysis methods, making our plasma-driven method a potentially easier and faster technique.

The Geometry of Colors in Van Gogh’s Sunflowers

“…Possibly the most striking difference we observed when comparing the hues of yellow in the flower depiction of the three version of Sunflowers in a vase with a yellow background with the hues of yellow in real sunflowers observed in modern photographs is an increase in the amount of blue coordinates in those yellow hues in the paintings, leading to a less vibrant and brownish yellow, in agreement with chemical analyses of the aging of chrome yellow [9,10,11,13,27]. This observation hints at an opportunity to restore algorithmically the original colors of the Sunflowers: reducing the amount of blue for all pixels in the images of the paintings.…”

“…By comparing those color spaces with those occupied by similar ROIs in photographs of real sunflowers, we identified shifts in all three color coordinates, R, G, and B, with the shift in the blue coordinate being the more salient. This shift in blue that leads to hues of yellow that are faded and even brownish are consistent with the fading of the chrome yellow, the pigments used by van Gogh for representing yellow in Sunflowers, observed by chemical spectroscopic methods [9,10,11,13,27]. We have proposed two algorithms, PCR-1 and PCR-2, for correcting that shift in blue and generate representations of the paintings that aim to restore their original conditions (see Figure 11).…”

“…He painted F454 in late august 1888, when sunflowers usually start to fade [26]. Second, van Gogh used the yellow chrome pigment to color the sunflowers and it is known that this pigment darkened as it ages [9,10,27]. Finally, we analyze the colors of the paintings using digital images of those paintings.…”

“…Recent X-ray fluorescence spectrometry analyses of F454 [29] and F458 [14] revealed that indeed van Gogh was using chrome yellows to render yellow in those paintings; it is very likely that he was using the same pigments for F457. The aging of chrome yellow pigments used in paintings has been studied in details [9,10,11,13,27], including studies on van Gogh's Sunflowers [15]. Those studies have highlighted that the lightest hues in the chrome yellow family contain sulfate groups, which reduce the pigments' stability under light: bright yellow on canvases then turns to brownish green.…”

Integrated quantitative PIXE analysis and EDX spectroscopy using a laser-driven particle source

“…These fields could substantially benefit from the adoption of a flexible, multi-particle tool for X-ray emission spectroscopy with different capabilities. Laser-driven sources (10,11) are worth of consideration for materials science applications (12)(13)(14)(15). A common acceleration scheme exploits the interaction between ultra-short (tens fs) super-intense (I>10 18 W/cm 2 ) laser pulses and micrometric solid targets to accelerate electrons and protons to energies ranging from few MeVs up to 100s MeV.…”