Raman study of different crystalline forms of PbCrO₄ and PbCr_1−xS_xO₄ solid solutions for the noninvasive identification of chrome yellows in paintings: a focus on works by Vincent van Gogh

Abstract

Chrome yellows, a class of pigments frequently used by painters of the Impressionism and Post‐impressionism period, are known for their different chemical stability; the latter depends on the chemical composition (PbCrO4, PbCr1−xSxO4) and crystalline structure (monoclinic or orthorhombic) of the material. The possibility to distinguish among different forms of this pigment is therefore relevant in order to extend knowledge on the corresponding degradation process that is observed on several original paintings. For this purpose, three paintings conserved at the Van Gogh Museum (Amsterdam) were analyzed using noninvasive Raman spectroscopy, while equivalent investigations employing bench‐top instrumentation were performed to obtain information from micro‐samples originating from these works of art. In each painting, the chrome yellow was identified either as monoclinic PbCrO4 or in the form of monoclinic PbCr1−xSxO4 (x < 0.25) or S‐rich orthorhombic PbCr1−xSxO4 (x ~ 0.5). Our ability to make this fairly subtle distinction is based on a Raman study of several oil paint model samples made up of monoclinic and/or orthorhombic crystalline forms of PbCrO4 and PbCr1−xSxO4 (0.1 ≤ × ≤ 0.8). These paints were studied using several excitation wavelengths (namely 785.0, 532.0, 514.5, and 488 nm). Because of the absence of the resonance Raman effect, which strongly enhances the chromate symmetric stretching band, and the absence of any laser‐induced photodecomposition, it is advantageous to acquire data at 785.0 nm. The band‐shape and the position of the chromate bending modes proved to be more sensitive to the solid solution composition and crystalline structure than the stretching modes and can be used as distinctive spectral markers to discriminate among the different chrome yellow forms that are present. Copyright © 2014 John Wiley & Sons, Ltd.

Reflectance imaging spectroscopy and synchrotron radiation X-ray fluorescence mapping used in a technical study of The Blue Room by Pablo Picasso

“…These phases can be identified using µRaman spectroscopy and µXPD [38,39]. The chromate stretching band at 841 cm −1 increases as the amount of sulfate in the pigment increases, along with changes in the intensities of the chromate bending modes.…”

Characterization of Haku Maki prints from the “Poem” series using light-based techniques

“…Interestingly, the Raman features of the orange paint matched that of the yellow paint, attributing the primary pigment to chrome yellow in its predominantly monoclinic phase [26]. Considering that both the orange and yellow paints were applied at the same time and aged together in the same environment over the same period, it is surprising that the orange paint darkens much faster than the yellow paint.…”

“…Table 2 shows the possible chrome yellow variants based on PbCr 1−x S x O 4 . Using Raman spectroscopic analysis with 785 nm laser excitation, Monico et al [26] were able to discriminate between monoclinic and orthorhombic PbCrO 4 , whereas peaks at 411, 384, 362 and 341 cm −1 are characteristic of the orthorhombic phase [26]. , all point to a predominantly monoclinic phase with low sulfur content of x ≤ 0.1.…”

“…However, it is possible that mixtures of different phases exist in the paint. When the orthorhombic and sulfur-rich (x > 0.4) phase is mixed in substantial amounts with the monoclinic phase, the peaks appear at 841, 402, and 359 cm −1 , resembling monoclinic features due to a higher Raman scattering coefficient for the monoclinic phase, and the sulfate peak occurs at 978 cm −1 [26]. Noting that the yellow pigment used in Haku Maki's print gives a weak sulfate peak at 970 cm −1 , we can deduce that it is predominantly monoclinic with low sulfur content (x < 0.25).…”

Raman microscopic detection of chromium compounds

“…Neither for decades admired Sunflowers by Van Gogh, do not remain the same. Old Masters before the end of the 18th century used mineral pigments for shiny yellow colour, containing toxic elements such as chromium, lead or cadmium [2]. These pigments subject to reduction of hexavalent chromium in time and are changing the shiny yellow to green.…”