Degradation Process of Lead Chromate in Paintings by Vincent van Gogh Studied by Means of Spectromicroscopic Methods. 4. Artificial Aging of Model Samples of Co-Precipitates of Lead Chromate and Lead Sulfate

Abstract

Previous investigations about the darkening of chrome yellow pigments revealed that this form of alteration is attributable to a reduction of the original Cr(VI) to Cr(III), and that the presence of sulfur-containing compounds, most often sulfates, plays a key role during this process. We recently demonstrated that different crystal forms of chrome yellow pigments (PbCrO(4) and PbCr(1-x)S(x)O(4)) are present in paintings by Vincent van Gogh. In the present work, we show how both the chemical composition and the crystalline structure of lead chromate-based pigments influence their stability. For this purpose, oil model samples made with in-house synthesized powders of PbCrO(4) and PbCr(1-x)S(x)O(4) were artificially aged and characterized. We observed a profound darkening only for those paint models made with PbCr(1-x)S(x)O(4), rich in SO(4)(2-) (x ≥ 0.4), and orthorhombic phases (>30 wt %). Cr and S K-edge micro X-ray absorption near edge structure investigations revealed in an unequivocal manner the formation of up to about 60% of Cr(III)-species in the outer layer of the most altered samples; conversely, independent of the paint models' chemical composition, no change in the S-oxidation state was observed. Analyses employing UV-visible diffuse reflectance and Fourier transform infrared spectroscopy were performed on unaged and aged model samples in order to obtain additional information on the physicochemical changes induced by the aging treatment.

2D X-ray and FTIR micro-analysis of the degradation of cadmium yellow pigment in paintings of Henri Matisse

“…Several examples of the discoloration of synthetic yellow pigments from the turn of the twentieth century (e.g. zinc yellow (K 2 OÁ4ZnCrO 4 Á3H 2 O) [1], chrome yellow (PbCrO 4 ) [2][3][4][5][6], and cadmium yellow (CdS) [7,8]) have been recently reported.…”

Nanoscale Investigation of the Degradation Mechanism of a Historical Chrome Yellow Paint by Quantitative Electron Energy Loss spectroscopy Mapping of Chromium Species

“…They are known to be sensitive towards darkening when exposed to sunlight. [5] However, in a recent study, [6] it was found that the speed of the alteration process of the lead chromate based yellow paints is influenced both by their chemical composition (in terms of sulfate amount) and crystalline structure (monoclinic and/or orthorhombic): a profound darkening was observed only for those compounds containing a sulfur-rich solid solution of PbCr 1Àx S x O 4 (x ! 0.5) and those in which this compound is mainly present in the orthorhombic crystalline form.…”

“…0.5) and those in which this compound is mainly present in the orthorhombic crystalline form. [6] Since different forms of the chrome yellow pigments were found to be present in several paintings by van Gogh, [3] this could provide a possible explanation why only some of the areas painted with this class of pigments appear darkened today. In recent years, a growing interest in the conservation of these paintings has arisen.…”

“…By means of X-ray absorption nearedge structure (XANES) measurements at the micrometer scale, we previously demonstrated that the alteration of chromium yellow is associated with the reduction of the original Cr VI centers to Cr III . [1,6] Recently, Casadio and Rose characterized painting materials used by Picasso through XRF nanoprobe measurements; [12] the same authors also studied zinc-potassium chromate compounds in a painting by Seurat. [7] However, almost no other studies have been carried out to investigate at the nanoscale level the exact nature of the progression of the alteration mechanism of chrome yellows; nevertheless such information could yield more detailed insights into the sequence of chemical transformations that drive the degradation process, thereby providing additional and/or complementary information with respect to the knowledge already acquired at the microscale level by synchrotron radiation-based XANES methods.…”

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

“…Sulfur-rich orthorhombic forms are more susceptible to browning. It has been postulated that PbCrO 4 and PbCr 1−x S x O 4 are more soluble when in the orthorhombic phase, and thus more chromate ions are available for redox reactions [25]. Table 2 shows the possible chrome yellow variants based on PbCr 1−x S x O 4 .…”