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.
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.
Centre SMAArt and Dipartimento di Chimica, Università degli Studi di Perugia, via Elce di Sotto 8, I-06123 Perugia, Italy.
Letizia
L
Monico
Koen
K
Janssens
Costanza
C
Miliani
Geert
G
Van der Snickt
Brunetto Giovanni
BG
Brunetti
Mariangela
M
Cestelli Guidi
Marie
M
Radepont
Marine
M
Cotte
D002840:Chromates / Q000737:chemistry*; D007854:Lead / Q000737:chemistry*; D008958:Models, Molecular; D010151:Paintings; D010316:Particle Size; D017550:Spectroscopy, Fourier Transform Infrared; D013059:Spectrum Analysis, Raman; D013499:Surface Properties; D013997:Time Factors; D014961:X-Ray Diffraction
D002840:Chromates; D007854:Lead; C031895:lead chromate; C032722:lead sulfate