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
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
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DOI：
10.1021/ac3021592
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发表时间：
2013-01-15
影响因子：
7.4
通讯作者：
Cotte, Marine
中科院分区：
文献类型：
--
作者：
Monico, Letizia;Janssens, Koen;Cotte, Marine
先前关于Chrome黄颜料变暗的研究表明，这种形式的改变可归因于原始Cr（VI）还原为Cr（III），并且含硫化合物（最常见的是硫酸盐）的存在在该过程中起着关键作用。我们最近证明，不同的晶体形式的Chrome黄颜料（PbCrO 4和PbCr 1-xSxO 4）是目前在绘画的文森特货车高。在目前的工作中，我们展示了铬酸铅颜料的化学组成和晶体结构如何影响其稳定性。为此，油模型样品与内部合成的粉末PbCrO 4和PbCr 1-xSxO 4进行人工老化和表征。我们只观察到用PbCr_（1-x）S_xO_4、富SO_（42-）（x >= 0.4）和正交相（> 30wt%）制成的涂料模型的深度变暗。Cr和S K-边缘微X-射线吸收近边缘结构的调查显示，在一个明确的方式形成高达约60%的Cr（III）-物种在外层的最改变的样品;相反，独立的油漆模型的化学成分，没有变化的S-氧化态观察。采用紫外-可见漫反射和傅里叶变换红外光谱分析未老化和老化的模型样品进行，以获得额外的信息的物理化学变化引起的老化处理。
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 (PbCrO4 and PbCr1-xSxO4) 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 PbCrO4 and PbCr1-xSxO4 were artificially aged and characterized. We observed a profound darkening only for those paint models made with PbCr1-xSxO4, rich in SO42- (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.