SMAArt Centre and Department of Chemistry, Biology and Biotechnology, University of Perugia, Via Elce di Sotto 8, 06123, Perugia, Italy.
CNR-Institute of Molecular Science and Technologies (ISTM), Via Elce di Sotto 8, 06123, Perugia, Italy.
Chemistry. 2018 Aug 9;24(45):11584-11593. doi: 10.1002/chem.201801503. Epub 2018 Jul 25.
Cadmium yellows (CdYs) refer to a family of cadmium sulfide pigments, which have been widely used by artists since the late 19th century. Despite being considered stable, they are suffering from discoloration in iconic paintings, such as Joy of Life by Matisse, Flowers in a blue vase by Van Gogh, and The Scream by Munch, most likely due to the formation of CdSO ⋅n H O. The driving factors of the CdYs degradation and how these affect the overall process are still unknown. Here, we study a series of oil mock-up paints made of CdYs of different stoichiometry (CdS/Cd Zn S) and crystalline structure (hexagonal/cubic) before and after aging at variable relative humidity under exposure to light and in darkness. Synchrotron radiation-based X-ray methods combined with UV-Vis and FTIR spectroscopy show that: 1) Cd Zn S is more susceptible to photooxidation than CdS; both compounds can act as photocatalysts for the oil oxidation. 2) The photooxidation of CdS/Cd Zn S to CdSO ⋅n H O is triggered by moisture. 3) The nature of alteration products depends on the aging conditions and the Cd/Zn stoichiometry. Based on our findings, we propose a scheme for the mechanism of the photocorrosion process and the photocatalytic activity of CdY pigments in the oil binder. Overall, our results form a reliable basis for understanding the degradation of CdS-based paints in artworks and contribute towards developing better ways of preserving them for future generations.
镉黄（CdYs）是指一类硫化镉颜料，自 19 世纪末以来一直被艺术家广泛使用。尽管它们被认为是稳定的，但在标志性画作中，如马蒂斯的《欢乐》、梵高的《花瓶里的花》和蒙克的《呐喊》，它们正在遭受变色，这很可能是由于 CdSO ⋅n H O 的形成。CdYs 降解的驱动因素以及这些因素如何影响整个过程仍不清楚。在这里，我们研究了一系列由不同化学计量比（CdS/Cd Zn S）和晶体结构（六方/立方）的 CdYs 制成的油模拟漆，这些油模拟漆在可变相对湿度下暴露于光和黑暗中进行老化前后。基于同步辐射的 X 射线方法结合紫外可见和傅里叶变换红外光谱表明：1）Cd Zn S 比 CdS 更容易光氧化；这两种化合物都可以作为油氧化的光催化剂。2）CdS/Cd Zn S 光氧化为 CdSO ⋅n H O 是由水分引发的。3）变质产物的性质取决于老化条件和 Cd/Zn 化学计量比。基于我们的发现，我们提出了一个关于 CdY 颜料在油基中的光腐蚀过程和光催化活性的机制方案。总的来说，我们的研究结果为理解艺术品中基于 CdS 的涂料的降解提供了可靠的基础，并为开发更好的保护方法以造福后代做出了贡献。
