Time-Resolved Photoluminescence Microscopy Combined with X-ray Analyses and Raman Spectroscopy Sheds Light on the Imperfect Synthesis of Historical Cadmium Pigments.

Recent studies have shown that modern pigments produced after the Second Industrial Revolution are complex systems characterized by a high level of heterogeneities. Therefore, it is fundamental to adopt a multianalytical approach and highly sensitive methods to characterize the impurities present within pigments. In this work we propose time-resolved and spectrally resolved photoluminescence (PL) microscopy for the mapping of luminescent crystal defects and impurities in historical cadmium-based pigments. PL analysis is complemented by X-ray diffraction, X-ray fluorescence and Raman spectroscopies, and by scanning electron microscopy to determine the chemical composition and crystal structure of samples. The study highlights the heterogeneous and complex nature of historical samples that can be associated with the imperfect manufacturing processes tested during the period between the 1850s and 1950s. The results also allow us to speculate on a range of synthesis processes. Since it is recognized that the stability of paints can be related to pigments synthesis, this research paves the way to a wider study on the relationship between synthesis methods and deterioration of cadmium pigments and paints. This rapid and immediate approach using PL can be applied to other semiconductor pigments and real case studies.