Understanding Photo-physical Properties of Inorganic Pigments Using Pump-probe Microscopy

Abstract

In cultural heritage science, art characterization and conservation encounter several complications. Artworks typically contain mixtures of multiple pigments along with impurities, additives, and binders, which leads to many undesired alterations and causes difficulties in identifying individual pigments. Conventional analytical techniques typically suffer from the trade-off between chemical specificity, spatial resolution, and invasive sampling. This trade-off limits the characterization, identification, and the current understanding of degradation of historical pigments in works of art. To address these limitations, this dissertation introduces pump-probe microscopy as a noninvasive approach for imaging various historical pigments by accessing their intrinsic photophysical dynamics with high chemical specificity and spatial resolution.In this dissertation, I present the application of pump-probe microscopy in creating three-dimensional virtual cross-sections to visualize the artificial degradation of cadmium sulfide (CdS, also known as cadmium yellow) oil paints. Pump-probe microscopy can nondestructively detect the onset of early-stage CdS degradation of small particles and on the surface of large particles. This artificial degradation only occurs on CdS reproduced by following historical recipes but is not seen on commercial CdS. These findings suggest that there is an influence from the particle dimension and the pigment quality on the evolution and extent of CdS degradation.Furthermore, the pump-probe technique can provide stoichiometry information on cadmium red (cadmium sulfoselenide) pigments by analyzing a series of pump-probe dynamics at multiple wavelength combinations. The mathematical fitting of pump-probe dynamics allows us to correlate the dynamic lifetimes with the chemical compositions of the corresponding pigments, indicating the potential utility of pump-probe microscopy to reveal the stoichiometry of unknown cadmium pigments.In addition, I extend the application of pump-probe analysis to copper-based green pigments, including malachite, verdigris, and copper resinate, which exhibit similarities in color and chemical composition. The chemically specific contrast provided by pump-probe microscopy highlights its potential for nondestructive identification of these green pigments in artworks. Moreover, this technique has proved useful for tracking the degradation of ultramarine blue, with the signal amplitude showing a strong dependence on the quantity of chromophores present in the pigment.With all the above applications, I have demonstrated that pump-probe microscopy is a powerful tool to study artists’ pigments. Future work should focus on applying the insights from pigment analysis to investigate cultural heritage objects.