Nanoscale morphological and spectroscopic mapping of zinc carboxylate formation in oil and tempera paintings

Background: This study explores the application of infrared scattering-type Scanning Near-field Optical Microscopy (IR s-SNOM) to analyze zinc white paint models at the nanoscale, focusing on the formation and crystallization of zinc carboxylates. Zinc carboxylates are critical degradation products in oil and tempera paintings, causing brittleness, delamination, and color changes. Our research applies IR s-SNOM to paint samples prepared with oil and egg binders, subjected to both natural and artificial aging. Results: The impact of environmental conditions and binding mediums on inter-sample variability was highlighted using a chemometric approach. Notably, zinc carboxylates form more rapidly in oil binders under artificial aging, while natural ageing results in more ordered crystalline structures in egg binders. While the analysis of individual IR s-SNOM spectra did not reveal an unambiguous correlation between morphology and chemistry at the nanoscale, it allowed to disentangle the causes of intra-sample variability. The surface sensitivity guaranteed by IR s-SNOM was the key element for disclosing spectral features hidden by micro-approaches, such as the identification of the broad band in the 1700-1500 cm- 1 spectral region in the egg medium, which had never been reported before. Significance: This study highlights the critical role that nanoscale analysis may play in advancing the understanding of degradation mechanisms, offering valuable insights for art conservation. By transitioning from average, bulk analysis to nanoscale investigations, IR s-SNOM proved to be a powerful tool for developing targeted conservation strategies, ultimately enhancing the preservation of cultural heritage.