Application of Micro-spatially Offset Raman Spectroscopy to Street Art Paintings

Recently, the micro-Spatially Offset Raman Spectroscopy (micro-SORS) technique has been developed to extend the applicability of Raman spectroscopy to probing thin, highly diffuse, scattering layers such as stratified paint samples. The method is a conceptual evolution of its parent technique, (macro-scale) SORS, combining SORS with microscopy and it can be applied in situations where the high turbidity of layers prevents the deployment of conventional confocal Raman microscopy with its depth-resolving capability. The method was applied successfully to detect non-invasively the presence of thin, highly turbid layers within polymers, wheat seeds, paper and cultural heritage materials. In this work two developed variants, defocusing and full micro-SORS, are described and applied to discriminate between the chemical composition of micrometric layers below the surface of street art paintings. A micro-invasive, cross-sectional analysis confirmed and provided a deeper understanding of the micro-SORS findings. Micro-SORS represents a new Raman imaging modality, expanding the portfolio of non-invasive, chemically specific analytical tools in archaeology and art history.