Deep inside the color: How optical microscopy contributes to the elemental characterization of a painting

In the context of paintings characterization, X-Ray Fluorescence (XRF) combined with Fibre Optics Reflectance Spectroscopy (FORS) is a well-known methodology. The latter allows to classify the pigments constituting the external layers (few microns) of the painting while the former provides the fingerprint of the main chemical elements present in almost all layers (tens of microns) of the sample. The synergetic application of XRF and FORS is a key turn for non-invasive scientific analysis of works of art since it exhaustively describes the pigments employed by the masters and supplies a first insight into their distribution on the panels. In this scenario, Optical Microscopy (OM) applied directly on the painting surface, without any type of sampling, is traditionally deemed as a descriptive tool for showing microscopic features such as craquelure, brushstrokes or painting material distribution. It can be eventually expendable as a qualitative help during the discussion of spectroscopic data, but not considered fundamental. Nonetheless, the application of image processing protocols can make OM a concrete support for the characterization of paintings. In this work a new image analysis strategy has been introduced, its potential has been demonstrated studying different flesh tones from the panel \u201cThe Holy Family with St. Anne and the young St. John\u201d by Bernardino Luini (Pinacoteca Ambrosiana Collection of Milan) and, finally, it has been discussed how OM could quantitatively complete XRF and FORS analysis. The results show that optical microscopy, other than provide a glance of the investigated surface spots, effectively integrates the description given by the spectroscopic techniques and, properly managed, can be a useful resource for solving ambiguous cases