Mineralogical interpretation of multispectral images: The case study of the pigments in the frigidarium of the Sarno Baths, Pompeii

Wall paintings are invaluable archives of pigments, techniques, and artistic expressions of past civilizations, and Pompeii is an excellent example of such archives for Roman art from mid-first century AD. Assessing wall paintings’ state of preservation requires characterizing the pigments on a large scale and documenting alteration processes which will influence conservation interventions. The problem is that large scale characterization requires delicate high resolution chemical and spectral instrumentation that is limited to the museum environment. Here, we present a new methodology for non-invasive characterization of wall paintings on-site, based on analyzing stacked photos as multi-spectral data. The photos were imaged using a portable modified digital camera that registers the ultraviolet, visible, and infrared spectral regions through external band-pass filters. Combined with portable X-ray fluorescence and fiber optic reflectance spectrometers, predetermined mineral-specific band ratios were developed on fragments found below the wall paintings, and large scale mineralogical maps were constructed. The mineralogical maps show the distribution of hematite, goethite and Egyptian blue in a wall painting of the frigidarium of the Sarno Baths complex of Pompeii, documenting iconographic figurines of plants, pygmy people, and animals from a degraded depiction of a river scene. The applications are in conservation and archaeological sciences, showing the ancient technology of the wall paintings using non-invasive measurements, and developing restoration strategies that are matching the ancient materials.