Unveiling street art: A multimodal and multitechnique approach for analyzing and mapping painting materials on large murals

Street art murals are increasingly recognized as valuable contemporary artworks, often attaining significant artistic, historical, and social importance. As these murals become integral parts of cultural heritage, finding efficient strategies for their conservation is crucial. However, their typical large surface areas, heterogeneous materials, and high variability in exposure to environmental and pollution factors pose significant challenges in establishing appropriate analytical strategies to obtain the necessary information. This study proposes a multiscale and multitechnique noninvasive approach to investigate and monitor street art murals in situ. By combining portable point techniques-such as external reflection Fourier transform infrared spectroscopy, Raman spectroscopy, visible, near infrared, and short-wave infrared reflectance spectroscopy, and X-ray fluorescence spectroscopy-with visible and near infrared hyperspectral imaging, the mural's composition across a square meter surface could be analyzed. Additionally, multispectral imaging mounted on a drone provided a global reconstruction and characterization of the overall mural. This method was complemented by microdestructive laboratory analyses of selected samples, using pyrolysis gas chromatography-mass spectrometry and liquid chromatography coupled with diode array detector and tandem mass spectrometry, to further investigate selected samples and support noninvasive results. The approach was applied to the iconic mural "Musica Popolare" (2017) by Orticanoodles in Milan, Italy, revealing detailed information about its pigments, binders, fillers, and degradation. The findings demonstrate the potential of this integrated methodology for the effective material identification, conservation assessment, and short-and long-term monitoring of urban heritage.