Multimodal and multiscale non-invasive study of Leonardo’s mural painting in Sala delle Asse (Milan): A tool supporting cleaning evaluation

Sala delle Asse in the Milan Sforza Castle was decorated by Leonardo da Vinci and his workshop at the end of the XV century. Due to historical events involving the Duchy of Milan, the decoration of the room was interrupted in 1499 and some areas remained devoid of polychrome paintings. For this reason, the painted surfaces were whitewashed and forgotten for a long-time, but during the 19th and 20th centuries, they were rediscovered and restored twice, uncovering the remnants of original layers. In recent years, the hall has been the focus of a comprehensive project aimed at thoroughly assessing the conservation issues of its walls and optimizing the necessary intervention methods. One of the primary objectives of this project is to restore the legibility of the original painting remnants. For this purpose, cleaning tests have been conducted with different methodologies on specific sections of the walls. This study employs a multimodal and multiscale approach conducted in situ on both cleaned and uncleaned polychrome areas of the walls to provide an overview of the pigments used by Leonardo da Vinci and his workshop. Additionally, it evaluates the effectiveness of cleaning procedures in removing 19th and 20th century overpainting, thereby revealing the underlying painted layers. The non-invasive approach includes portable micro-X-Ray Fluorescence (point-based measurements and micro-mapping), portable Raman spectroscopy (point measurements and micro-spatially offset Raman spectroscopy), and mobile remote standoff spectral imaging, carried out at a stand-off distance of ∼ 10 m. The integration of imaging and point-based data collected at remote and close distance, with centimetre scale field of view and sub-millimetres spatial resolution (remote standoff imaging) and millimetres and micrometres spot size (point measurements), provided valuable insights on elemental and molecular composition of the materials present both in the uncleaned and cleaned areas of the mural painting, permitting a partial reconstruction of the stratigraphy and thus shedding light on the 15th century paint residues; moreover, the study expands the up-to-date protocols for the non-invasive identification of pigments and for an integrated evaluation of cleaning effectiveness.