Reconstruction of lost architectural decorations by integrating archival photographs with 3D survey of the status quo: an investigation into Gaetano Vaccani’s grisaille technique

Abstract. The research issue underlying this article stems from the will to recover memory and visually restore, through the potential offered by virtual reconstruction techniques, the lost image of the rich decoration that adorned the ancient Salone Napoleonico, the lecture hall of the Brera Academy of Fine Arts, destroyed following the devastation of the Second World War. The perimeter walls withstood the impact of the bombings, while the barrel vault was completely compromised: during the post-war reconstruction, the rich pictorial decoration was deemed irrecoverable, and efforts were concentrated on faithfully restoring the hall in terms of volume. This premise allowed us to adopt the photogrammetric model of the existing structure as a solid base to ensure metric accuracy and correspondence to the original architectural layout. The 3D model was then mapped and assembled with decorative registers extracted from historical black-and-white photographs, subjected to a preliminary orthorectification process, adopting criteria aimed at recognizing hierarchies between portions with varying degrees of detail and reliability. To approximate the original visual and chromatic characteristics, a comparative analysis of contemporary monochrome decorations by the ornamental painter Gaetano Vaccani, preserved in the Palazzo di Brera, led to the formulation of color hypotheses through RGB sampling, achieving an informed chromatic and stylistic interpretation. The multidisciplinary framework, based on historical, archival, and bibliographic sources, integrated into the workflow, ensured the achievement of results that balanced methodological rigor with cultural and historical sensitivity, suitable for online publication and access to the scientific community and the wider public.