Digital holographic speckle pattern interferometry in support of ongoing conservation: the case of Giotto’s wall paintings in the Bardi Chapel, Florence

• Digital Holographic Speckle Pattern Interferometry (DHSPI) supports the conservation of the Bardi Chapel by providing non-invasive, non-contact and real-time in-situ insights into the subsurface condition of Giotto’s wall paintings. • DHSPI reveals hidden subsurface features that could evolve in the future, which are not yet detectable by conventional methods, aiding in monitoring and risk assessment strategies. • DHSPI provides new historical insights by uncovering hidden Giotto’s original fresco mortar layers and thus supporting conservators’ decisions on the cleaning and removal of previous plaster restorations. Effective conservation begins with a deep understanding of an artwork’s structural condition before restoring the pictorial layer. This is especially important for multi-layered painted surfaces like wall paintings, where hidden issues must be carefully evaluated to ensure long-term preservation. An ongoing conservation campaign (2022–2025) is dedicated to preserving Giotto’s irreplaceable wall paintings in the Bardi Chapel, Basilica di Santa Croce, Florence (Italy). The project faces significant challenges due to gaps in detailed documentation regarding past restorations. Conservators and heritage scientists were called to assess the subsurface condition of the wall paintings and accurately distinguish areas requiring interventions of varying urgency. A comprehensive diagnostic examination was conducted, incorporating advanced methodologies, including Digital Holographic Speckle Pattern Interferometry (DHSPI). This paper focuses on advantages and challenges of the DHSPI, operated under both thermal excitation and natural conditions, for detecting critical subsurface defects and supporting the study of the restoration history of Giotto’s wall paintings. DHSPI enables revealing hidden micro-deformations and monitoring direct surface responses to environmental changes in a quantitative full-field real-time non-contact examination. Thus, it provides valuable insights into structural stability, invisible overlapping of detachments and cracks, hidden structured layers, and through defect propagation to trace the future deterioration risks. By incorporating DHSPI into an interdisciplinary diagnostic strategy, we demonstrate how this approach enhances conservation decision-making, supports informed and minimally invasive interventions, and, ultimately, contributes to the long-term preservation of Giotto’s unique wall paintings.