Non-invasive quantitative investigation of varnish stratigraphy in historical artifacts using line-field confocal OCT

Abstract Optical Coherence Tomography (OCT) is a powerful non-destructive and non-invasive 3D imaging technique for cultural heritage artifacts. It provides morphological information, such as in-depth layer mapping and particle presence. Line-field Confocal OCT (LC-OCT) combines OCT with confocal microscopy to achieve improved spatial resolution (~1 µm) and fast imaging, while maintaining a similar penetration depth to standard OCT. LC-OCT combined with automated data processing is used to map varnish layers and characterize their removal during conservation treatments. It is applied to a 17th century painting, to document previous restorations, and to a 17th century violin by the renowned Italian violin-maker Nicolo Amati, to assess the presence of its unique original varnish and optimize the conservation process for the selective removal of the overlying non-original varnish. This demonstrates the effectiveness of LC-OCT as a new technique for the quantitative characterization and conservation guidance of varnished cultural heritage artifacts.