Insights into the stratigraphy of gelatin-based emulsion in historical dry plate photographic negatives: A multi-analytical approach

• Many efforts focus on preserving photographic collections, but research on dry plate negatives remains scarce. • Multianalytical approach allowed investigating three types of dry plate negatives. • Studying the composition and stratigraphic structure is crucial for understanding emulsion degradation. • Light absorption hampers many optical techniques, but NLOM-MPEF can overcome this limitation. This research investigates historical gelatin dry plate negatives’ chemical composition and stratigraphic structure through a comprehensive multi-analytical approach. Our methodology integrated optical and laser-based techniques, with microscopies, including Optical and Scanning Electron Microscopy – Backscattered Electrons (SEM-BSE) and Non-linear Optical Microscopy (NLOM) in its Multi-photon Excitation Fluorescence (MPEF) modality to analyze stratigraphy. MPEF enabled us to overcome the limitations associated with light absorption encountered in other optical techniques. Chemical analysis using Laser-Induced Breakdown Spectroscopy (LIBS), Scanning Electron Microscopy – Energy Dispersive X-ray Spectroscopy (SEM-EDS), and micro-Raman spectroscopy allowed for the classification of negative types based on specific compositional materials, such as synthetic dyes. Our findings contribute to the knowledge of dry plate negative composition and stratigraphy, the aspects that have seldom been investigated. The information retrieved is crucial for understanding the realization and degradation mechanisms of these valuable artifacts, often prone to delamination and flaking, and can inform conservators to better treat historical dry plate photographic negatives.