High-lateral resolution image recovery and degradation mapping of daguerreotypes using mobile reflectance, μ-XRF, MA-XRD, and benchtop μ-vibrational spectroscopies

The study of daguerreotypes, one of the earliest photographic process, requires analytical methods that address both their metallic composition and the high spatial resolution of their images. This work presents results obtained with research-grade laboratory instruments for image recovery and chemical characterization of degraded daguerreotype plates. Our methodology combines two transportable instruments with high lateral resolution for the image recovery: reflectance imaging spectroscopy (RIS) in the visible and near-infrared (Vis-NIR) ranges (400-2500 nm, lateral resolution 125 μm), which was used to study the optical properties of the plate, and micro-X-ray fluorescence (μ-XRF) mapping (lateral resolution ≤40 μm), which provided chemical information at spatial resolution comparable to synchrotron-based μ-XRF (10-50 μm) while offering the practical advantage of laboratory and field analysis, free from the limitations of large-scale facilities. Additionally, macro-X-ray diffraction (MA-XRD) and micro-vibrational spectroscopy (μ-Raman and μ-FTIR in reflection mode) mapping were applied to investigate, at a mesoscale, specific regions of interest (ROIs) to deepen the chemical characterization of the copper corrosion products, such as formates and oxides, present on the surface. Spot-wise micro-confocal-XRF (C-XRF) was used to study the stratigraphic distribution of elements in the plate to understand the corrosion mechanism. Our findings demonstrate the capabilities of image recovery with RIS, particularly in the NIR spectral range, and μ-XRF in tracing the Hg-L spatial distribution. This multi-scale methodology, combining macro-, meso-, and spot-wise scales, is particularly useful for documenting the image condition across the entire plate and for mapping the distribution of degradation products in regions of interest (ROIs). Such information is crucial for guiding conservation strategies in daguerreotype collections.