Multi-analytical assessment of dry plate negative cleaning using non-invasive optical methods

Cleaning has long been recognised as one of the most delicate operations in conservation, given its irreversible and potentially invasive nature. Despite its relevance to the conservation of photographic documents, the cleaning of mould-contaminated gelatine glass-plate negatives remains poorly documented. This study aims to assess the outcomes of cleaning treatments applied to mould-contaminated dry plate negatives using non-invasive and non-contact optical techniques. Both dry cleaning alone and combined with a hydroalcoholic solution were tested on fungal contamination. To assess cleaning efficacy, a novel acquisition setup for Reflectance Imaging Spectroscopy (RIS) was developed, enabling multispectral measurements on glass-plate negatives. These data were employed to calculate colour variation across the entire object surface. This approach combined with Optical Coherence Tomography (OCT), optical microscopy-based profilometry, FTIR chemical imaging in reflectance mode, and stereomicroscopy, enabled systematic monitoring of the cleaning process. The results indicate only partial removal of mould from the emulsion layer, yielding important insights for conservators and highlighting the importance of preventive strategies against biological contamination. In addition, the conservation status of the negatives was investigated prior to cleaning using ATR-FTIR spectroscopy and OCT, providing additional information on the stratigraphy and degradation mechanisms of dry plate negatives and contributing to the advancement of knowledge of these still understudied cultural heritage objects.