Dramatic condition changes were noted in a group of daguerreotypes during a 10 month-long exhibition featuring masterworks of the prominent Boston-based studio of Southworth & Hawes. The changes took the form of a hazy, extended white surface layer and of white spots that partially obscured the images after between four weeks and 10 months of exhibition. Raman spectroscopy, complemented by in situ scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDS), was used to nondestructively characterize this form of image deterioration in eight representative daguerreotypes from the studio. Raman proved to be a sensitive technique to noninvasively identify the compounds on the plates due to the fact that their Ag surfaces behave as surface-enhanced Raman scattering (SERS) substrates. A band at ca 242 cm −1 assigned to the Ag-Cl stretching mode was observed in the areas where a white surface layer or white spots are present. In some of the areas probed, substituted aromatic compounds were also detected. The presence of Ag-Cl bonds on daguerreotype surfaces has profound implications for their exhibition and preservation due to the photosensitivity of silver chloride in the ultraviolet-visible range, which can generate metallic silver that would redeposit on the surfaces. The possible sources of the compounds detected on the surfaces of the plates are discussed.
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