Non invasive micro-Raman spectroscopy for investigation of historical silver salt gelatin photographs

At the end of the 19th century, photography rapidly spread and technical developments allowed for a wide range of artistic expressions. The application of micro-Raman spectroscopy on historical photographs permitted to identify the silver salt gelatin process and to characterize a typical surface alteration, known as “silver mirroring” or “silver tarnish”. The Raman spectra show diagnostic peaks of silver halide as an evidence of the use of the silver salt gelatin technique, whereas on the areas that are affected by silver mirroring the presence of silver sulfide could be attested by comparison with standards. This application resulted especially useful for those photographs with no evidence of silver tarnish although a silver salt technique had been used. Moreover, this study proves the suitability of micro-Raman spectroscopy on fragile materials such as photographs, without causing any damage.