Causes of Silver Mirroring and Yellowing on a Nineteenth-century Photographic Gelatin Glass Plate Negative

Glass plate negatives were used to produce photographs during the Qajar era (from 1789 to 1925). It is the oldest photographic method to become popular in Iran. Considering the historical and artistic value and vulnerability of these kinds of negatives, their preventive conservation is vital. Silver mirroring and yellowing are the most common and serious forms of damage on historical silver gelatin photographic glass plate negatives. These damages typically appear under varying conditions and in conjunction with different materials. This study aimed to aid the identification of possible sources and causes of the yellowing and mirroring damage on glass plate negatives through investigating in depth the damage on a sample historic glass plate negative of the Qajar era. The emulsion and a foam used in packaging of the object were thoroughly analyzed. Results are presented based on FTIR and FE-SEM-EDX analysis as well as precise visual assessments of the object. It was confirmed that the emulsion used was gelatin. At the same time, the foam used as a cushion next to the object was identified as polyurethane. Finally, the main cause of both silver mirroring and yellowing damage in the investigated object was found to be the polyurethane foam used in the packaging, which should not be used in such cases in the future under any circumstance.