Influence of image density and interfacial surfaces on ER-FTIR spectra of 19th century photographic print processes

External reflectance (ER)-FTIR spectroscopy is a non-contact analytical technique particularly suitable for the study of cultural heritage materials having delicate surfaces, such as photographs. Challenges to implementing ER-FTIR spectroscopy in the field of photograph conservation are difficulty in interpreting spectra and a lack of published reference spectra. To investigate the applicability of the technique to characterize diverse types of photographs, archetypes of major photographic print processes common in the 19th and early 20th century were analyzed. The effects of image density, surface gloss, and layer structure were evaluated. Spectra acquired from plain salted paper and albumen prints do not change substantially with image density; cyanotypes show the presence of infrared-active pigment image material, Prussian blue. Beyond identifying materials, such as cellulose and protein, present at the surface of photographs, ER-FTIR spectroscopy can be sensitive to components deeper within the print's structure. For glossy prints on baryta paper, such as gelatin and collodion printed-out prints, interference is generated between the specular surface reflection and the reflection from the binder-baryta interface. Interference patterns in the FTIR spectra are modulated by the presence of silver image material, as shown in spectra from developed-out silver gelatin prints in which information from the baryta layer is completely blocked in the darkest areas of the image. This preliminary study shows that it is possible to classify the photographic process used to make a print and obtain information related to layer structure from multilayered photographs using ER-FTIR spectroscopy. Further work to develop understanding of the multiple effects of complex systems will reveal greater insight into photographic structures and assist in the care and appreciation of prints. & COPY; 2023 Published by Elsevier Masson SAS on behalf of Consiglio Nazionale delle Ricerche (CNR).