Spectroscopic characterization of selected French paper negatives (1843-1856): how to see through many processes?

Photographic negative images on paper are certainly the earliest types of photographs ever produced, giving access to multiple photographic prints using a unique matrix. As early as 1841 in France, many variants of the paper negatives processes have been developed by skilled chemists and photographers covering a wide spectrum of practical and aesthetic concerns. Nowadays, due to their inherent historical and esthetical values, those negatives are getting an increasing interest from the art and museum community. However, their materiality has been poorly studied despite a large variety of processes involving many different chemicals and organic coatings; limiting our possibilities of identification, attribution or even preservation. The aim of this work was to develop a methodology to better assess the way those images were produced based on physical and chemical characteristics. A non-invasive approach combining optical, vibrational and X-ray spectroscopies was implemented on a collection of 138 historical images between 1841 and 1856 from French collections. A survey was carried out on these negatives to provide a series of relevant physical data (dimensions, thickness, weight, etc). Non-invasive reflectance FTIR showed great potentiality in revealing the presence of organic sizing (gelatin) or impregnation (wax). Multivariate analysis was applied on XRF data to help clustering negatives that have similar elemental composition and highlight relationships between makers. Finally, the morphological, physical and chemical results were all combined to answer historical questioning on the paper negative process.