Towards a methodological approach to identify the main components used in historic photographs

In this work, a methodology is proposed to identify the components used in historic photographs using non-invasive and non-destructive characterisation techniques. The methodology provides an approach for analysing photographs by following three steps represented in a flowchart: firstly, the type of binder is determined by identifying morphological and structural characteristics; secondly, the presence or absence of a baryta layer is established; and finally, the presence of organic components used as part of a protective layer is determined. The characteristics and properties revealed using this methodology identify the type of historic photograph through different analytical techniques that provide science-based evidence for any qualitative observations first made. These techniques are interchangeable as they identify specific characteristics in the photographs by using infrared spectroscopy, Raman spectroscopy, infrared spectroscopic ellipsometry, confocal microscopy and atomic force microscopy, among others. As such, the methodology proposed facilitates a secure evidence-based classification of photographs and provides valuable information for their conservation.