Seeing through the surface –micro-Spatially Offset Raman Spectroscopy Imaging on paper-based archival documents

Raman spectroscopy is a well-established method for the characterization of Cultural Heritage (CH) materials such as inorganic pigments, modern synthetic dyes, and gems [1-3]. However, the technique is often limited to materials that are found on the uppermost surface. For hidden information, other techniques are often considered. One of the most used techniques may be MA-XRF which, with its ability to probe through the upper layer, allows to visualize the distribution and hint at the nature of the pigment(s) through their elemental composition. However, XRF does not allow for a complete pigment identification, which can be overcome using molecular spectroscopy such as Raman. Consequently, both techniques are often used in combination, allowing for a complete understanding of the materials and their distribution. Micro-Spatially Offset Raman Spectroscopy (micro-SORS), which has been applied in the field of CH for almost a decade [4], allows access to the molecular characterization of pigments through opaque layers. Micro-SORS has been applied to several types of materials in the field of Cultural Heritage such as painting, polychrome sculptures, painted plasters, street art mural paintings and decorated porcelain cards [5] but has never been applied to archives-type materials such as closed letters, and or glued pages. Associated with mapping possibilities, the technique allows for both the materials characterization and its spatial distribution to be done at the same time, with the same instrument, rather than relying on two different techniques [6]. With this study, we used micro-SORS for reconstructing hidden images or letters in several case studies representative of what can be found in archives and libraries. Case studies include a 1847 patented playing cards for which the unknown figure is glued to a page, unopened letters and naval notebooks from the 1600s containing blackboard-like pages with friable chalk writings covered by protective paper pages. Here, we will present the results obtained on these objects as well as the limitations that were encountered.