Unravelling hidden text and figures in paper-based archival documents with micro-spatially offset Raman spectroscopy imaging

The preservation of paper-based archival documents is crucial for safeguarding historical and cultural heritage. Some records possess visually inaccessible text or images because of previous conservation measures, their method of construction, or historic damage. Micro-spatially Offset Raman Spectroscopy (micro-SORS) has emerged as a promising method for probing below or through opaque material substrates non-invasively. This study explores the potential of micro-SORS to image hidden text and figures in paper-based archival documents, utilizing Raman signals, fluorescence emissions, and overall spectral intensity reflecting also sample absorption. We present case studies involving sealed letters and playing cards from historical collections, demonstrating the efficacy of micro-SORS in identifying pigments and deciphering hidden ink writings. Results show the successful mapping of vermilion pigment in playing cards and reconstruction of hidden iron gall ink text in sealed letters. Chemometric analysis further enhances the visualization of hidden text. Despite challenges such as the absence of Raman signal of the target materials, micro-SORS proves to be a valuable tool for accessing hidden information in paper-based artifacts, aiding in preservation efforts and historical research.