Imagine uncovering the secrets hidden beneath layers of paint on a centuries-old fresco or reading the contents of a sealed 18th-century letter without ever opening it. Thanks to physics and innovative applications of spectroscopy, what was once invisible is now within our reach. Here, Dr Sara Mosca shares the fascinating work of a collaborative team from STFC’s Central Laser Facility (CLF) in Oxfordshire and ISPC-CNR in Milan, Italy. Together they have bridged the gap between macro- and micro-Spatially Offset Raman Spectroscopy (SORS), revolutionising its applications in cultural heritage.
Micro-SORS was originally developed by the team to enable non-invasive analysis of cultural heritage artefacts. This cutting-edge technique allows researchers to uncover the chemical composition of hidden layers of paint in frescoes, paintings and statues without causing damage to the objects. This information is crucial for effective and safe restoration efforts and it provides valuable information about the techniques used by historical artist. Micro-SORS builds upon macro-SORS, a technique developed earlier at CLF for investigating larger, diffusely scattering objects, such as opaque plastic bottles, pharmaceutical products or human tissue. Notably, this technology has been deployed at airports for scanning liquid samples through unopened containers.
Until recently, the transition between the macro- and micro-SORS regimes was not well understood. This posed a significant challenge when applying the technique to mesoscale samples such as Limewash layers and diffusion of salts inside plasters and mortars (~1 mm thick). The team has recently bridged this gap, clarifying when micro- and macro-SORS methods are most effective, providing straightforward guidelines for selecting the optimal sampling approach.
In an exciting extension of this work, the team has partnered with The National Archives (TNA) in London, a Tranche 1 Facilities RICHeS funded project, applying micro-SORS to analyse 18th century historical documents and letters including sealed letters seized on ships. This study was conducted at TNA using open-geometry Horiba Raman microscopes, funded by the Arts and Humanities Research Council Capability for Collections Fund (CapCo), which were adapted for micro-SORS sampling. By leveraging the optical properties of the sample such as absorption, fluorescence and photon diffusion, the team has successfully recovered invisible texts through layers of paper in a non-invasive manner. This remarkable achievement underscores the potential of micro-SORS to reveal hidden information from historical artefacts while preserving their integrity.
Visible and micro-SORS images of a measured section of a sealed letter from TNA, with a schematic of the micro-SORS configuration used to retrieve the sub-surface imaging
Image copyright: The National Archives, https://doi.org/10.1016/j.saa.2024.125591