Clinical computed tomography and surface-enhanced Raman scattering characterisation of ancient pigments

A systematic and complete chemical and physical characterisation of painted pigments on wood samples was carried out using multislice X-ray computed tomography (MSCT) and surface-enhanced Raman scattering (SERS) techniques. Inorganic and organic pigments present on the wooden tablets were differentiated by MSCT determinations of Hounsfield units, a semi-quantitative method for measuring X-ray attenuation that, in turn, offers an indirect estimation of a material's density. However, the MSCT technique is not as reliable as traditional spectroscopic techniques for recognising and classifying organic pigments. Nevertheless, a strength of the MSCT approach was its ability to simultaneously provide a volumetric view of the wood and segment the layers of the specimen using suitable reconstruction methods such as is generally done for biomedical applications. Furthermore, the SERS technique made it possible to identify the type of material present in the pigments (for both inorganic or organic materials) with a high spatial resolution, even pigments in mixtures or those applied directly on the investigated wooden support. The combined MSCT and SERS data obtained through this systematic investigation constitutes the basis for the assembly of larger reference databases that will ultimately support the development of long-term conservation protocols.