Neutron imaging investigation of fossil woods: non-destructive characterization of microstructure and detection of in situ changes as occurring in museum cabinets

This paper discusses the applicability of neutron imaging techniques for probing the internal microstructure of several fossil woods upon wetting and drying, two phenomena occurring in museum cabinets and endangering the fossil woods. Investigations were carried out using lignites (fossil woods) from two French localities (Rivecourt, Parisian Basin, Oise – Paleogene; Angeac, Aquitanian Basin, Charente – Cretaceous), which present different macroscopic behavior upon drying. Thanks to the high sensitivity of neutrons to hydrogen content, it was possible to track water diffusion through 3 mm thick samples and to follow in situ changes related to either supply or withdrawal of water without any special preparation and in a relevant time range (from 1 min to a few hours). Classical image analysis allows discriminating between the behavior of the two fossil woods with regard to their interaction with water. Further analysis based on a Fourier transform of projection images provides additional information regarding the existence of large pores in one of the samples. Differences in pore network and internal structures have important mechanical consequences as one of the samples retains its integrity upon drying, whereas the other one shatters into pieces. A better understanding of the underlying processes will clearly require multi-scale analyses, using additional techniques that could probe the materials at a lower scale. Such a combination of multi-scale analyses should provide valuable information for a better conservation of wood remnants, which is crucial for both paleobotanical research and museum exhibits.