Insight into the manufacturing and conservation state of skin-based collections by nonlinear optical microscopy

Session

Analysis and Imaging in Heritage Science

Authors

Dr Laurianne Robinet (1), Dr Margaux Schmeltz (2), Mrs Sylvie Heu-Thao (1), Mrs Giulia Galante (2, 1), Dr Marie-Claire Schanne-Klein (2), Dr Gaël Latour (2)

Affiliations

1. Centre de Recherche sur la Conservation
2. Laboratoire d’Optique et Biosciences

Keywords

Non linear optical microscopy, cultural heritage, parchment, degradation, collagen,

Abstract text

Nonlinear optical microscopy (NLO) is a promising technique for the investigation of artworks as it allows non-invasive three-dimensional imaging with micrometer-scale resolution. This technique can combine two-photon excited fluorescence (2PEF) from exogenous and endogenous fluorophores and second harmonic generation (SHG), which enables the visualization of unstained fibrillar collagen. In biomedical researches, SHG microscopy has emerged as a powerful technique to image and quantify collagen 3D organization in unstained tissues, in particular in skin dermis and cornea. 

Recently, the technique was successfully applied to investigate the structure and the degradation of skin-based materials in cultural heritage collections, which main constituent is collagen. The technique offers the possibility to investigate non-invasively the impact of the manufacturing process on the collagen structural organization. In the case of parchments, which are untanned dried animal skins used mainly as writing support material in the Middle Ages, NLO can be used to assess the collagen degradation state, which is characterized by the loss of the SHG signal and the onset of a 2PEF signal.

The combination of NLO microscopy and infrared nanospectroscopy (nanoIR) was used to interpret the collagen degradation at the fiber scale by correlating chemical information to the NLO signals collected. To quantify intermediate states of degradation, two parameters were further implemented: the ratio of two-photon excited fluorescence to second harmonic generation signals sensitive to severe degradation, and the anisotropy parameter extracted from polarization-resolved SHG (P-SHG) measurements, sensitive to early degradation. For P-SHG microscopy, the SHG intensity is recorded as a function of the linear polarization orientation of the excitation for each pixel of the image, and two quantitative information are collected: the main orientation and the degree of orientation disorder at the submicrometer scale. These quantitative approaches were applied to investigate the collagen degradation state in two historical parchments: the UNESCO Memory of the World Register Mappa Mundi of Albi (8th century, France), as well as medieval manuscripts from the Chartres’ library exposed to fire and water as a result of the library bombing at the end the 2nd World War. These results show the high potential of NLO microscopy for in situ quantitative measurements of the conservation state of historical parchments, methodology that may be extended to other cultural heritage materials.