The contribution of multiphoton microscopy to our knowledge of parchments

Multiphoton microscopy, also called non-linear optical (NLO) microscopy, is an optical imaging technique widely developed in the biomedical field, but relatively new and promising for the non-invasive analysis of heritage materials. This technique is particularly efficient to investigate skin-based materials at the microscopic scale, especially parchments, since it can visualise the collagen fibre organisation within the material from the collection of the second harmonic generation (SHG) signal. The simultaneous collection of fluorescence signals by the same NLO microscope can also reveal the presence of interacting fluorescent materials such as fat, elastin and keratin, remaining from the skin preparation, or gelatin, resulting from collagen degradation. When applied to historical parchments and manuscripts, NLO microscopy sometimes reveals very different morphology of collagen fibres and signals collected, likely associated to various factors in the life of the object. Based on experiments set up on modern parchments and measurements made on historical objects, we will demonstrate how the technique can contribute to our knowledge on the manufacturing, the degradation and the impact of some restoration treatment of parchments.