Noninvasive quantitative imaging of historical parchments by polarization-resolved SHG microscopy

Advanced nonlinear optical (NLO) microscopy enables quantitative in situ mapping of parchment degradation at the micrometer scale. We show that measurements of the ratio of fluorescence over second harmonic generation (SHG) signals probes severe collagen degradation and could help to identify the parchments most at risk, close to the irreversible collagen denaturation into gelatin. Conversely, the anisotropy parameter obtained from polarization-resolved SHG measurements probes the earlier stages of degradation. This approach is first validated by comparing NLO quantitative parameters to thermal measurements by differential scanning calorimetry on artificially altered contemporary parchments. We then analyze invaluable parchments from the Middle Ages.