Retardance and axis orientation measurements for characterization of the collagen fiber network in parchments

Parchment is a fibrous material which was the primary writing support in Europe during the Middle Ages. Its optical anisotropy (manifested as birefringence) can serve as a valuable indicator of the material's structure, which gives useful information for conservation and restoration purposes. Non-invasive transmission polarized light microscopy can reveal distinct collagen fiber orientations and areas where gelatinization (collagen denaturation) occurred. However, this imaging technique faces limitations, such as the sample thickness and light depolarization caused by light scattering within the fibrous structure. A home-made experimental setup was used to measure two-dimensional spatial distributions of collagen fibers' retardance and axis orientations in a thick deer parchment and a thin calf membrane used for restoration. Homogenous distribution of fiber orientations was observed in the thin membrane sample, whereas in the thick parchment fibers were found to be aligned along the animal spine. These results are discussed in terms of parchment's animal origin and manufacturing process.