Identification of commonly used papermaking bast fibers in East Asia by polarized light microscopy

Plant fibers are the core raw materials of traditional East Asian handmade papers. For their identification, microscopic observation has been conventionally conducted relying on fiber staining combined with optical microscopy. Comparatively, polarized light microscopy (PLM) has long been underutilized, especially in the study of ancient East Asian paper relics. Thus, this study aims to evaluate the feasibility of PLM in characterizing fiber optical properties and identifying morphological features, which can be used to distinguish bast fibers in traditional Chinese handmade paper production. As a result, the differences in microfibril angle (MFA) and the effects of alkali treatment on the samples suggest that color changes in the sign of elongation (SE) can serve as indicators for fiber evaluation and characterization, though nuance differences await further exploration. Methodologically, the cross-polarized light (CPL) mode detects optical characteristics (SE and Herzog tests), while the single-polarized light (SPL) mode captures morphological details. This combined approach can effectively distinguish different papermaking bast fibers, offering a micro-destructive technique for identifying plant fibers in East Asian handmade papers and fibrous relics in antiquity.