Revealing the Light Angle-Dependent Visibility Mechanism of Unearthed Charred Historical Papers for Rescue Conservation

Charred historical papers carry invaluable cultural heritages but are extremely fragile, making rescue research and rescue conservation urgent and important tasks. To clarify the light angle-dependent visibility mechanism of charred historical papers dated 603-655 AD unearthed from the Tangchaodun Ancient City Site in northwest China and develop effective rescue conservation strategies, we conduct comprehensive analyses including radiocarbon dating, FTIR, Raman spectroscopy, SEM observation, optical characterization, and reinforcement experiments. The Charred historical papers were made from bast fibers with kaolinite fillers, showing complete degradation of cellulose. Light angle-dependent visibility of characters originates from synergistic optical differences such as whiteness, reflectance, glossiness and distinct reflection modes between paper and character regions, including specular for paper substrates and scattering for character regions. Specifically, the enhanced scattering in the character regions is attributed to the rough ink layer coating the surface of paper fibers. A 1.0 wt % sodium carboxymethylcellulose (CMC-Na) solution effectively reinforces the fragile papers without altering their appearance or obscuring character information, outperforming common Japanese Tengujo papers. This study provides scientific insights into charred paper relics and a practical conservation method, herein having been successfully applied to over 3000 fragments, which holds broad significance for the study and conservation of charred cultural relics.