Dimensional Stability of Sanskrit Manuscript Supports Exposed to Environmental Humidity: Tibetan Paper, Palm Leaf, and Birch Bark

Abstract Environmental humidity can induce irreversible deformation of manuscript supports (expansion, contraction, bending), threatening long-term preservation. Using a controlled-variable design, we exposed Tibetan paper, palm leaf, and birch bark supports to five constant relative humidities (30.0 %, 55.3 %, 76.2 %, 86.0 %, and 95.0 %) and monitored moisture content, dimensional change, and deformation for 145 days. Structural and compositional differences were considered to compare underlying deformation mechanisms. Deformation in all supports increased significantly with humidity, and the magnitude, rate, and patterns matched their hygroscopic/desorption behaviour. Palm leaves showed the largest expansion–contraction, followed by birch bark, while Tibetan paper exhibited the best dimensional and planar stability. Palm-leaf deformation is attributed to highly hygroscopic constituents, a loose microstructure and oriented vascular bundles. Birch-bark lateral curling is governed by structural anisotropy and contrasting hydrophilicity between inner and outer surfaces. Tibetan paper remains stable due to its dense fibre network and traditional manufacturing. Based on the results, ca. 55 % RH is identified as an optimal general storage humidity for all three supports, where projection and expansion ratios fall within relatively stable ranges. These findings establish a unified humidity benchmark for mixed-support collections and support targeted preservation strategies.