Effect of neutral AKD surface sizing on the physicochemical properties and durability of Xuan paper

Traditional bone-glue/alum water sizing is acidic and compromises the long-term stability of Xuan paper. Here, we evaluate a neutral alternative—alkyl ketene dimer (AKD) emulsion applied by conventional dip surface-sizing—and assess its effects on physicochemical properties and durability. Dry-heat and damp-heat artificial aging were used to quantify changes in brightness, chromaticity, tensile strength, and surface pH across a concentration series (0.0–1.2 %vol). Results indicate that AKD surface sizing increased tensile strength, and maintained the surface pH within a weakly alkaline range. Both aging processes reduced brightness and increased color difference, with damp-heat aging causing significantly greater degradation. Although pH decreased upon aging, it remained weakly alkaline; the lowest concentrations (0.4 %vol0.6 %vol) showed the best pH stability. Dry-heat aging produced negligible changes in tensile index and elongation at break, whereas damp-heat aging led to significant losses. These findings demonstrate that neutral AKD sizing benefits the physicochemical performance of Xuan paper. Aging behavior is co-modulated by environmental humidity and sizing concentration. For calligraphy and painting conservation, high-concentration AKD surface sizing should be avoided, and high humidity in storage or exhibition environment should be strictly prevented.