Impact of Animal-Based Glues on the Surface Characteristics of Traditional Wood-Supported Polychrome Coatings

Animal-derived adhesives have historically played a critical role in East Asian polychrome artworks, yet their influence on coating properties remains insufficiently understood. In this study, three traditional animal glues—bone glue, hide glue, and fish glue—were systematically compared as binding media for cinnabar (red), orpiment (yellow), and lazurite (blue) coatings on pinewood substrates. FTIR spectra of all glues exhibited characteristic collagen-related absorption bands, with bone glue showing a stronger hydroxyl peak at 3280 cm−1. Viscosity analysis revealed bone glue as the most stable, reaching 120 ± 6 mPa·s at 20% concentration, compared with lower values for fish glue and hide glue. Colorimetric analysis indicated that cinnabar coatings were primarily affected by glue concentration, while orpiment coatings showed significant glue-type effects on chroma, with hide glue producing the highest saturation. Lazurite coatings displayed the strongest adhesive-related differences: hide glue enhanced saturation, whereas fish glue increased lightness. Gloss remained consistently low (<3 Gloss Units (GUs)), in line with the matte appearance of historical coatings, although bone glue produced smoother cinnabar surfaces (Ra = 1.72 µm, GU = 3.09). Two-way ANOVA confirmed that both the glue type and concentration significantly influenced chroma, gloss, and roughness. These findings demonstrate that although all animal glues share a collagenous origin, their distinct physicochemical properties shape the optical and microstructural qualities of polychrome coatings, offering a scientific basis for adhesive selection in cultural heritage conservation.