Before acidification: Wet-heat alternation and alum synergistically inducing rapid brittle fracture of gelatin coated artworks

Brittle fracture of GAW-treated Xuan paper occurs earlier than acidification. • Wet-heat alternations induce the rapid brittle fracture of gelatin-alum film. • Wet-heat alternations disrupt the coordination network in gelatin-alum film. • Gelatin-alum film is prone to oxidation due to the increase in random coil content. • The exposed Al 1 and Al 3+ are prone to hydrolysis and accelerates acidification. The deterioration of paper artworks treated with gelatin alum solution (GAW) is conventionally attributed to cellulose hydrolysis catalyzed by alum-derived protons. However, our observations and aging evaluation of GAW treated Xuan paper under alternating hygrothermal conditions revealed a discrepancy that mechanical properties declined faster than acidity accumulation, challenging this acid-centric view. To elucidate the underlying mechanism, this work focused on investigating the conformational changes of gelatin molecules under alternating hygrothermal conditions, as well as the presence or absence of alum. Results showed that gelatin-alum films (GAF) initially exhibited better flexibility than gelatin films (GF) due to the coordination crosslinking effect of Al 3+ species, which also inhibited the recovery of gelatin's triple-helix structure. However, after accelerated aging, the GAF underwent more pronounced brittle fracture. This is mainly attributed to the intense movement of molecular segments in hygrothermally sensitive gelatin, which disrupted the coordination cross-linking network, resulting in the formation of more disordered structures such as random coil and accelerated oxidation of gelatin. Subsequently, the exposed Al 3+ and Al(H 2 O) 6 3+ (Al 1 ) became more prone to hydrolysis and polymerization, yielding acidic species and [Al 30 O 8 (OH) 56 (H 2 O) 24 ] 18+ (Al 30 ), thereby accelerating acidification and brittle fracture. Thus, this work provides a more comprehensive perspective to understand the damage mechanism of GAW-treated Xuan paper, valuable for conservation and restoration of GAW-treated paper artworks. Download: Download high-res image (372KB) Download: Download full-size image