Gelatine adhesives from mammalian and fish origins for historical art objects conservation: How do microstructural features determine physical and mechanical properties?

Gelatine adhesives aka ‘animal glues’ are water-soluble biopolymers used in historic objects such as wooden cabinets and panel paintings since ancient times. This paper investigates the correlations between microstructural features, namely triple helices, and macroscopic properties of four different types of gelatine adhesives, prevalently used in conservation practices, irrespective of the animal origin. These adhesives include bovine bone, bovine skin, rabbit skin, and fish glues. Thin adhesive films were produced via solution casting methods in controlled climate conditions and their thermal and mechanical properties, and moisture sensitivity were investigated. XRD as a non-destructive characterisation method demonstrated good agreement with DSC in the quantification of gelatine adhesive (animal glue) triple helix content irrespective of the animal origin. Linear correlations between triple helices and gel (Bloom) strength and tensile strain energy to failure (toughness) were found for all adhesive types. Dynamic vapour sorption experiments demonstrated that lower triple helix content is correlated with higher moisture sensitivity of the adhesives. Moreover, the effect of environmental RH on the thermal behaviour of adhesives was investigated by DSC. The results demonstrated that the increase in environmental RH causes a reduction in the adhesives' glass transition and denaturation temperatures whilst triple helix content did not alter. Bovine bone glue with the lowest triple helix content showed the least toughness and highest moisture sensitivity, whilst fish glue with the highest triple helix content was identified as the most flexible glue.