Opuntia ficus-indica (L.) Mill. and Opuntia stricta (Haw.) Haw. Mucilage-Based Painting Binders for Conservation of Cultural Heritage

The possibility of using materials from the waste of agricultural products for the conserva- tion of cultural and artistic heritage has led to important technological developments on mortars, plasters, colors, and other applications. In this experimental work, we investigated the binding properties of mucilage obtained from two different species of the genus Opuntia, both collected in Sicily, Italy: Opuntia ficus-indica (L.) Mill. and Opuntia stricta (Haw.) Haw. Through chemical acid hydrolysis, and subsequent spectroscopic analysis conducted at 13C-NMR, the main monosaccharide composition of both mucilage was studied, identifying considerable compositional differences. In fact, the mucilage of O. ficus indica had similar total amounts of arabinose (23.65%), galactose (20.87%), and glucose isomers (23.89%), while that of O. stricta was characterized by significant amounts of arabinose (36.48%) and galactose (32.31%) units. The samples were obtained by dispersing pigments on the mucilage and applying the obtained tempera by a brush onto both paper and chalk supports, in order to observe if the colors changed with different substrates. Colorimetric analysis, measuring ∆E, showed how the same pigment modifies its aspect depending on the binder used. After a two-week UV ageing process, pigments that had dispersed in O. stricta changed their aspect more than those dispersed in O. ficus-indica. Overall, it is also evident how ∆E data for organic pigments are higher than those for inorganic ones.