New insights into the stability of organic binders in wall paintings on gypsum

CAREAGA, VALERIA; SIRACUSANO, GABRIELA; GOTTAS, GERARDO; MAIER, MARTA S.; CASTELLANOS RODRÍGUEZ, DIANA M.; FRECIA MAUDET, GALA

Brujas

Congreso; Technart 2019; 2019

Previous studies on relevant mural paintings in 18th century Andean churches in Chile and Bolivia indicated a secco painting technique using a mixture of egg and linseed oil as pigment binders applied on a preparation layer of gypsum primed with animal glue [1-3]. These results prompted us to study the degradation processes of lipids and proteins in binders as siccative oils, egg and animal glue in pictorial replicas prepared on glass slides and gypsum as support. The mock-ups were naturally aged in the laboratory conditions for one and two years. Then, microsamples extracted with a scalpel were analysed by FTIR-ATR through analysis of the second derivative of the spectra. The ratio of the areas of the bands associated to the carbonyl ester group and the carboxylic acid group indicated a higher stability of the lipidic components in the mock-ups with gypsum as support. This was further confirmed by GC and GC-MS analysis of the fatty acid methyl esters of the oils, which showed higher concentrations of dicarboxylic acids, such as azelaic and sebacic acids, in the glass slides replicas in comparison to the mock-ups with gypsum as support. These results allowed us to hypothesize a better preservation of lipids in the gypsum matrix. The protein binders showed greater stability than the lipid components and the amino acid compositions were similar in both supports. Results obtained by FTIR and GC were analysed applying multivariate statistical models, allowing a better interpretation of the data. Further analysis of microsamples extracted from mural paintings from three churches in Peru confirmed the presence of gypsum as the preparation layer as well as the same painting technique characterised in the Andean churches in Chile and Bolivia