Micro FTIR imaging for the investigation of deteriorated organic binders in wall painting stratigraphies of different techniques and periods

This work deals with the detection and imaging of metal oxalates/carboxylates in the paint layers of ancient wall paintings and with the investigation of their possible association with the ageing of organic substances used as binding media in the original painting. This assumption is investigated employing Fourier transform infrared (FTIR) imaging on several case studies; starting with a thin section of the paint stratigraphy extracted from extensively deteriorated post-Byzantine murals, painted with an oil binder. Metal carboxylates and oxalates show their maximum concentration at the interface of free fatty acids and pigment particles, indicating a possible origin of their formation. Then, the methodology was applied in paint thin sections, originating from Prehistoric, Roman and Hellenistic wall paintings. In these case studies, metal oxalates detected within the paint layers were explored regarding their relation to the use of an organic binder in the wall paintings. The chemical images of calcium oxalates in the paint layers of the Prehistoric wall paintings was a trigger for further laborious investigation on numerous samples, which resulted in revealing the spectrum of an aged proteinaceous binding medium. Thereafter, the distribution of metal oxalates within the paint layers of the samples from the Roman and Hellenistic wall paintings, unaffected by any later intervention, was assumed as an indirect indication of the use of an original organic binder.