Abstract
A stony sculptural composition of the Nativity Scene is preserved in Altamura’s Cathedral (Apulia, Italy). This commonly called Apulian “presepe”, attributed to an unknown stonemason, is composed of polychrome carbonate white stone sculptures. While earlier stratigraphic tests have unveiled a complex superimposition of painting layers—meaning that several editions of the sculptures succeeded from the 16th to 20th century—a chemical investigation intended to identify the organic binding media used in painting layers was undertaken. Drawing on current literature, two strategies were exploited: a non-invasive in situ digestion analysis and an approach based on micro-removal of painting film followed by the Bligh and Dyer extraction protocol. Both peptide and lipid mixtures were analyzed by matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) and reversed-phase liquid chromatography coupled to mass spectrometry by electrospray ionization (RPLC-ESI-MS). Attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) examinations were also performed on micro-samples of painting films before lipids and proteins extraction. While human keratins were found to be common contaminants of the artwork’s surfaces, traces of animal collagen, siccative oils, and egg white proteins were evidenced in different sampling zones of the sculptures, thus suggesting the use of non-homogeneous painting techniques in the colored layers.
Highlights
IntroductionMilk, egg, animal glue, siccative oils (i.e., linseed oil, poppy seed oil), and plant gums from Acacia, Tragacanth, and Arabic were employed by artists in painting formulations [1,2,3]
Since ancient times, milk, egg, animal glue, siccative oils, and plant gums from Acacia, Tragacanth, and Arabic were employed by artists in painting formulations [1,2,3]
ATR-FTIR spectra were processed with the software used for acquisition; resulting data were exported in txt format to plot the spectrum in OriginPro 2016
Summary
Milk, egg, animal glue, siccative oils (i.e., linseed oil, poppy seed oil), and plant gums from Acacia, Tragacanth, and Arabic were employed by artists in painting formulations [1,2,3]. Diagnostic activity on the mobile sculptures began in 2019 and involved stratigraphic tests as well as optical microscopy investigation of pictorial film’s cross-sections These studies revealed the existence of several overlapped layers of painting, suggesting numerous renovations and restorations occurred between the end of the 16th and 20th centuries. The peptide mixture absorbed by the gel’s porous structure was released into a suitable extraction solution This novel strategy based on in situ multi-enzyme digestion of proteinaceous material of paint layer binders allowed a more confident binder identification ensuing a bottom-up proteomic approach. By this protocol it was likely to exploit a non-invasive or minimally invasive sampling of proteins taking advantage of the occurrence of trypsin and chymotrypsin which synergistically work to increase protein coverage.
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