First evidence of Opuntia Ficus-Indica leaves used in ancient surface finishing treatments of stone architecture in Salento (Apulia, Italy)

Abstract The application of hydrophobic coatings on the Lecce stone is a widespread practice today to preserve stone monuments from decay. The practice of covering the stone facades of the buildings was also used in the past in Salento (Apulia, Italy): in fact, ancient sources report that natural waterproofing agents were used for esthetic, protective, hygienic and conservative purposes. The materials and methods of preparation of these finishes became often secret of the workshop and were not handed down, and when known, they may have undergone changes over time. The identification of the materials and methodologies used on historical buildings for their protection, plays a major role to establish correct protocols for their restoration. For this purpose, selected religious and civil buildings (based on requirements, such as historical reasons, conservation status, restoration interventions and presence of visible or documented surface treatments), made of Lecce stone and located in the territory of the Salento, were studied and the presence of surface treatments was investigated. Samples were analyzed by pyrolysis-gas chromatography/mass spectrometry analysis (Py-GC/MS) with and without thermally assisted hydrolysis and methylation. The results of the analyses showed that these buildings have been protected with different natural products. This paper, indeed, gives evidences of the presence of surface treatments, i.e. identifies the ancient finishing techniques through diagnostic analyses. Furthermore, for the first time, the identification of chemical biomarkers of prickly pear leaves confirms the use of Opuntia Ficus-Indica in finishing treatments of Lecce stone.