An interdisciplinary approach to a knowledge-based restoration: The dark alteration on Matera Cathedral (Italy)

•The dark alteration was identified as a calcium oxalate film.•Cyanobacteria, bacilli, heterotrophic and cocci type bacteria were identified.•The film is probably due to both biological activity and residues of past treatments.•Results of the campaign highlighted the best chemical cleaning method.•The best protective agent was a non-filmy silane monomers aqueous emulsion. An interdisciplinary analytical campaign was carried out on the exterior walls of the Santa Maria della Bruna and Sant’Eustachio Cathedral in Matera. Large areas of these walls have become darkened and the main objective was to evaluate the state of conservation of the stone material (a very porous, organogenic limestone called “Pietra di Matera”), and to suggest the best strategy for the current restoration. Several techniques were used in situ and ex situ-in laboratory analyses: X-ray diffraction, infrared spectroscopy, ion chromatography, pyrolysis/gas chromatography coupled with mass spectrometry, colour change measurements, laser-induced fluorescence together with biological techniques. Ex-situ and in situ cleaning tests were also carried out on the stone surface. The results showed the presence of chlorophyll and bacteria on the surface, together with sulfation and calcium oxalate films as the main decay phenomena. In addition, the determination of saccharide and egg residues suggest both biological activity and past conservative treatments as the cause of oxalate films. Data obtained from the analyses proved to be very useful for the conservation work; a complex plan of restoration was adopted, including both traditional and innovative techniques (such as biocleaning, bacterial-gel and a laser system) together with a final evaluation of several protective methods.