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.