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

An interdisciplinary analytical campaign was carried out on the exterior walls of theSanta Maria della Bruna andSant’EustachioCathedral in Matera. Large areas of these walls have become darkened and the main objective wasto evaluate the state of conservation of the stone material (a very porous, organogenic limestone called“Pietra diMatera”), and to suggest the best strategy for the current restoration. Several techniques were usedin situandexsitu-in laboratory analyses: X-ray diffraction, infrared spectroscopy, ion chromatography, pyrolysis/gas chro-matography coupled with mass spectrometry, colour change measurements, laser-inducedfluorescence togetherwith biological techniques.Ex-situandin situcleaning tests were also carried out on the stone surface.The results showed the presence of chlorophyll and bacteria on the surface, together with sulfation andcalcium oxalatefilms as the main decay phenomena. In addition, the determination of saccharide and eggresidues suggest both biological activity and past conservative treatments as the cause of oxalatefilms. Dataobtained from the analyses proved to be very useful for the conservation work; a complex plan of restoration wasadopted, including both traditional and innovative techniques (such as biocleaning, bacterial-gel and a lasersystem) together with afinal evaluation of several protective methods.