Alteration of marble stones by red discoloration phenomena

Red discoloration on marble showed particular interest between 1980-2000, when red stain phenomena were investigated on different monuments such as the façade of the Certosa di Pavia, the Siena Cathedral, the Orvieto Cathedral, the marble Cenotaph in the Monumental Cemetery of Pisa, the Labyrinth Fountain in the Garden of Villa di Castello in Florence, the Fountain of Galatea in Villa Litta (Lainate-Milan), the Pretoria Fountain in Palermo. New examples of red stains were recently observed in Genoa (columns of Carlo Felice Theatre , Basilica of Santa Maria Assunta in Carignano, cloister of Genoa University in Balbi Street, etc.) and Florence Cathedral of Santa Maria del Fiore, the Baptistery, basement of the statuary complex of Ratto delle Sabine by Giambologna, the statue of Giovanni delle Bande Nere, etc.). Comparative macroscopic observations on these new case studies are presented, showing that water plays an essential role in the formation of red stains, together with the presence of microorganisms and a source of lead. This work was therefore focused on the characterization of such red discolorations observed on the marble cladding of the apse side of the Florence cathedral. A multi-analytical approach was chosen for the red stain investigations, using the following techniques: microscopic analysis (OM), Raman spectroscopy, X-ray microtomography and XRF. The red stains display heterogeneous aspect, the wideness of the covered area and their chromaticity being variable. They were observed both on horizontal and vertical slabs, and often the presence of microorganisms was detected (mainly chasmolitic cyanobacteria and chromogenic bacteria). The results revealed the presence of lead as well. The Raman spectra showed the presence of minium (lead oxide) in both phases - crystalline and amorphous. The possible source of lead is located in the joins between the marble slabs and/or the traffic pollution, while the experiential microorganisms, lead-resistant, join in the process of red stain formation.