A large number of sculptures and decorative pieces made of Carrara marble show a typical red staining; this staining is mentioned in the 1848 general inventory of the Certosa. Microscopic examinations have allowed the identification of unicellular algae, fungal hyphae, and heterotrophic bacteria. Some of these bacteria, isolated in a culture, produce a red pigment like that observed on the marble surface. The red stains are probably due to the pigments made by bacteria in the marble microcracks.