Chromatic alteration on marable surfaces analysed by molecular biology tools

The surfaces of works of art exposed to specific environment with which they interact, are subject to a high number of exchanges and reactions leading to the creation of the “natural patina”, through the surface alteration. This definition already explains its genesis and stands as the consequence of a natural phenomenon of surface alteration of the matter. Cesare Brandi defined patina as “that particular characterization of the surface of a historical, cultural, artistic work deriving exclusively from the natural and usual stabilization and modification process that the materials of the surface undergo because of the interaction with outdoor agents characterizing the surrounding environment, in particular with the light energy, the air and its components, the temperature and the humidity that characterize it”. The legislation called NorMal 1/88 defined it as “the alteration strictly limited to natural modifications of the surface that is not linked to an obvious phenomenon of degradation that can be noticed through a change in the original colour of the matter”.The patina is considered a very complex process due to an exchange of matter and energy between two open systems, i.e the work of art and the environment; thus the patina arouses an additional fascination onto the surface of the work, except if it alters the interpretation of the work too much. This is often the case when we talk about “biological patina” or bio-films, usually generated by macro and/or micro-organic colonization (fungi, bacteria, algae) which contribute to the surface bio-deterioration and thus lead to the formation of pigmented areas. The presence of chromatic alterations (rose- coloured areas), as a consequence of bacterial colonization, were particularly pointed out in different sites, such as in the marble slabs on the facades of both the Cathedral of Siena (Duomo di Siena) and the Certosa of Pavia. The present study shows an example of chromatic alteration of the surfaces of marble due to some bacterial species, such as Micrococcus.