Assessment of the conservation state of stone materials in relation to the level environmental pollution in the conservation place

The patina represents a superficial natural alteration of the constituting matter of the work of art. It emerges from the natural and usual stabilization process that the materials of the surface undergo because of the interaction with outdoor agents characterizing the surrounding environment. Besides, it is not linked to an obvious phenomenon of degradation that can be noticed through the change in the original colour of the matter. This is what we intend when we talk about biological patina usually generated by macro and/or micro-organic colonization (fungi, bacteria, alga) which contributes to surface bio-deterioration and thus lead to the formation of orange, red or even brown and dark pigmented areas. The presence of chromatic alterations (rose-coloured areas), as a consequence of bacterial colonization, was most 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 surface of marble works due to bacterial colonization