Chromatic alteration of Florentine sandstone: only a natural phenomen?

The stone materials mainly used in Florentine architecture are two sandstones: Pietraforte, mostly used during the Middle Ages as building structures and during the Renaissance as facing, and Pietra Serena used for ornamental purposes. The first one is found in abundance in the hills at south of the city, whereas Pietra Serena is outcropping out in the hills near Fiesole (north of the city). Pietraforte is a sandstone belonging to the turbiditic formation present in the allocthonous complex of the External Liguridi superposed on the Tuscan Series; Pietra Serena, instead, belongs to the sandstones of the Macigno Formation which consists of beds of turbiditic sandstones separated by pelitic levels which are the finest components of single turbidity current. Petrographically, Pietraforte is a fine-grained lithic sandstone made in the same proportion by silicatic grains (quartz, feldspars and magmatic fragments) and carbonatic grains (dolostones). The grains are bounded by a mainly calcitic matrix that makes the rock particularly strong. Pietra Serena can be defined as a medium coarse- grained greywacke made by quartz, feldspars, micas, fragments of metamorphic and magmatic rocks. The matrix is quite abundant and is made by illite, kaolinite and chlorite-vermiculite (present only in some outcropping areas). It is well known that the processes of decay of the sandstones are related to the type of matrix, the amount of cement, the kind of clay minerals present and to the distribution of porosity, which lead to water infiltrations, resulting in swelling and peeling. The sandstones have a bluish-grey color in fresh cut, but is easily oxidized acquiring an ochraceous - reddish brown color on buildings. Such changes in color, appear to be due in part to the oxidation of iron, proceeding very quickly from the surface to the inside, without cohesion decrease. Indeed the chromatic changes not necessarily implies a deterioration of characteristics of the material, but they often seem to represents a natural patina acquired with the time. However, in some cases the thickness of oxidized layer and its hardness could also be the result of treatments performed in the past. In Florence, several monuments and buildings are affected by such phenomenon, in particular it possible to note an intense and diffuse reddish coloring on bell tower of Valdese Church entirely constituted by Pietra Serena sandstone, in the exterior columns of the Church Santissima Annunziata and in the historical building of Santa Croce square etc. About the origin of the color change, the hypothesis most reliable is a natural decay of stone, due to iron oxidation. On the other hand the presence of an intense and homogeneous distribution of the red color could be due to the application of conservative treatments, as a possible consequence of a recent restoration. This study intends to verify the real and more probable causes of this frequent chromatic alteration and the processes which determine it.