Non destructive chemical analyses of the "Red Marble" from Santa Maria del Fiore Cathedral (Florence (Italy).

The Cathedral of Santa Maria del Fiore in Florence (XII-XV centuries) plays a fundamental role in the historic architecture of the city; it was decorated by using three stones of the different colour: white marble, red limestones (so-called "red marble") and green serpentine (so-called "green marble"). White and green marbles have been widely studied, whereas not sufficient data are available on the red marbles; different types of this latter have been used as a cladding of the Cathedral, and it is difficult to distinguish them and consequently to obtain information on their provenance. A research carried out at the Archive of the Florence Cathedral, made it possible to know that since the XV century, stones of different provenance were used both for the original decoration and for the following restoration works until today. As a matter of fact, many of these stones come from three geological formations of the Central Apennine: Scaglia Toscana and Rosso Ammonitico (Tuscan Sequence), Scaglia Rossa (Umbrian Sequence). It is easy to differentiate at naked eye the claddings belonging to Rosso Ammonitico showing nodular aspect from those belonging to the groups Scaglia Toscana and Scaglia Rossa which instead show massive appearance. Furthermore, different supply quarries were used for each of the three formations and macroscopically it is not possible to recognize the single quarry of provenance. To this regard the chemical composition and, in particular, the trace element abundances could help in the identification but, due to the architectural and historical importance of the Cathedral, it was not possible to collect samples. Therefore XRF analyses with the portable instrument (model TRACER III SD by Bruker) have been carried out in situ on the red claddings; the traditional XRF technique (model WD-XRF Rigaku Primus II) was, in addition, used to determine the composition of the stones coming from the different quarries sites. The results permitted to obtain a chemical database of the different quarries; these data have been compared with those acquired through the portable X-ray Fluorescence. The comparison permitted to achieve an important result: the different typologies of red marbles can be distinguished through the abundances of some trace elements (e.g. Sr and Ba) which are thus discriminant of the various typologies of red stones.