Combined use of Synchrotron based X-ray techniques and micro-Raman spectroscopy for Pb compounds mapping of red stains in heritage marbles

Heritage marbles are frequently affected by different alteration phenomena. Among them, the presence of chromatic discolouration, such as the red stains, is one of the most widespread on Cultural Heritage buildings. Previous multi-analytical studies demonstrated that the red stains in some heritage marbles are mainly due to the presence of minium (lead tetraoxide) concentrated prevalently in the calcite crystal boundaries [1]. In order to reveal the presence and distribution of other Pb compounds at the micro-scale level, small fragments of several red stains originated in different environmental contexts and marble types were analysed. The samples under study come from Florentine historical buildings (San Giovanni Baptistery and Santa Maria del Fiore Cathedral) and monumental fountains. The investigation combines structural information from 2D high lateral resolution X-Ray Powder Diffraction (XRPD performed at ID13 ESRF synchrotron facility) [2] and micro-Raman spectroscopy, with the elemental distribution performed with micro-X-Ray Fluorescence mapping (µ-XRF performed at ID21, ESRF) and EDX-SEM. Both structural and elemental information, a part from minium, highlight the presence of cerussite and hydrocerussite distributed in different ways depending on the sample and copresent in some cases with light elements like P and S. All these information suggest the conditions of the solution from which these compounds precipitated [3] and help us to shed light on the environmental factors that affect the formation of the red stains.