Red stains on heritage marbles: application of micro-scale analyses to assess the presence and distribution of lead compounds

Abstract

Stone cultural heritage buildings are frequently affected by different alteration phenomena and in particular, on heritage marbles the presence of chromatic discolouration, as the red stains, is one of the most widespread. In this paper, small fragments of red stains originated on marble exposed to different environmental contexts were analysed to reveal the presence and distribution of lead compounds at the micro-scale level. The samples come from slabs of historical religious buildings in Florence (Santa Maria del Fiore Cathedral and the San Giovanni Baptistery) and from the monumental fountains conserved in the Medicean Villa La Petraia (Florence). The presence and distribution of lead compounds: minium (Pb₃O₄), carbonates such as cerussite (PbCO₃) and hydrocerussite (Pb₃(CO₃)₂(OH)₂) and plattnerite (PbO₂), was revealed using 2D high lateral resolution micro-X-Ray Powder Diffraction (μ-XRPD) and μ-Raman spectroscopy. Additional information were provided by Scanning Electron Microscopy, the elemental distribution performed with micro-X-Ray Fluorescence mapping enable to verify the possible presence of light elements and the use of FTIR chemical imaging confirmed the absence of organic compounds.