Compositional characterization of encrustation on marble with laser induced breakdown spectroscopy

This study deals with the analysis of encrustation on marble by laser induced breakdown spectroscopy (LIBS), with the aim to obtain quick in-situ information on the in-depth profiling of the encrustation before advancing to conservation treatments. The encrustation examined is formed on exposed marble: (a) as products of the interaction between the stone surface and atmospheric pollutants (dendritic black and thin black encrustation, of approximately 300 and 200 μm thicknesses, respectively); (b) from deposition of soil–dust on marble surfaces (soil–dust crust, 300 μm thick); and (c) from treatments conducted in the past for aesthetic and/or protective purposes (patina samples, 300 μm thick). The crusts examined are multilayer encrustations on un-weathered marble, as revealed by studying cross sections with optical microscopy and scanning electron microscopy coupled with energy dispersive X-ray analysis. The elemental LIBS profiles of black encrustation based on relative spectral line intensity values show that the Fe, Si, Al and Ti content relative to Ca content decrease significantly with depth, expressing, thus, contamination decreasing within the alteration layers, since these elements originate from atmospheric pollution and deposition. In the cases of soil–dust encrustation and patina samples Si I and Al I emissions identified throughout the analyzed crust, indicate deposition of soil–dust and remnants of previous treatments, respectively. Therefore, LIBS, a micro-destructive technique can be used as an autonomous in-situ diagnostic technique to obtain in-depth elemental profiling of encrustation even in cases of highly in-homogeneous layered crusts, such those of un-weathered Pentelic marble.