Compositional characterization of encrustation on marble with laser induced breakdown spectroscopy

This study deals with the analysis of encrustation on marble by laser induc ed breakdown spectroscopy (LIBS), with the aim to obtain quick in-situ info rmation on the in-depth profiling of the encrustation before advancing to c onservation treatments. The encrustation examined is formed on exposed marb le: (a) as products of the interaction between the stone surface and atmosp heric pollutants (dendritic black and thin black encrustation, of approxima tely 300 and 200 mum thicknesses, respectively); (b) from deposition of soi l-dust on marble surfaces (soil-dust crust, 300 mum thick); and (c) from tr eatments conducted in the past for aesthetic and/or protective purposes (pa tina samples, 300 mum thick). The crusts examined are multilayer encrustati ons on un-weathered marble, as revealed by studying cross sections with opt ical microscopy and scanning electron microscopy coupled with energy disper sive X-ray analysis. The elemental LIBS profiles of black encrustation base d on relative spectral line intensity values show that the Fe, Si, Al and T i content relative to Ca content decrease significantly with depth, express ing, thus, contamination decreasing within the alteration layers, since the se elements originate from atmospheric pollution and deposition. In the cas es of soil-dust encrustation and patina samples Si I and A] I emissions ide ntified throughout the analyzed crust, indicate deposition of soil-dust and remnants of previous treatments, respectively. Therefore, LIBS, a micro-de structive technique can be used as an autonomous in-situ diagnostic techniq ue to obtain in-depth elemental profiling of encrustation even in cases of highly in-homogeneous layered crusts, such those of un-weathered Pentelic m arble. (C) 2001 Elsevier Science B.V. All rights reserved.