Effects of LIBS Measurement Parameters on Wall Paintings Pigments Alteration and Detection

Abstract

Laser-induced breakdown spectroscopy (LIBS) is an efficient tool for pigment analysis since it is a rapid, noncontact, and nearly nondestructive technique. This work focused on the particular context of wall paintings analysis. Six common pigments were studied: ultramarine blue, red lead, green earth, charcoal, red and yellow ocher. Two complementary approaches were tested: macro- and micro-LIBS. Micro-LIBS made it possible to verify pigment distribution on a small area, thanks to its excellent spatial resolution and analytical capabilities. For macro-LIBS, the influence of laser energy and focal length on the crater size, induced by laser-material interaction and on plasma emission signal, were studied to evaluate their importance on sample alteration and pigment detection. It appeared that varying the focal length induced modification of the crater size without a change in signal. Moreover, all pigments showed similar behavior in terms of analytical signal. Laser energy and focal length also induced variations on crater diameters, suggesting a beam treatment to obtain better control on crater dimension. Raman microscopy was used as a diagnostic tool to check the preservation of the pictorial layer after a LIBS analysis.