Experimental evidence of the thermally dominated effect of CW NIR laser irradiation for the restoration of darkened red lead in wall paintings

In wall paintings, the widely used red lead pigment (Pb3O4) may darken due to its transformation to black-brown plattnerite (β-PbO2). No well-established conversion methods to restore darkened red lead in wall paintings have been reported yet. Only recently, the use of Nd:YAG Continuous Wave (CW) laser irradiation has been proposed and tested on mock-ups and on a wall painting to recover darkened red lead. Although very promising, the method still needs to be fully characterized (penetration depth, stability, by-products etc.). The understanding of the main mechanisms is crucial for the identification of the best operative conditions and to assess the possible risks associated to this method. To identify the laser-induced products and their distribution in the paint layer a combination of synchrotron based (SR) micro X-ray powder diffraction mapping (µ-XRPD), Near-edge X-ray absorption spectroscopy (XANES) at the Pb LIII edge, µ-Raman spectroscopy, and VIS-NIR and short wave infrared (SWIR) hyperspectral imaging (400–2000 nm) has been used to characterize the bulk, the surface and the stratigraphy of a plattnerite wall painting mock-up treated with a CW 1064 nm laser. New compounds in the laser-induced treatment of plattnerite have been identified, representing the key evidence of a predominant thermal effect of the laser treatment. These compounds are non-stoichiometric lead oxides that are an intermediate step in the reduction process of plattnerite. A characteristic Raman spectrum of these non-stoichiometric lead oxides was identified with a main band at 419 cm−1. These compounds are found to be surprisingly stable over the 2-year observation period.