Raman Study of 532-Nanometer Laser-Induced Degradation of Red Lead.

Abstract

Red lead is commonly employed as a red pigment in numerous valuable cultural artifacts. Raman spectrometry has been widely employed as the primary tool in many nondestructive studies on red lead. Therefore, it is necessary to evaluate and study the impact of lasers on the pigment. The degradation of red lead induced by a 532 nm laser is investigated using micro-Raman spectroscopy. At room temperature, red lead begins to degrade into β-PbO when the power density of the 532 nm laser reaches approximately 5.1 × 104 W/cm2 (laser: 532 nm, objective: 50×). At this point, the temperature at the focus of the sample is estimated to be at least 500 °C, aided by the Raman peak shift of β-PbO. Furthermore, the power density of the laser-induced degradation decreases as the temperature of the red lead increases. Hence, the degradation of red lead can be attributed to the photothermal effect. The temperature rise can be explained by two factors. First, red lead exhibits a high absorbance of approximately 0.5942 at 532 nm. Second, red lead has significantly low thermal diffusivity and conductivity, measuring 0.039 mm2·s-1 and 0.078 W·m-1·K-1, respectively, which leads to heat accumulation at the focal point of the laser beam. To better preserve cultural heritage, the appropriate laser power should be prioritized when the degradation process is caused by the thermal effect of laser irradiation.