Susceptibility of different pigments to irradiation by two lasers with different pulse duration and wavelength

The effects of available laser treatments on paint pigments must be taken into consideration for effective cleaning of wall painting. Selection of the most appropriate parameters of the laser is essential for optimal cleaning. This study analysed the effects of two lasers operating at different wavelength and pulse duration (Avia Nd:YVO₄ emitting at 355 nm, with a pulse duration of 25 ns, and Spirit system laser at 1040 nm and 380 fs) on pigments used in wall paintings (tempera and rock art). Specifically, pressed tablets of carbonates (azurite, malachite and lead white), sulphides (cinnabar and orpiment), oxide (hematite) and a natural mixture of silicates and oxides (red earth) were prepared and irradiated for initial measurement of laser damage thresholds and then, higher fluence levels were applied to induce severe changes on surfaces.

Despite of fluences used with the Spirit (IR-fs) were higher than those with Avia (UV-ns), colour changes were generally higher in the latter. Regardless of the laser, sulphides were the most susceptible pigments, and cinnabar underwent intense darkening even at minimum fluence, whereas lead white was the most resistant pigment. Colour changes were attributed to mineralogical and chemical changes in some cases and to micromorphological changes in others.