Laser damage on painting pigments & CN vibrations in prussian blue Prussian blue is a pigment highly sensitive to electromagnetic radiation, visible light included. This photosensitivity, associated with a complex redox behavior, causes a vulnerability even to Raman lasers, with the possibility of sample alteration or irreversible damage. In this study, we systematically explored the influence of the laser wavelength and laser power on different types of Prussian blue pigments, soluble and insoluble. The use of different laser wavelengths does not influence the position of the characteristic peak, though it affects the signal‐to‐noise ratio. The latter can be improved by increasing the number of accumulations and/or the acquisition time. Furthermore, we evaluated a safe level of Raman laser excitation or “safe zone” with laser power between 0.005 and 0.06 mW, where Raman analysis can be performed without laser‐induced artifacts or damage for the sample. These artifacts may affect the characteristic spectral signature of the two different Prussian blue, leading to a wrong identification of the pigments. Moreover, artifacts can also hide features arisen from fading in objects presenting non‐visible alterations of Prussian blue. Prussian blue pigment is significant since it was the first stable and relatively lightfast blue pigment to be widely used following the loss of knowledge regarding the synthesis of Egyptian blue. European painters had previously used a number of pigments such as indigo dye, smalt, and Tyrian purple, which tend to fade, and the extremely expensive ultramarine made from lapis lazuli. Japanese painters and woodblock print artists, likewise, did not have access to a long-lasting blue pigment until they began to import Prussian blue from Europe. Prussian blue Fe 7(CN) 18 (also (Fe 4[Fe(CN) 6] 3) · xH 2O) was probably synthesized for the first time by the paint maker Diesbach in Berlin around 1706.[3] |
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