HERAS: A helium jet to prevent damage on works of art in Raman experiments

Abstract

Raman spectroscopy is often used for non-destructive analysis of works of art, polymers and biological materials, but in some cases, the laser beam can cause damages on the surface being studied due to the deposited beam energy. When possible, such damage can be prevented by minimizing laser intensity or acquisition time, but this is usually available only on high sensitivity bench-top spectrometers. Portable Raman spectrometers are commonly not so flexible and an alternative is needed to ensure the safe study of sensitive works of art and other fragile materials. A helium jet aimed directly at the laser spot may prevent this damage from occurring and, in some cases, helps improving the Raman spectra. We designed a simple system (HERAS, Helium Raman System) consisting of a pinhole collimator, coupled to a helium line and a gas mass flux control and tested it on pyroxylin, vermilion and ochre paint references and pigment samples, using a 785 nm portable Raman spectrometer at various laser powers. Experimental conditions slightly differ for each sample, but small burns on the surface were avoided in all cases and only some ablation was observed on the most sensitive materials. The use of this coupled system allows the non-destructive application of Raman to a wider variety of materials, while the technique remains portable. This setup may be used in bench-top apparatus as well.

Spectroscopic characterization of sixteenth century panel painting references using Raman, surface-enhanced Raman spectroscopy and helium-Raman system for in situ analysis of Ibero-American Colonial paintings

“…The first one is that helium creates an inert atmosphere, avoiding sample combustion together with its associated light emission. The second one is that helium cools the analysed surface, reducing the production of fluorescent species from combustion, reductive and oxidative processes, laser-induced degradation or photochemical reactions [20]. Asphalt (Y4-10), cochineal (Y4-14), indigo (Y4-20) and carbon black (Y4-24) were other pigments that suffered laser-induced damages.…”

“…In a previous work [20], we reported the use of a helium flux (HERAS) to avoid these thermal effects, inspired by the application of He to reduce the ion and X-ray energy loss and to decrease the damage caused by the ion irradiation heating on the sample [21,22]. A later development of the same technique allows the simultaneous use of a microscope.…”

“…A less addressed subject in Raman spectroscopy is the heating effect from the laser on the analysed surfaces, which becomes important when the laser power is above certain limits [16][17][18][19]. In a previous work [20], we reported the use of a helium flux (HERAS) to avoid these thermal effects, inspired by the application of He to reduce the ion and X-ray energy loss and to decrease the damage caused by the ion irradiation heating on the sample [21,22]. A later development of the same technique allows the simultaneous use of a microscope.…”

Recent advances in linear and nonlinear Raman spectroscopy. Part VIII

“…The use of this coupled system allows the nondestructive application of Raman to a wider variety of materials, whereas the technique remains portable but may be used in bench-top applications as well. [187] Shi and Li used Raman to investigate the fiber and pigment composition of 15th and 19th century Chinese paper currencies. Both microscopic examination of stained fibers and Raman analysis allowed, for the first time, an understanding of fiber and pigment use in the production of ancient Chinese paper currencies.…”

“…Ruvalcaba‐Sil and coworkers reported the development of a helium jet to prevent damage on works of art in Raman experiments called helium Raman system. The use of this coupled system allows the nondestructive application of Raman to a wider variety of materials, whereas the technique remains portable but may be used in bench‐top applications as well . Shi and Li used Raman to investigate the fiber and pigment composition of 15th and 19th century Chinese paper currencies.…”