Visible and Near-Infrared Reflectance Spectra of modern watercolours in the UV-Vis-NIR range
Analysing paper conservation materials is often quite difficult, due to the fact that the paint layers in delicate art pieces are usually very thin and small in size. In these situations, micro-sampling is a complicated task . The use of in situ techniques is thus recommended as the most appropriate analytical tool [2, 3]. Reflectance spectroscopy (RS) is an advantageous methodology for such cases. It provides measurements on site, the spectra of which can be obtained quickly and analysed in real time. For an identification of the pigments and dyes of a specific case study, it is always necessary to build a suitable database based on the creation of standard mock-ups for purposes of comparison . The database presented here was prepared by focusing on the pigments and lakes most used by the Portuguese modernist painter Amadeo de Souza-Cardoso (1887-1918). It was then used to identify the pictorial materials employed by the artist in the livre d'artiste "La Légende de Saint Julien l'Hospitalier" (1912), which now belongs to the collection of the Centro de Arte Moderna - Fundação Calouste Gulbenkian (Lisbon, Portugal). The mock-ups were prepared by grinding each single commercial pigment into a solution of 10% of Arabic gum in water (w/v). Five different dilutions were obtained: 5%, 10%, 20%, 30% and 40%. The paints of each dilution were spread using a brush on Whatman laboratory paper. Note that each pigment was previously characterized by means of X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF) and Fourier transform infrared spectroscopy (FTIR) techniques. Despite the fact that the reflectance spectra of the database present a strong signal of paper in the NIR region, they make it possible to identify the main spectral features of the pigments and lakes in question. Equipment Reflectance spectra were acquired with the use of two techniques, the first one employed a portable device equipped with optical fibers (FORS, Fiber Optic Reflectance Spectroscopy), while the second one used a laboratory spectrophotometer: 1) FORS measurements in the 350-2000 nm range were performed using a portable device equipped with optical fibre bundles constituting of two Zeiss spectroanalysers (models MCS601 UV-Vis and MCS611 NIR 2.2 WR) and a 20-W halogen tungsten lamp (model CLH600). The MCS 601 UV-Vis model consists of a 1024 silicon photodiode array detector and operates in the 200-1000 nm range with an acquisition step of 0.8 nm/pixel. The MCS 611 NIR 2.2 WR model has a 256 InGaAs photodiode array detector that operates in the 900-2200 nm range with an acquisition step of approximately 5 nm/pixel. The spectroanalysers were mounted on a single chassis together with the CLH600 model light source. A 99% Spectralon® diffuse reflectance standard was used to calibrate the spectroanalysers. Two different probe-heads with geometries of 0º/2x45º and 8º/8º were used. In the first case, the aperture at 0º was used to illuminate the investigated areas and the two 45° apertures received the back-scattered radiation from the surface analysed and sent it to the two spectroanalysers. By using this probe geometry, a diameter area of approximately 2 mm was investigated. In the second case, the illumination was sent almost perpendicularly to the surface (8°), and the back-scattered radiation was collected in the same direction as the illumination, so as to reduce the area effectively measured on the surface to a very small spot that had a diameter of about 1 mm. Both configurations were designed to remove the specular components from the reflectance measurements. 2) Reflectance spectra were also acquired in the 330-2500 nm range by using a UV-Vis-NIR double beam spectrophotometer, Perkin-Elmer model lambda 1050, equipped with a 60-mm integrating sphere with a 0º/d geometry. The bench was provided with two light sources: a pre-aligned deuterium lamp and a tungsten-halogen lamp (UV-Vis and Vis-NIR ranges, respectively), as well as a source-doubling mirror for ultra-high sensitivity. The integrating sphere had two detectors: a R6872 photomultiplier for high energy in the entire UV-Vis range, and a high-performance Peltier-cooled PbS detector for the NIR range. The reflectance spectra were acquired excluding the specular component.
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spettri riflettanza UV-Vis-NIR
Ana Margarida Silva1,2, Marcello Picollo3, Lorenzo Stefani3 1 Department of Conservation and Restoration, Faculdade de Ciências e Tecnologia - Universidade Nova de Lisboa, Campus Caparica, 2829-516 Caparica, Portugal; 2 REQUIMTE-LAQV, Faculdade de Ciência