No abstract

Raman spectroscopy (RS) is a powerful non-invasive tool for the characterization of materials. However, the fluorescence effect often hampers the detectability of the relatively weak vibrational Raman signal. Several approaches were exploited to overcome this limit. This work, in particular, evaluates the performance of an in situ portable sequentially shifted excitation (SSE™) Raman spectrometer applied to the examination of artistic historical pigment powders enclosed in glass vials. The explored handheld spectrometer employs a dual, temperature-shifted, 785 nm and 852 nm laser excitation to optimize both spectral coverage and fluorescence subtraction. The study demonstrates the feasibility of the SSE RS approach for non-invasive identification of art materials, and its applicability in complex situations where the examined material cannot be removed from its container. Laboratory measurements using benchtop dispersive micro-Raman spectroscopy at 785 nm are reported for comparison.

Copper alloy artworks are particularly subjected to chloride attack, which may trigger bronze disease. Therefore, early identification of the phenomenon is crucial in order to stabilize the reactive copper chloride (CuCl) and remove the harmful corrosion products (atacamite and polymorphs). Confocal Raman Microspectroscopy (CRM) has proven to be effective for the detection of small amounts of atacamite, ascribable to the initial phases of corrosion. The handling of bronze artworks is often difficult or even impossible given their large size and weight, and sampling is not always allowed, making the use of portable instruments mandatory for on-site diagnostics. This paper proposes a method for the early detection of corrosion using non-invasive approaches. In this work, we present the results obtained from a set of artificially aged bronze samples with a suite of either laboratory (bench-top) or field (portable/transportable) instruments with the aim of highlighting their characteristics and performances in the diagnosis of bronze disease. Raman spectroscopy, Fiber Optics Reflectance Spectroscopy (FORS), Optical Coherence Tomography (OCT), and Scanning Electron Microscopy (SEM) were applied for chemical and morphological characterization of the samples.

Madder lake is the most utilized anthraquinone dyestuff in artworks. Its main constituents (alizarin and purpurin, together with other natural dyes) form an organometallic complex (the lake) insoluble in water by precipitation or adsorption of the dye onto an inorganic substrate. Raman spectra of madder lake are strongly affected by fluorescence, making its identification by Raman spectroscopy difficult or impossible. In this work, we prepared different lakes according to historical recipes and characterized them with X-ray diffraction (XRD), IR, and UV-Visible spectroscopies. Additionally, painting mock-ups were prepared and the lakes were extracted with hydrated and ethanol-rich agar gel beads. This work is aimed at showing the advantages and limits for the identification of the madder lake constituents by Raman and SERS (Surface-Enhanced Raman Spectroscopy) methods in the hydrogels used for sample micro-extraction. The main advantage of SERS for madder lake detection is to allow the user to acquire spectra in random spots in the gel bead, without the need for recrystallized grain localization in the dried hydrogel.