The Degradation of Daguerreotypes and the Relationship with Their Multi-Material Structure: A Multimodal Investigation.

Preserving and analytically examining daguerreotypes is particularly challenging because of their multi-material and multi-component structure. Various sensors have been exploited to examine mainly the image plates of the daguerreotypes even though the degradation goes beyond this component. Micro-analyses have been the preferred method due to the nanoscale structure of the image particles. In this work, we propose comprehensive multi-modal non-invasive sensing to investigate the corrosion products present in nine daguerreotypes from the Fondazione Alinari per la Fotografia (FAF, Florence, Italy). The methodology proposed includes chemical and morphological analyses: portable X-ray fluorescence spectrometry (pXRF), Raman microspectroscopy (µ-Raman), and micro-Fourier transform infrared spectroscopy in reflection mode (µ-rFTIR) for the chemical identification. For the first time, optical coherence tomography (OCT) was deployed to record the cross-sectional and morphological data of the relevant corrosion formations on daguerreotypes in a contactless way. The results allowed the characterization, in a non-invasive mode at a microscopic level, of a wide range of degradation products produced by the interaction of the different elements present in the structure of the daguerreotypes. The aim was to verify the performance of the proposed methodology and to link the chemical and physical complexity of the entire structure, disclosed by the state-of-art sensors, to the daguerreotype degradation. The results draw attention to the need to monitor not only the image condition but the whole object as a partially closed system in constant interaction internally and with the environment.

Portable Sequentially Shifted Excitation Raman Spectroscopy to Examine Historic Powders Enclosed in Glass Vials.

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.

Nonlinear imaging and vibrational spectroscopic analysis of cellulosic fibres treated with COEX® flame-retardant for tapestry preservation.

Cellulose-based fabrics are widely used in the preservation and storage of historic tapestries. Their ease of flammability is a serious concern that greatly limits their applications and requires the development of effective and safe flame-retardant treatments. In this work, we analysed linen and cotton textile samples before and after COEX® treatment, a patented green technology imparting anti-flame properties by functionalizing the cellulose molecules with phosphorus and sulphur groups. Some of the samples were also exposed to photo-induced ageing after the treatment. The resulting structural and chemical changes in both fibres were characterized by nonlinear optical imaging modalities, namely Second Harmonic Generation (SHG) and Two-Photon Excited Fluorescence microscopies (TPEF), and Raman and Attenuated Total Reflection - Fourier Transform Infrared (ATR-FTIR) spectroscopies. Complementary results evidenced a reduction in microfibril crystallinity, attributed mainly to the reduction of hydrogen bonding among cellulose macromolecules, with a concomitant increase in fluorescence possibly due to the introduction of ester groups into cellulose chains and to decomposition of lignin into fluorescent by-products.

Non-invasive contactless analysis of an early drawing by Raffaello Sanzio by means of optical methods.

Studying highly valuable and fragile Renaissance drawings requires a non-invasive contactless analytical approach. In this work, we study an early drawing by Raffaello Sanzio, one of the most important artists of the Italian Renaissance, realized during his stay in Florence (1504-1508). Our analyses aimed to reveal the features of the paper support significant for its dating, identify and map the drawing media to understand the artist's technical approach, and document the drawing condition with imaging and single-sited optical methods. Reflectance Vis-NIR multispectral imaging spectroscopy elaborated with False-Colour and Principal Component Analysis provided information about the paper support and the drawing media. Laser scanning micro-profilometry and Optical Coherence Tomography allowed revealing the superficial micro-scale features of the support. The chemical composition of the different drawing materials was characterized by µ-Raman spectroscopy, which provided also some evidence of the conservation history of the drawing. Integration of spectroscopic and imaging data shows that Raffaello used different dry drawing media (carbon-based and Pb stylus) to sketch the figure and then refined the details and shadows with iron-gall ink. The paper presents a methodological approach for the analytical examination of fragile paper artworks.