In situ Raman spectroscopy analysis combined with Raman and SEM‐EDS imaging to assess the conservation state of 16th century wall paintings

Abstract

A multianalytical methodology based on Raman spectroscopy was proposed to carry out the analysis of two wall paintings located in Saint Andrew Church (Biañez, Biscay) and Saint John the Baptist Church (Axpe, Biscay). On the one hand, Raman, assisted with energy-dispersive X-ray fluorescence and diffuse reflectance infrared portable spectrometers, was used in the in situ analysis of original support materials and pigments as well as in the identification of the degradation products. Such portable spectroscopic techniques allow identifying the areas to be sampled in order to perform deeper analyses by means of Raman chemical imaging and scanning electron microscope equipped with an energy dispersive spectrometer. Cross-sections were performed with the micro-samples taken in the areas of interest. Thereby, the colour palette, the mortar, the restoration processes and the degradation products were determined. The Raman analyses revealed that the transformation of gypsum into anhydrite in the intonaco layer was responsible for the detachment observed in the wall paintings from Axpe. Several hypotheses such as the harmful effect of previous restorations have been considered as precursor of the damage. Besides, decaying products, such as nitrates and oxalates, derived from physicochemical processes in the raw materials were also detected.

Raman and SEM‐EDX analyses of the ‘Royal Portal’ of Bordeaux Cathedral for the virtual restitution of the statuary polychromy

“…Working outside the sunlight can be a problem because some spectral interference can arise. [21] In the laboratory, in contrast, Raman spectra are acquired in dark.…”

“…open atmosphere, vibrations, sunlight and low working temperatures) and usually working within a limited time and space. [21,22] There are some studies where advantages and drawbacks of modern portable Raman instruments are described in the field of cultural heritage, [23][24][25][26] as well as in the field of geosciences, [27] pointing out the role of the focal plane precision, how the exposure time of an individual scan affects the signal-to-noise ratio and how the intensity of the background ambient light may be reflected in the spectral quality and instrumental performance.…”

“…In those cases, it has to be supported by cross-sectional analysis in the laboratory. [21] Despite the fact that sampling for cross-sectional analysis is micro-destructive, it is often necessary as it provides the chance to obtain more information regarding the raw materials used in the original artwork. But even in this case, in situ analysis is very useful because it helps in the selection of the more interesting sampling points.…”

Spectroscopic analysis used to uncover the original paint colour of the Helsinki Government Palace tower clock faces

“…Therefore, if the composition of individual layer of a multi-layered sample is required, e.g., paint chip, then, destructive sampling techniques is required [15]. This provides an opportunity to obtain information regarding the raw materials used in the original artwork [16].…”

“…In addition, chemical imaging on polished paint cross-sections is an effective technique capable of visually displaying the distribution of numerous chemical components in a sample [22,23]. For instance, the different layers of a painting can be identified and characterised [15]. These techniques are not sample consuming techniques and allow the opportunity for further analysis to be undertaken with other techniques.…”