Non-invasive identification of organic materials in wall paintings by fiber optic reflectance infrared spectroscopy: a statistical multivariate approach

Abstract

The aim of this study is to develop a method for the non-invasive and in situ identification of organic binders in wall paintings by fiber optic mid-FTIR reflectance spectroscopy. The non-invasive point analysis methodology was set-up working on a wide set of wall painting replicas of known composition and using statistical multivariate methods, in particular principal component analysis (PCA), for the interpretation, understanding, and management of data acquired with reflectance mid-FTIR spectroscopy. Results show that PCA can be helpful in managing and preliminary sorting of the large amount of spectra typically collected during non-invasive measurement campaigns and highlight further avenues for research. The developed PCA model was finally applied to the case of a Renaissance wall painting by Perugino assessing it predictability as compared to the interpretation of the single spectrum.

Identification and Preservation of Cultural Heritage

“…These band characteristics seem to originate in a contribution of diffusely reflected radiation, which is increased, if the surface roughness is comparable to the infrared wavelength. 4,10 Below 1700 cm -1 absorption index spectra showed more similarities to the spectra measured in the transmission mode, as shown in the Figures 2 and 3.…”

“…4,8 Actually the reflection spectrum of azurite with Arabic gum clearly showed the aforementioned bands and the ν 3 vibration was registered inverted, as it is the case for other typical bands of azurite at 956 and 838 cm -1 ( Figure 13). Contrary, the absorption index spectrum showed no typical bands for azurite and it may be presumed that the diffuse reflection had a high influence during the measurement of this sample.…”

“…Additionally, the reflection spectra showed two weak bands at 3698 and 3692 cm -1 , which are related to O-H stretching of calcium dihydroxide. 4 The strongest band of ultramarine is the Si-O asymmetric stretching band, showing a maximum at 1012 cm -1 in the IRUG database spectrum MP0008 and 1008 cm -1 in the µ-FTIR spectrum, appeared as a reststrahlen band (inverted) with the maximum at 1008 cm -1 in the reflection spectrum and at 1039 cm -1 in the absorption index spectrum (Figure 8). The second strongest band, which is located outside the range of the database spectrum, is the Si-O bending mode with a maximum at 450 cm -1 .…”

“…In recent years, reflection FTIR instruments for non-destructive in-situ analysis of art objects have been developed and applied, controlling the optical path either by mirrors 2,3 or fibre optics. [4][5][6][7][8][9][10][11][12][13] Other non-destructive techniques such as X-ray fluorescence analysis (XRF) or UV/Vis spectroscopy have been used for the identification of pigments as well, but both have their limitations. On one hand, XRF (in air) only reveals elements with higher atomic numbers than silicon.…”

“…14 For these reasons, FTIR in the reflection mode seems to be the ideal complementary technique to the aforementioned, providing analytical information about pigments and binding media. However, previous studies 4,8,10,13,15 demonstrate some drawbacks of this method mainly concerning the distortion of the spectra in the fingerprint region. These distortions are caused by anomalous dispersion due to variations in the refractive index across an absorption band.…”