Fiber-Optic Fourier Transform Mid-Infrared Reflectance Spectroscopy: A Suitable Technique for in Situ Studies of Mural Paintings

Abstract

A prototypical in situ noninvasive study of ancient mural painting materials has been carried out using an easily manageable fiber-optic Fourier transform mid-infrared (mid-FT-IR) reflectance spectrophotometer. The reported object of the study is the Renaissance fresco by Pietro Vannucci, called il Perugino, located in the church of Santa Maria delle Lacrime (1521, Trevi, Perugia Italy). For the first classification and interpretation of infrared spectra, principal components analysis was used. Spectral artifacts due to lacunas, restoration materials, or alteration products have been identified, as well as two different secco refinements bound in a tempera medium. For the characterization of inorganic pigments, mid-FT-IR spectra have been integrated with other data obtained through in situ X-ray fluorescence (XRF) elemental analysis. This complementary noninvasive approach led to the characterization of Perugino's pigments, even in the presence of complex mixtures. The mid-FT-IR noninvasive technique, in combination with XRF, is thus recommended as a valuable first approach for the examination of mural paintings, permitting the assessment of the execution technique as well as contributing to the evaluation of the conservation state.

Identification and Preservation of Cultural Heritage

“…gold metallic decorations on paintings. 8 In our experiments, the spectra usually showed typical features of specular reflection and subjecting them to Kramers-Kronig transformation yielded the reflection spectrum at 3535 and 1739 cm -1 as well as in the absorption index spectrum at 684 and 600 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.…”

“…The literature showed that Kramers-Kronig transformation yielded good results processing reflection spectra obtained in a FTIR microscope (0°/0° optical geometry). 15,20,[21][22][23] Evaluation of reflection spectra calculating "pseudo absorbance" A' = log(1/R) was preferably used in research studies applying fibre optic instruments [7][8][9][11][12][13][14]16 combined with PCA (principal component analysis). 4,8…”

“…1 The diamond cell in a FTIR microscope as well achieved with current chalcogenide optical fibres absorbing radiation below 900 cm -1 . 8,16 2 Experimental…”