On the Use of Overtone and Combination Bands for the Analysis of the CaSO₄—H₂O System by Mid-Infrared Reflection Spectroscopy

Abstract

With the aim of characterizing ground preparations of paintings by infrared reflection spectroscopy, the CaSO(4)-H(2)O system (gypsum/bassanite/anhydrite) has been re-investigated, evaluating and assigning the SO(4)(2-) and OH overtone and combination bands, respectively, in the ranges 1900-2700 cm(-1) and 5000-6000 cm(-1) resulting from reflection and high concentration transmission spectra. The second-order modes have been proven to be highly specific, reliable, and less affected by overlap with bands of organic binders and can hence be exploited for the identification of the sulfate hydration phase using infrared (IR) reflection spectroscopy. Subsequently, the characterization and identification of hydration phases in unknown sulfate-based ground preparations on authentic artworks have been carried out noninvasively by fiber-optic reflection IR spectroscopy and on cross-sections by infrared reflection micro-spectroscopy. The spectroscopic data collected both on standards and artworks have been cross-validated by X-ray diffraction.

Mapping of egg yolk and animal skin glue paint binders in Early Renaissance paintings using near infrared reflectance imaging spectroscopy

“…2a. 21,25 The sharpness of the bands results from water that is in specic crystallographic sites. The most recognizable features are likely the well-dened bands at 1446, 1490, and 1535 nm due to the rst overtone OH stretch [2n(OH)], 23,24 and the combination band near 1943 nm with a distinct shoulder at 1975 nm due to stretching (asymmetric, n a ) and bending modes of water [(n a + d)-OH].…”

Non-invasive identification of metal-oxalate complexes on polychrome artwork surfaces by reflection mid-infrared spectroscopy

“…For this latter class of materials positive outcome arises from the possibility to employ the combination and/or overtone bands as markers for their identification in artworks [23,29,31]. However, this is not always possible, such as it is for example the case of metal-oxalate complexes.…”

Further Insight into Mesoamerican Paint Technology: Unveiling the Colour Palette of the Pre‐Columbian Codex Fejérváry‐Mayer by Means of Non‐invasive Analysis

“…The gypsum combination bands of OH bending and stretching placed at 5040 and 5150 cm −1 (Rosi et al . ) are always present, although very weak in the grey spectrum dominated by the signals of anhydrite (Fig. (b)).…”

“…In the 2000–2500 cm −1 spectral range, where sulphate combination and overtone bands occur (Rosi et al . ), the light grey spectrum is dominated by the signals of gypsum and the grey one by those of anhydrite, while the black one shows a profile due to the contribution of both (Fig. (b)).…”

“…(b), dashed rectangle) related to the ν 3 antisymmetric stretching of SO 4 2− (Farmer ), appears inverted due to the reststrahlen effect; thus it does not allow for discrimination among different types of calcium sulphate (Rosi et al . ). Diversely, the bands between 600 and 700 cm −1 (Fig.…”