The potential of fibre optic reflection infrared spectroscopy for the non-invasive identification of artists’ pigments is presented. Sixteen different carbonate, sulphate and silicate-based pigments are taken into account considering their wide use during the history of art and their infrared optical properties. The infrared distortions mainly generated by the specular reflection are discussed on the basis of experimental measurements carried out on reference samples. The study on pure materials permitted the definition of marker bands, mainly combination and overtone modes, enhanced by the diffuse reflection component of the light, functional for the non-invasive pigment identification in real artworks. Several case studies are reported, including wall, easel, canvas paintings and manuscripts from ancient to modern art demonstrating the strengths of the technique on the identification of pigments even in the presence of complex mixtures of both organic (binders, varnishes) and inorganic (supports, fillers and other pigments) compounds.

Reflection infrared spectroscopy for the non-invasive in situ study of artists’ pigments

In situ chemical imaging techniques are being developed to provide information on the spatial distribution of artists' pigments used in polychrome works of art such as paintings. The new methods include reflectance imaging spectroscopy and X-ray fluorescence mapping. Results from these new methods have extended the knowledge obtained from site-specific chemical analyses widely in use. While these mapping methods have aided in determining the distribution of pigments, there is a growing interest to develop methods capable of identifying and mapping organic paint binders as well. Near infrared (NIR) reflectance spectroscopy has been extensively used in the remote sensing field as well as in the chemical industry to detect organic compounds. NIR spectroscopy provides a rapid method to assay organics by utilizing vibrational overtones and combination bands of fundamental absorptions that occur in the mid-IR. Here we explore the utility of NIR reflectance imaging spectroscopy to map organic binders in situ by examining a series of panel paintings known to have been painted using distemper (animal skin glue) and tempera (egg yolk) binders as determined by amino acid analysis of samples taken from multiple sites on the panels. In this report we demonstrate the success in identifying and mapping these binders by NIR reflectance imaging spectroscopy in situ. Three of the four panel paintings from Cosimo Tura's The Annunciation with Saint Francis and Saint Louis of Toulouse (ca. 1475) are imaged using a highly sensitive, line-scanning hyperspectral imaging camera. The results show an animal skin glue binder was used for the blue skies and blue robe of the Virgin Mary, and egg yolk tempera was used for the red robes and brown landscape. The mapping results show evidence for the use of both egg yolk and animal skin glue in the faces of the figures. The strongest absorption associated with lipidic egg yolk features visually correlates with areas that appear to have white highlights. The results are in agreement with prior site-specific amino acid analysis, underscoring the synergy of both methods. The work here demonstrates that NIR reflectance imaging spectroscopy is a useful technique that can identify and map paint binding media based on differences in chemical composition.

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

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

In this work non-invasive reflection infrared spectroscopy was used to discriminate between different Cu-based green pigments (namely, malachite, verdigris, chrysocolla, emerald green and phthalo green). The pig- ments, chosen because of their historical widespread use in artworks, were investigated as pure powders and in situ on painted models by reflection FTIR spectroscopy. The dis- tortion arising as a result of the specular and diffuse com- ponent of reflected radiation was evaluated as function of the optical and surface properties of the investigated mock- ups. Use of the Kramers-Kronig (KK) algorithm to correct for the distortion arising from the surface reflection gave k index maxima shifted by more than 20 cm �1 relative to those obtained in conventional transmission mode. These findings stress the need to carefully manage use of the KK algorithm on reflection spectra to avoid erroneous assignment. On the other hand, combination and overtone bands (which are enhanced by the diffuse reflection) were proved to enable reliable and sensitive identification of most of the pigments studied. Knowledge acquired by study of pure compounds and painted models was subsequently applied to interpret the spectra acquired from paintings and manuscripts.

In-situ identification of copper-based green pigments on paintings and manuscripts by reflection FTIR

Interpretation of mid and near-infrared reflection properties of synthetic polymer paints for the non-invasive assessment of binding media in twentieth-century pictorial artworks

With the aim of characterizing ground preparations of paintings by infrared reflection spectroscopy, the CaSO4–H2O system (gypsum/bassanite/anhydrite) has been re-investigated, evaluating and assigning the SO42− 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.

Sabrina Nazzareni

Papers

On the use of overtone and combination bands for the analysis of the CaSO4-H2O system by mid-infrared reflection spectroscopy.