Francesca Rosi

In Situ Noninvasive Study of Artworks: The MOLAB Multitechnique Approach

Driven by the need to study precious and irreplaceable artworks without compromising their integrity, researchers have undertaken numerous efforts to develop noninvasive analytical tools and methodologies that can provide a chemical description of cultural heritage materials without any contact with the object. The challenge is that artworks are made of complex mixtures, often with heterogeneous and unknown layered materials. Their components must be identified over a range of size scales, from the molecular identification of constituent compounds to the mapping of alteration phases. In this Account, we review recent research in spectroscopic techniques accessible from the mobile laboratory (MOLAB). The lab is equipped with an array of state-of-the-art, portable, and noninvasive instruments specifically tailored to tackle the different issues confronted by archaeologists, curators, and conservators. The MOLAB approach is suitable for studying a variety of objects, from ceramics to manuscripts or from historical wall paintings to contemporary canvases. We begin by discussing issues related to the acquisition and interpretation of reflectance or backscattered spectra from the surface of heterogeneous materials. Then we show how the selectivity needed for the noninvasive identification of pigments in paintings, even in mixtures or in layered matrices, can be acquired by combining elemental information from X-ray fluorescence with molecular and structural insights from electronic and vibrational spectroscopies. Discriminating between original pigments and restoration retouches is possible, even when both comprise similar chromophores, as highlighted in the study of paintings by Jordaens and Raphael. The noninvasive approach permits the examination of a very large number of artworks with a virtually limitless number of measurements. Thus, unexpected and uncommon features may be uncovered, as in the case of a lead pyroantimonate yellow doped with zinc that was discovered by micro-Raman and X-ray fluorescence on an Italian Renaissance majolica. For characterizing binding media, we discuss the strengths and limitations of using mid- and near-FTIR (Fourier transform infrared) spectroscopies supported by a multivariate statistical analysis, detailing the study of organic materials in a wall painting by Perugino and a survey of the painting technique on 18 contemporary paintings by Burri. In Michelangelos David, we show how the noninvasive mapping of contaminants and alteration phases might inform decisions on preventive conservation plans. The multitechnique MOLAB approach overcomes the intrinsic limitation of individual spectroscopic methods. Moreover, the ability to analyze artworks without the need to move them is an invaluable asset in the study and preservation of cultural heritage.

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

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.

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

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 Peruginos 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.

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

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.

Noninvasive Analysis of Paintings by Mid‐infrared Hyperspectral Imaging

In recent years, in situ noninvasive spectroscopy has played an increasingly important role in art conservation. Spectroscopic methods can be used to gain a deep understanding of the material composition of art objects while fully respecting their integrity and value. Considerable improvements in detector technology, instrument–computer interfacing, focusing optics, and the performance of radiation sources have been made in the spectroscopic field and have led to the development and successful application of a series of noninvasive and portable analytical tools for point examination. In this context, current scientific interest is focused on the development of mapping/imaging multi-/hyperspectral methods, since area examination naturally meets the demands of a holistic art-historical approach by revealing not only the chemical composition of painting materials but also their semiquantitative spatial distribution with respect to what is visible to the naked eye. Recently, the possibility of mapping elemental distribution on paintings by means of a portable scanning macro X-ray fluorescence device was demonstrated to be useful for the investigation of the materials used by artists. The molecular identification and spatial distribution of a number of pigments can be inferred from reflection imaging in the visible (Vis: 400–750 nm) and near-infrared regions (NIR: 750–2500 nm) by combining information on electronic transitions in the visible range with overtone and combination vibrational bands in the near-infrared range which are associated with inorganic pigments containing hydroxy groups, such as lead white, azurite, and gypsum. The identification and mapping of organic compounds is a challenge for the noninvasive analysis of artworks. In comparison with inorganic pigments and fillers, there is a larger variety of organic compounds, and they are present in smaller amounts and more subject to chemical alteration. If both traditional and modern art are taken into consideration, just some of the organic compounds to be identified and localized may include: drying oils, proteins, acrylics, alkyds, polyvinyl acetates, natural and synthetic waxes, terpene resins, and natural and synthetic dyes. Recently, Ricciardi et al. demonstrated the potential of hyperspectral imaging in the NIR region (10000–4000 cm ) to discriminate between lipid and proteinaceous binders on illuminated manuscripts on the basis of the vibrational combination/overtone bands of methylene groups and amides. In contrast, the mid-infrared (MIR) range has not yet been exploited for the spatially resolved remote study of artworks, although the MIR range (4000–600 cm ) has proved to be very informative for the identification of artists materials by noninvasive point analysis 12] or microinvasively by cross-section imaging. Herein, we describe the potential of imaging MIR spectroscopy for painting analysis through the application of a novel hyperspectral imaging system (HI90, Bruker Optics). The system developed for the remote identification and mapping of hazardous compounds was adapted in this study for reflection measurements of paintings by the use of an external infrared radiation source. The subject of our study was a painting by Alberto Burri, Sestante 10 (1982), which is currently exhibited at the Ex-Seccatoi del Tabacco (Perugia, Italy). Three areas (ca. 9 9 cm) of the large painting (250 360 cm) were analyzed on site with the HI90 system. We also investigated several points within the same areas noninvasively with a portable FTIR spectrometer (Alpha-R, Bruker Optics) to validate the assignment made on the basis of the HI90 spectral data. In Figure 1a, the visible image of the investigated area I of Sestante 10 is shown. The resulting brightness temperature difference images in Figure 1b,c clearly highlight the use of a different binder for the orange sector to that used for the other sectors. More precisely, the image depicted in Figure 1b shows the false-color representation of the difference in the mean brightness temperature for a peak in the signature of the orange area (1154–1167 cm ) and the mean brightness temperature for a frequency range in which the reflectance is lower (1197–1209 cm ; see range (b) in Figure 1d). The area in the brightness temperature image in Figure 1b indicates the presence of an acrylic binder, as suggested by a comparison with the spectrum recorded with the HI90 instrument [*] Dr. F. Rosi, Dr. C. Miliani Istituto di Scienze e Tecnologie Molecolari CNR-ISTM, SMAArt Via Elce di sotto, 9 Perugia 06123 (Italy) E-mail: costanza.miliani@cnr.it

Multivariate chemical mapping of pigments and binders in easel painting cross-sections by micro IR reflection spectroscopy

AbstractPaintings are composed of superimposed layers of inorganic and organic materials (pigments and binders). Knowledge of the stratigraphic sequence of these heterogeneous layers is fundamental for understanding the artist’s painting technique and for conservation issues. In this study, micro-IR mapping experiments in reflection mode have been carried out on cross-sections taken from simulations of ancient easel paintings. The objective was to locate both organic binders and inorganic pigments. Chemical maps have been re-constructed using the common approach based on the integration of specific infrared bands. However, owing to the complexity of painting materials, this approach is not always applicable when dealing with broad and superimposed spectral features and with reststrahlen or derivative-like bands resulting from acquisition in reflection mode. To overcome these limitations, principal-component analysis has been successfully used for the re-construction of the image, extracting the relevant information from the complex full spectral data sets and obtaining reliable chemical distributions of the stratigraphy materials. Different pigment–binder combinations have been evaluated in order to understand limitations and strengths of the approach. Finally, the method has been applied for stratigraphic characterization of a cross-section from a 17th century wooden sculpture identifying both the original paint layer and the several overpaintings constituting the complex stratigraphy. Figurea) OM image of an easel painting cross-section with the grid indicating the IR mapped area; b) t1 vs. t2 score plot including the color bar referred to the score values along each components; c) t2-score map; d) p1 and p2 profiles from PCA (labels: Gy-gypsum, L-lipid, I-amide I, II-amide II, U-ultramarine blue)

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

In this work a reflection mid-infrared spectroscopy study of twelve metal-oxalate complexes, of interest in art conservation science as alteration compounds, was performed. Spectra of the reference materials highlighted the presence of derivative-like and/or inverted features for the fundamental vibrational modes as result of the main contribution from the surface component of the reflected light. In order to provide insights in the interpretation of theses spectral distortions, reflection spectra were compared with conventional transmission ones. The Kramers-Kronig (KK) algorithm, employed to correct for the surface reflection distortions, worked properly only for the derivative-like bands. Therefore, to pay attention to the use of this algorithm when interpreting the reflection spectra is recommended. The outcome of this investigation was exploited to discriminate among different oxalates on thirteen polychrome artworks analyzed in situ by reflection mid-infrared spectroscopy. The visualization of the νs(CO) modes (1400-1200 cm(-1)) and low wavenumber bands (below 900 cm(-1)) in the raw reflection profiles allowed Ca, Cu and Zn oxalates to be identified. Further information about the speciation of different hydration forms of calcium oxalates were obtained by using the KK transform. The work proves reflection mid-infrared spectroscopy to be a reliable and sensitive spectro-analytical method for identifying and mapping different metal-oxalate alteration compounds on the surface of artworks, thus providing conservation scientists with a non-invasive tool to obtain information on the state of conservation and causes of alteration of artworks.

Material analyses of ‘Christ with singing and music-making Angels’, a late 15th-C panel painting attributed to Hans Memling and assistants: Part I. non-invasive in situ investigations

In cultural heritage science, compositional data is traditionally obtained from works of art through the analysis of samples by means of various bench-top instruments (scanning electron microscope, Raman spectrometer, etc.). Alternatively, the object can be transported to a laboratory where it may be examined, usually by spectroscopic methods working in reflection mode. However, this paper describes how a complementary set of mobile and portable instruments was deployed in situ to gain a comprehensive view on the materials and related ageing compounds of an (almost) unmovable 15th-C polyptych, prior to and in preparation of the extraction of a limited number of samples. In line with the methodological approach discussed, PXRF was first employed as an efficient screening tool. The ensuing elemental data was supplemented by more specific information on both organic as inorganic materials supplied by reflection near- and mid-FTIR spectroscopy and fluorimetry. In completion, a limited number of diffraction patterns were collected with a mobile XRD instrument in order to identify the constituent crystalline phases in pigments, grounding materials and degradation products. In this way, it could be demonstrated how a rich array of colours was obtained by means of a limited palette of pigments: lead white, lead tin yellow, azurite, natural ultramarine, bone black, vermillion, madder lake, and a green copper-organo complex were detected and situated on the panels. Remarkably, next to chalk also gypsum was found in the ground layer(s) of this Western European easel painting. The relatively large surface of the background was covered with gold leaf; the analyses seem to point towards the labour-intensive water gilding technique. The versatility of this combination of analytical techniques was further illustrated by the accurate characterisation of degradation products affecting the readability and conservation of the painting: the overall presence of a calcium oxalate-based film of variable thickness was established. Nevertheless, further analysis of cross-sectioned samples was considered desirable in order to study the stratigraphy, to gain direct access to altered and sub-imposed layers and to allow highly detailed analysis of micrometric degradation products by state-of-the art techniques (i.e. synchrotron radiation).

Photoluminescence Properties of Zinc Oxide in Paints: A Study of the Effect of Self-Absorption and Passivation

Zinc oxide has been widely used as a white artist pigment since the end of the eighteenth century. The luminescence properties of this compound have received great interest during the last decades for promising applications in different fields of material science, but their diagnostic implications in the cultural-heritage context have been poorly exploited. This paper is intended to provide a clear picture of the luminescence behavior of zinc white in oil paintings. With this aim, three white pigments and three highly pure (analytical grade) zinc oxides were studied as powder substrates and as painting models by ultraviolet–visible (UV–VIS) fluorescence and Fourier transform infrared (FT-IR) spectroscopy. The quenching of the luminescence intensity of the UV excitonic emission due to self-absorption and multiple scattering phenomena has been investigated, pointing out the possible difficulty of detecting this signal with negative consequences in the diagnostics of works of art. By contrast, the UV emission is notably enhanced by interaction with the binder, whereas the visible emission decreases. This phenomenon is probably due to the formation of covalent bonds between zinc atoms and carboxylates from the lipidic medium that are chemisorbed on zinc oxide surfaces.

Modified Naples yellow in Renaissance majolica: study of Pb–Sb–Zn and Pb–Sb–Fe ternary pyroantimonates by X-ray absorption spectroscopy

X-Ray absorption spectroscopy investigations combined with ab initio structural simulations of ternary forms (Pb–Sb–Zn and Pb–Sb–Fe) of the pigment Naples yellow (Pb2Sb2O7) allowed to achieve a detailed insight into the incorporation route of Zn and Fe ions inside the pigment lattice. The study of newly synthesised yellow pyroantimonates provided direct evidence that Zn and Fe ions enter the pyrochlore octahedral sites replacing Sb ions. Experimental results were discussed considering, also, the information provided by Raman spectroscopy used in this study as a sensitive probe of the pyrochlore lattice distortions in modified yellow pyroantimonates. Finally, by exploiting the non-invasiveness of the X-ray absorption spectroscopy technique, the same structural arrangement has been unambiguously observed for Zn cations in a modified Naples yellow pigment of a Renaissance ceramic shard.