Raman microscopy and x-ray fluorescence analysis of pigments on medieval and Renaissance Italian manuscript cuttings

High-resolution compound-specific mapping in works of art via data fusion of MA-XRPD with hyperspectral data (part 1: Method evaluation)

BACKGROUND

Hyperspectral imaging techniques have emerged as powerful tools for non-invasive investigation of artworks. This paper employs either reflectance imaging spectroscopy (RIS) or macroscopic X-ray fluorescence (MA-XRF) imaging in combination with macroscopic X-ray powder diffraction (MA-XRPD) for state-of-the-art chemical imaging of painted cultural heritage artefacts. While RIS can provide molecular information and MA-XRF can offer elemental distribution maps of paintings of high lateral resolution, the unique advantage of MA-XRPD lies in its ability to visualize the distributions of specific pigments and estimate in a quantitative manner the relative concentrations of the crystalline phases at the surface of artworks. However, MA-XRPD is more time-consuming and offers a lower lateral resolution than RIS and MA-XRF.

RESULTS

This study introduces a machine learning (ML) approach to obtain the distribution of specific compounds on the surface of artworks with a resolution that is comparable to that of RIS and MA-XRF data but with the compound specificity of MA-XRPD. The general aim is to expedite non-destructive artwork imaging analysis by fusing data from different imaging modalities via machine learning models. The effect of preprocessing techniques to enhance the predictive accuracy of the models is explored. The paper demonstrates the method's efficacy on a 16th-century illuminated manuscript, showcasing the feasibility of predicting compound-specific distribution maps. Three evaluation methods-visual examination of the predicted distribution, root mean square errors (RMSE), and feature permutation importance (FPI)-are employed to assess model performance. Fusing MA-XRF with MA-XRPD led to the best RMSE scores overall. However, fusing the RIS and MA-XRPD data blocks also yield very satisfactory and easily interpretable high-resolution compound maps.

SIGNIFICANCE

While MA-XRPD allows for highly specific imaging of artworks, its time-consuming nature and limited resolution presents a bottleneck during non-invasive imaging of painted works of art. By integrating data from more time-efficient hyperspectral techniques such as MA-XRF and RIS, and employing machine learning, we expedite the process without compromising accuracy. The fusion process can also denoise the distribution maps, improving their readability for heritage professionals and art historical scholars.

Chemical characterisation of artists' spray-paints: A diagnostic tool for urban art conservation

In this study the chemical characterisation of 24 commercial spray-paints in different colours as used in contemporary public murals, street art, and graffiti is presented. The analyses were focused on the identification of the binding media, pigments, and additives. In addition, four spray-paint samples were analysed in the form of bi-layered paint films to explore the possibility of determining the composition of multi-layered samples. The aim of the study was to provide a useful diagnostic tool for the conservation of spray-paints and the removal of overpaintings from both commissioned murals and any other form of cultural heritage. To achieve this goal, a multi-analytical approach was developed using Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) for the identification of the main binder, pigments, and fillers/extenders, while Raman spectroscopy and Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy (SEM/EDS) were used as complementary tools for the determination of organic and inorganic pigments, and fillers. Five kinds of binders were detected in this work: (1) acrylic resins combined with nitrocellulose, (2) acrylic resins modified with styrene and combined with nitrocellulose, (3) alkyd resins modified with styrene and combined with nitrocellulose, (4) combined acrylic and alkyd resins modified with styrene and blended with nitrocellulose, and (5) combined polystyrene and acrylic resins. Also, a wide variety of organic pigments and inorganic components were detected.

Analysis of Iron Complexes of Tannic Acid and Other Related Polyphenols as Revealed by Spectroscopic Techniques: Implications in the Identification and Characterization of Iron Gall Inks in Historical Manuscripts

In this work, a structural analysis of the polyphenol complexes with iron and copper at several conditions is reported. The investigated polyphenols were tannic acid, gallic acid, pyrogallol, and syringic acid, being components and molecular models of the gallnuts usually employed in the past in fabrication of iron gall inks (IGIs). Commercial tannic acid extracted from gallnuts, which is a complex mixture of different gallotannins and simpler galloylglucoses, was also employed in this analysis. This analysis comprised the use of Raman, Fourier-transform infrared (FTIR), UV-vis absorption, and fluorescence spectroscopy. The complexation of iron with these molecules leads to a strong change in color due to the deep restructuring of the polyphenol that can be clearly seen by Raman and FTIR spectra. Three main Raman bands appeared at 1450-1490 cm^-1 (ν₁), 1320-1345 cm^-1 (ν₂), and 400-650 cm^-1 (ν₃), which are characteristic of the metal complexes. The structural changes of the polyphenol complexes with iron were also investigated at different pHs and different polyphenol/iron stoichiometries. Other effects of the interaction of polyphenols with iron are the pH decrease of the mixture upon metal complexation and fluorescence quenching induced by the interaction of iron. This quenching is important since it facilitates the Raman inspection of manuscripts since polyphenols show a strong fluorescence emission that overlaps the Raman spectrum. Furthermore, DFT calculations were performed for the first time on the gallic acid complex with iron in order to elaborate a detailed assignment of the vibrational modes of polyphenols and their metal complexes, something that was missed in previous applications of Raman to IGIs.

Antimony and Nickel Impurities in Blue and Green Copper Pigments

Impurities in paint layers executed with green and blue copper pigments, although relatively common, have been studied only little to date. Yet, their proper identification is a powerful tool for classification of paintings, and, potentially, for future provenance studies. In this paper, we present analyses of copper pigments layers from wall paintings situated in the vicinity of copper ore deposits (the palace in Kielce, the palace in Ciechanowice, and the parish church in Chotków) located within the contemporary borders of Poland. We compare the results with the analyses of copper minerals from three deposits, two local, and one historically important for the supply of copper in Europe, i.e., Miedzianka in the Holy Cross Mountains, Miedzianka in the Sudetes, and, as a reference, Špania Dolina in the Slovakian Low Tatra. Optical (OM) and electron microscopy (SEM-EDS), Raman spectroscopy, and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) have been used for a detailed investigation of the minute grains. Special attention has been devoted to antimony and nickel phases, as more unusual than the commonly described iron oxides. Analyses of minerals from the deposits helped to interpret the results obtained from the paint samples. For the first time, quantitative analyses of copper pigments’ impurities have been described.

Review of Methods for Documentation, Management, and Sustainability of Cultural Heritage. Case Study: Museum of King Jan III’s Palace at Wilanów

All countries around the world are blessed with particularly rich cultural heritage. Nowadays, many researchers are exploring different methods for documentation, management, and sustainability of cultural heritage. The aim of this article is to review the state-of-the-art documentation, management, and sustainability techniques in the field of cultural heritage based on the case study in the Museum of King Jan III’s Palace at Wilanów. Various 2D/3D image and range-based methods are discussed demonstrating their applications and drawbacks. The geographical information system (GIS) is presented as a method for management, storage, and maintenance of cultural heritage documentation.

Minimally-destructive atmospheric ionisation mass spectrometry authenticates authorship of historical manuscripts

Authentic historic manuscripts fetch high sums, but establishing their authenticity is challenging, relies on a host of stylistic clues and requires expert knowledge. High resolution mass spectrometry has not, until now, been applied to guide the authentication of historic manuscripts. Robert Burns is a well-known Scottish poet, whose fame, and the eponymous 'Burns Night' are celebrated world-wide. Authenticity of his works is complicated by the 'industrial' production of fakes by Alexander Smith in the 1890s, many of which were of good quality and capable of fooling experts. This study represents the first analysis of the inks and paper used in Burns poetry, in a minimally destructive manner that could find application in many areas. Applying direct infusion mass spectrometry to a panel of selected authenticated Burns and Smith manuscripts, we have produced a Support Vector Machine classifier that distinguishes Burns from Smith with a 0.77 AUC. Using contemporary recipes for inks, we were also able to match features of each to the inks used to produce some of Burns' original manuscripts. We anticipate the method and classifier having broad application in authentication of manuscripts, and our analysis of contemporary inks to provide insights into the production of written works of art.

Looking for Common Fingerprints in Leonardo's Pupils Using Nondestructive Pigment Characterization

Non-invasive, portable analytical techniques are becoming increasingly widespread for the study and conservation in the field of cultural heritage, proving that a good data handling, supported by a deep knowledge of the techniques themselves, and the right synergy can give surprisingly substantial results when using portable but reliable instrumentation. In this work, pigment characterization was carried out on 21 Leonardesque paintings applying in situ X-ray fluorescence (XRF) and fiber optic reflection spectroscopy (FORS) analyses. In-depth data evaluation allowed to get information on the color palette and the painting technique of the different artists and workshops . Particular attention was paid to green pigments (for which a deeper study of possible pigments and alterations was performed with FORS analyses), flesh tones (for which a comparison with available data from cross-sections was made), and ground preparation.

Pigment analysis by Raman microscopy and portable X-ray fluorescence (pXRF) of thirteenth to fourteenth century illuminations and cuttings from Bologna

Non-destructive pigment analysis by Raman microscopy (RM) and portable X-ray fluorescence (pXRF) has been carried out on some Bolognese illuminations and cuttings chosen to represent the beginnings, evolution and height of Bolognese illuminated manuscript production. Dating to the thirteenth and fourteenth centuries and held in a private collection, the study provides evidence for the pigments generally used in this period. The results, which are compared with those obtained for other north Italian artwork, show the developments in usage of artistic materials and technique. Also addressed in this study is an examination of the respective roles of RM and pXRF analysis in this area of technical art history.This article is part of the themed issue 'Raman spectroscopy in art and archaeology'.

Circularly polarized luminescence reveals interaction between commercial stains and protein matrices used in paintings

Here we show that circularly polarized luminescence (CPL) can give unique insight into interactions between fluorescent commercial stains and protein-based materials used in painting. CPL can complement information from ECD and fluorescence.

Exploiting external reflection FTIR spectroscopy for the in-situ identification of pigments and binders in illuminated manuscripts. Brochantite and posnjakite as a case study

In the present work, the use of portable instrumentation allowing in-situ reflection FTIR analyses is exploited to identify the coloring matters of northern-Italian illuminations dating to the XVI century. In order to build a database of spectra, reference paint samples were prepared spreading the pigments on parchment with two different binders, i.e. gum arabic and egg white, used in antiquity. Pigments for the database were chosen considering their use in the Middle Ages and in the Renaissance and their response in the mid- and near-IR region. The reflection FTIR spectra obtained resulted to be dominated by the specular reflection component, allowing the use of the Kramers-Kronig transform to convert them to the more conventional absorbance FTIR spectra. Several pigments could thus be identified in ancient illuminations, even if some green details showed a spectral pattern different with respect to the most common commercial green pigments of the database. Therefore, in addition, basic copper sulfates brochantite and posnjakite were synthesized and characterized. In three green details, posnjakite was identified, both as a pure compound and together with malachite.

Raman spectroscopy analysis of pigments on 16-17th c. Persian manuscripts

The palette of four Persian manuscripts of the 16th and 17th centuries were established by Raman microscopy to include lazurite, red lead, vermilion, orpiment, a carbon-based black, lead white, malachite, haematite, indigo, carmine and pararealgar. The first five pigments were identified on all four manuscripts, as previously found for other Islamic manuscripts of this period. The findings were compared with information available in treatises on Persian painting techniques. Red lead, although identified on all of the manuscripts analysed in this study as the main red pigment, is seldom mentioned in the literature. Two unusual pigments were also identified: the intermediate phase between realgar and pararealgar in the manuscript Timur namah, and carmine in the manuscript Shah namah. Although the established palette comprises few pigments, it was found that the illuminations were enhanced by the use of pigment mixtures, the components of which could be identified by Raman microscopy.