Fiber optics reflectance spectroscopy (FORS) is commonly used to non-invasively identify madder- and cochineal-based pigments on works of art, but the significant shifts sometimes observed in the position of their diagnostic absorption features can hinder correct interpretation of the spectra. To better understand these shifts, and improve the ability to confidently identify these pigments, a systematic study was carried out to evaluate the effects of different pigment recipes and laking substrates on reflectance spectra. Sixteen different madder- and cochineal-based pigments were synthesized using historical recipes. Each pigment, painted in four different binding media (gum Arabic, linseed oil, beeswax, and egg yolk), was fully characterized by FTIR and HPLC-DAD-MS prior to FORS measurements. The results of the study showed that, in contrast to the absorption features typically used for identification, features in the first derivative transformation of the FORS spectra provided a more robust means of primary identification. In addition, once it has been identified as cochineal, the absorption features in the spectra of cochineal-based pigments could be correlated to the recipe employed, providing a possible means for inferring the method of manufacture and laking substrate from a non-invasive analysis. The results of this study were used to create a decision tree for the identification of madder and cochineal pigments based solely on FORS.

Paints based on cadmium sulfide (CdS) were popular among artists beginning in the mid-19th century. Some paint formulations are prone to degrade, discoloring and disfiguring paintings where they have been used. Pablo Picasso’s Femme (Époque des “Demoiselles d’Avignon”) (1907) includes two commercial formulations of CdS: one is visibly degraded and now appears brownish yellow, while the other appears relatively intact and is vibrant yellow. This observation inspired the study reported here of the photoluminescence emission from trap states of the two CdS paints, complemented by data from multispectral imaging, X-ray fluorescence spectroscopy, micro-FTIR, and SEM-EDS. The two paints exhibit trap state emissions that differ in terms of spectrum, intensity, and decay kinetics. In the now-brownish yellow paint, trap state emission is highly favored with respect to near band edge optical recombination. This observation suggests a higher density of surface defects in the now-brownish yellow paint that promotes the surface reactivity of CdS particles and their subsequent paint degradation. CdS is a semiconductor, and surface defects in semiconductors can trap free charge carriers; this interaction becomes stronger at reduced particle size or, equivalently, with increased surface to volume ratio. Here, we speculate that the strong trap state emission in the now-brownish cadmium yellow paint is linked to the presence of CdS particles with a nanocrystalline phase, possibly resulting from a low degree of calcination during pigment synthesis. Taken together, the results presented here demonstrate how photoluminescence studies can probe surface defects in CdS paints and lead to an improved understanding of their complex degradation mechanisms.

The cultural heritage community is increasingly exploring synchrotron radiation (SR) based techniques for the study of art and archaeological objects. When considering heterogeneous and complex micro-samples, such as those from paintings, the combination of different SR X-ray techniques is often exploited to overcome the intrinsic limitations and sensitivity of the single technique. Less frequently, SR X-ray analyses are combined with SR micro-photoluminescence or micro-Fourier Transform Infrared spectroscopy, which provide complementary information on the molecular composition, offering a unique integrated analysis approach. Although the spatial correlation between the maps obtained with different techniques is not straightforward due to the different volumes probed by each method, the combination of the information provides a greater understanding and insight into the paint chemistry. In this work, we discuss the advantages and disadvantages of the combination of X-ray techniques and SR-based photoluminescence through the study of two paint micro-samples taken from Pablo Picasso's Femme (1907). The painting contains two cadmium yellow paints (based on CdS): one relatively intact and one visibly degraded. SR micro-analyses demonstrated that the two Cd-yellow paints differ in terms of structure, chemical composition, and photoluminescence properties. In particular, on the basis of the combination of different SR measurements, we hypothesize that the degraded yellow is based on nanocrystalline CdS with high presence of Cd(OH)Cl. These two characteristics have enhanced the reactivity of the paint and strongly influenced its stability.

The element‐specific distribution maps generated by scanning macro‐X‐ray fluorescence (XRF) spectroscopy are providing cultural heritage researchers with information about the composition of materials present in works of art and, more importantly, unprecedented insight into the techniques employed by artists in their creation. One of the advantages of macro‐XRF scanning is that the X‐rays probe materials in subsurface layers, allowing, for example, visualizations of hidden paintings to be produced. Consequently, macro‐XRF scanning has found wide use in the study of paintings, but the high spatial resolution also makes it particularly well suited for the study of the small‐scale painted illustrations and decorative elements found in illuminated manuscripts. The preliminary drawings made by manuscript illuminators to plan a painted composition—known as underdrawings—provide evidence relating to artists' creative vision and working process but are difficult to examine because they are generally hidden under the surface paint layer. Traditionally, underdrawings in a carbon‐based medium are visualized using infrared (IR) imaging. In this study, results of the analysis of painted illuminations from medieval illuminated manuscripts demonstrate that macro‐XRF scanning can visualize underdrawings in other materials, such as iron‐gall ink, metalpoint, and pigmented inks/paints, thus serving as a useful complement to traditional IR imaging. For manuscript illuminations in bound books, this study also discusses the use of interleaving materials to reduce unwanted signals from underlying folios. The ability to reveal manuscript underdrawings will help elucidate artistic intent and workshop practice and provide a new way to examine the history of medieval drawing.

Over the past several decades the painting An Old Man in Military Costume by Rembrandt Harmensz van Rijn (ca. 1630–31; J. Paul Getty Museum, 78.PB.246) has been the subject of a number of investigations carried out in order to better visualize a second painting beneath the surface figure. The underlying image – the head and shoulders of a man wearing a cloak – is oriented 180 degrees from the upper image and appears to be fairly complete. Scanning macro x-ray fluorescence (XRF) spectroscopy reveals the face is painted with lead white and a mercury-containing pigment (likely vermilion), and the cloak is painted with a copper-containing pigment. Following the revelation and digital color reconstruction of the underlying figure, a number of questions still remained. Here, through the use of infrared reflectance imaging spectroscopy (i.e., hyperspectral imaging) and macro-XRF imaging spectroscopy, together with cross-sections taken from targeted areas, the sequence of painting in both compositions was explored. Of particular interest was the discovery of evidence of multiple attempts to situate the lower figure, and the subsequent application of a blocking-out layer over the lower figure before the artist rotated the panel and executed the upper figure. In addition, examination of the placement of the two images on the panel adds to our understanding of the subtle complexities of Rembrandt's working process.

The size, shape, and application techniques of gold leaf in fourteenth- and fifteenth-century gold ground panel paintings attributed to artists working in Florence, Siena, and Fabriano was directly visualized using in situ scanning macro X-ray fluorescence spectroscopy (MA-XRF). The resulting gold (Au) Lα (9.712 keV) element distribution maps are of sufficient spatial resolution to enable an accurate and reproducible measurement of the shape and size of individual gold leaves. The resulting leaf measurements are discussed in the context of historical guild regulations, in particular, the 1403 reforms to the statutes of the Florentine Guild of Doctors and Apothecaries (Arte dei medici e speziale), which standardized the dimensions of gold leaf produced in Florence. The dimensions of the gold leaf from Florentine paintings created before and after the 1403 reforms were compared to each other as well as to gold ground paintings created in Siena and Fabriano. The results revealed the gold leaf squares in fourteenth-century Florentine and Sienese panels had side lengths averaging about 8 cm. In contrast, the gold leaf squares used in the fifteenth-century Florentine and Sienese paintings were smaller, with side lengths measuring about 7 cm. In addition, the degree of overlap between adjacent gold leaves was also measured. The amount of overlap was found to vary between artists, and the degree of overlap was consistent within the oeuvre of a specific artist. Taken together, these results suggest that the dimensions of the gold leaves found in panel paintings relate to the place of production on the Italian peninsula and the period in which they were created, while the degree of overlap relates to the individual hand of a gilder or artist/workshop.