Francesca Gabrieli

Disclosing Jackson Pollock’s palette in Alchemy (1947) by non-invasive spectroscopies

Alchemy (1947, Peggy Guggenheim Collection, Venice) is one of the most materic works by J. Pollock, whose palette is extensive, ranging from white to yellow, red, green, violet, blue, black, and silver. Each layer of color was laid on top of a previously dried one and effectively separated from the lower one forming a quite complex stratigraphy with colors intersecting each other. In this study, a non-invasive multi-technique method combining point analysis with Vis–NIR multispectral imaging has been exploited to give insights on the painting technique of the American abstract expressionist. The molecular identification of pigments, colorants and extenders contained in fifteen different paints has been achieved combining key spectral markers from elemental, electronic and vibrational spectroscopies. For those colors exhibiting similar hues but different chemical compositions, a mapping procedure based on false color rendering, obtained by properly mixing three spectral planes from the Vis–NIR multispectral imaging set, has been successfully applied. Relevant for the understanding of the evolution of Pollock’s drip technique is the identification of both traditional oil-based paints and oil-modified alkyd media. Point analysis by reflection FTIR scattered throughout the painting enabled mapping the distinct use of traditional and new binding media among painted, squeezed and dripped paints.

Revealing the Nature and Distribution of Metal Carboxylates in Jackson Pollock’s Alchemy (1947) by Micro-Attenuated Total Reflection FT-IR Spectroscopic Imaging

Protrusions, efflorescence, delamination, and opacity decreasing are severe degradation phenomena affecting oil paints with zinc oxide, one of the most common white pigments of the 20th century. Responsible for these dramatic alterations are the Zn carboxylates (also known as Zn soaps) originated by the interaction of the pigment and the fatty acids resulting from the hydrolysis of glycerides in the oil binding medium. Despite their widespread occurrence in paintings and the growing interest of the scientific community, the process of formation and evolution of Zn soaps is not yet fully understood. In this study micro-attenuated total reflection (ATR)-FT-IR spectroscopic imaging was required for the investigation at the microscale level of the nature and distribution of Zn soaps in the painting Alchemy by J. Pollock (1947, Peggy Guggenheim Collection, Venice) and for comparison with artificially aged model samples. For both actual samples and models, the role of AlSt(OH)2, a jellifying agent commonly added in 20th century paint tube formulations, proved decisive for the formation of zinc stearate-like (ZnSt2) soaps. It was observed that ZnSt2-like soaps first form around the added AlSt(OH)2 particles and then eventually grow within the whole painting stratigraphy as irregularly shaped particles. In some of the Alchemy samples, and diversely from the models, a peculiar distribution of ZnSt2 aggregates arranged as rounded and larger particles was also documented. Notably, in one of these samples, larger agglomerates of ZnSt2 expanding toward the support of the painting were observed and interpreted as the early stage of the formation of internal protrusions. Micro-ATR-FT-IR spectroscopic imaging, thanks to a very high chemical specificity combined with high spatial resolution, was proved to give valuable information for assessing the conservation state of irreplaceable 20th century oil paintings, revealing the chemical distribution of Zn soaps within the paint stratigraphy before their effect becomes disruptive.

An integrated approach based on micro-mapping analytical techniques for the detection of impurities in historical Zn-based white pigments

In this work we propose an integrated approach, based on synchrotron analysis with micrometric spatial resolution and sub-ppm sensitivity and μ-Raman mapping, for investigating impurities and heterogeneous inclusions in historical samples of Zn-based white pigments. Analysis was performed at the LUCIA beamline at the SOLEIL synchrotron radiation facility for the simultaneous detection of the elemental distribution in suitably prepared pigment samples using micro-X-ray fluorescence (μ-XRF) mapping and for the investigation of oxidation states and coordination of metals using micro-X-ray near edge absorption (μ-XANES) spectroscopy. The identification of specific molecular signatures and the detection of their spatial distribution throughout samples by μ-Raman measurements supported and complemented X-ray analysis, allowing the identification of Cr- and Fe-based inclusions in historical samples. In ZnO pigments, common impurities are due to the production process and include Fe and, depending on samples, Cd, Cl and Pb. In one of the Zn-containing pigments, identified as Lithopone, μ-XRF mapping revealed the presence of Co, both as highly concentrated micrometric inclusions and as impurities throughout the pigment.