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

Abstract

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), a jellifying agent commonly added in 20th century paint tube formulations, proved decisive for the formation of zinc stearate-like (ZnSt) soaps. It was observed that ZnSt-like soaps first form around the added AlSt(OH) 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 ZnSt aggregates arranged as rounded and larger particles was also documented. Notably, in one of these samples, larger agglomerates of ZnSt 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.

Time‐Dependent ATR‐FTIR Spectroscopic Studies on Fatty Acid Diffusion and the Formation of Metal Soaps in Oil Paint Model Systems

“…Such an ionomer medium contains clusters of metal carboxylate groups (identified by a broad ν a COO − band in infrared (IR) spectra) that, while potentially reactive towards long‐chain saturated fatty acids (SFAs), could contribute to the stability of the oil network on the short term 8. SFAs can either be formed by partial hydrolysis of the polymerized oil network, or be derived from paint additives such as aluminum stearate 10. Our current hypothesis, illustrated in Figure 1, is that the presence of free SFAs leads to the formation of amorphous metal soap complexes.…”

“…[8] SFAs can either be formed by partial hydrolysis of the polymerized oil network, or be derived from paint additives such as aluminum stearate. [10] Our current hypothesis,i llustrated in Figure 1, is that the presence of free SFAs leads to the formation of amorphous metal soap complexes.Subsequently,owing to the low solubility of metal soaps in oil, [11] these complexes will tend to crystallize and form metal-soap aggregates.W ith FTIR spectroscopy,this crystalline state of metal soaps can be distinguished from amorphous metal carboxylate species by their sharp COO À bands at 1510 cm À1 (Pb) or 1538 cm À1 (Zn).Linseed oil-based ionomer model systems and timedependent ATR-FTIR spectroscopy were used to investigate the diffusion of aS FA (palmitic acid) and its reaction with metal carboxylate clusters.A TR-FTIR spectroscopy has proven to be ap owerful tool to study dynamic processes in polymer films. [12][13][14][15][16][17] Mature oil paint model systems (Znpol and Pbpol) were synthesized by co-polymerization of linseed oil (LO) and metal sorbate (2,4-hexadienoate) at 150 8 8C (Supporting Information).…”

“…[8] SFAs can either be formed by partial hydrolysis of the polymerized oil network, or be derived from paint additives such as aluminum stearate. [10] Our current hypothesis,i llustrated in Figure 1, is that the presence of free SFAs leads to the formation of amorphous metal soap complexes.Subsequently,owing to the low solubility of metal soaps in oil, [11] these complexes will tend to crystallize and form metal-soap aggregates.W ith FTIR spectroscopy,this crystalline state of metal soaps can be distinguished from amorphous metal carboxylate species by their sharp COO À bands at 1510 cm À1 (Pb) or 1538 cm À1 (Zn).…”

Effects of zinc oxide filler on the curing and mechanical response of alkyd coatings

“…These effects are known to be quite important, including, for example the well-documented role of ZnO pigments on the degradation of oil-based paints resulting from the formation of metal soaps. [13][14][15][16][17][18].…”

Ineducable us: the applications and contexts of microscopy used for the characterisation of historic building materials

“…The use of microscopy (sensu stricto) is well known in the popular understanding of what might be considered 'high' cultural heritage-fine art conservation in particular (4,5], typified by the analysis of paint layers cross sections [6] and x-ray and infra-red analyses of canvases [7]. Petrographic analysis of cultural materials itself has an application history in archaeological work stretching back at least to near the advent of the use of thin sections for lithological characterisation.…”