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Conservation Issues in Several Twentieth-Century Canadian Oil Paintings: The Role of Zinc Carboxylate Reaction Products
Abstract
In an on-going study of the materials and techniques of twentieth-century Canadian painters, similar conservation issues in oil paintings by various artists have been noted. These include delamination and lifting paint, zinc soap protrusions and surface efflorescence or accretions. Examples of these phenomena are presented. Delamination in an oil painting from 1956 was found to be related to an underlayer with a high concentration of zinc fatty acid salts (zinc soaps). In two paintings that date from 1936 and 1937, zinc soaps have aggregated and formed protrusions that have broken through the paint surface. The protrusions were analysed using a combination of SEM-EDX, GCMS and FTIR. The FTIR spectra were compared to those of synthetic zinc palmitate, stearate, azelate and oleate. The combined GCMS and FTIR results indicate that the protrusions contain primarily zinc palmitate and stearate. Peak splitting in the FTIR spectrum, which is not observed in synthetic zinc palmitate, stearate or binary palmitate-stearate salts, is likely due to structural distortion. The final example describes a disfiguring surface accretion on a 1952–1954 painting caused by the reaction of zinc with a low molecular weight carboxylic acid (2-hydroxypropanoic or lactic acid).
... These so-called zinc soaps find application as fillers/plasticizers in polymers, [1][2][3][4] as stabilizers in polyvinyl chloride (PVC), 5 and as precursors for nanoparticle synthesis. 6,7 However, in the last decade, zinc soaps have also been studied extensively [8][9][10][11][12][13][14] as a degradation product in oil paintings that contain zinc white (ZnO) pigment, one of the most common white pigments used by artists in the late 19th and 20th centuries. 15 Forming as a result of chemical reactions that break down the ZnO pigment and the oil binder, 16 zinc soaps have been associated with many undesirable changes in the appearance and structural integrity of oil paintings, including increased paint transparency and delamination of paint layers. ...
... 22 For crystalline zinc soaps, however, some ambiguity remains. While the n a COO vibration shows a single band around 1538 cm À1 for zinc soaps of long-chain saturated fatty acids, the band has been described as a doublet for zinc soaps with unsaturated chains, 9,10 suggesting that there are at least two different coordination geometries possible. In oil paint samples, both the single and the double band types are regularly detected, 9,13,23 but a clear interpretation of these spectral differences in terms of crystal structure and fatty acid type has so far remained challenging. ...
... While the n a COO vibration shows a single band around 1538 cm À1 for zinc soaps of long-chain saturated fatty acids, the band has been described as a doublet for zinc soaps with unsaturated chains, 9,10 suggesting that there are at least two different coordination geometries possible. In oil paint samples, both the single and the double band types are regularly detected, 9,13,23 but a clear interpretation of these spectral differences in terms of crystal structure and fatty acid type has so far remained challenging. ...
The formation of crystalline zinc soaps (zinc salts of fatty acids) in oil paint layers is a common sign of paint degradation. In this study, we have used infrared spectroscopy to systematically identify differences in structure and composition of crystalline zinc soap phases, and report data analysis methods for structure attribution in challenging oil paint samples. Supported by reported crystal structures, it was possible to distinguish two distinct types of zinc soap geometry: a highly symmetrical packing for long-chain saturated soaps (type B) and an alternating packing for zinc soaps with short, unsat-urated, or dicarboxylic chains (type A). These two types of packing can be identified by a single or split asymmetric COO stretch vibration band. With this new information, we studied the structure and composition of zinc soaps formed in a zinc white model paint and in a cross-section from the painting Equations in Space by Lawren Harris. Using non-negative matrix factorization, band integration and band position maps, it was possible to clearly identify zinc azelate in the model paint and map its spatial distribution. The same methods showed that the paint cross-section contained both types of zinc soap structure within the same paint layer, with the less symmetrical structure appearing only at the interface with the ground layer. The results give valuable information on the internal chemistry of oil paint layers, and the demonstrated methods can find widespread application for in-depth analysis of infrared microscopy data.
... The organic components of the efflorescence were identified with GC/MS as primarily stearic and palmitic acids, with some azelaic acid, present as metal carboxylates; trace levels of these same fatty acids were detected in their free form. 7 Phosphate was also identified in the efflorescent material. and [14]. ...
... The lithopone-containing upper ground layer is a potential source of zinc carboxylates, aggregates of which are visible in the cross-sectional sample and on the surface of the paint layers. Zinc carboxylates in ground layers have been previously linked to the formation of inorganic surface crusts, migrating aggregates, and free fatty acid efflorescence, but few studies have discussed the presence of efflorescence composed primarily of crystalline zinc carboxylates [7,15]. 9 The potential presence of lead and zinc driers in the ground, paint, and coating layers may also be significant factors in efflorescence formation on The Spirit of Light. ...
... A paint flake sample was coated with carbon and analyzed with FE-SEM/EDS in secondary electron mode.6 The zinc stearate reference sample (purum, 10-12% Zn basis) was purchased from Sigma Aldrich, St. Louis, MO.7 Interpretation of the GC/MS results for efflorescence scrapings supports the proposed composition of metal carboxylates of stearic acid, palmitic acid, and azelaic acid, inorganic phosphate, as well as small amounts of free fatty acids. The analysis began with a two-step extraction and derivatization procedure and was created to fit the sample size limitations using ideas in[9,16], 23 Efflorescence on the Paintings of Edwin Austin Abbey: Examination, Analysis… ...
The Yale University Art Gallery (YUAG) collection contains more than 600 paintings by the American artist Edwin Austin Abbey (1852–1911). Over half of these works exhibit efflorescence that appears as a white surface haze. An upcoming exhibition focusing on the preparatory paintings of Abbey and his contemporaries during the American Renaissance has provided the opportunity to study these efflorescing paintings to gain insight into Abbey’s painting technique and to inform future conservation treatments. As part of a larger research project, this study aimed to understand how the observed efflorescence relates to the painting materials and techniques used in Abbey’s oil-on-canvas study The Spirit of Light and to explore approaches for reducing this efflorescence during treatment. The structure and surface of the painting were studied with a stereomicroscope, and multilayer samples were analyzed with field emission scanning electron microscopy – energy dispersive spectroscopy (FE-SEM/EDS). The morphology of the efflorescent material was imaged, and the elemental composition determined, with FE-SEM/EDS. FE-SEM/EDS revealed that the efflorescent material was composed of layers of lifted plates that contain carbon and zinc. The ground layers and efflorescent material were characterized with Fourier transform infrared spectroscopy (FTIR), FE-SEM/EDS, and gas chromatography/mass spectrometry (GC/MS). Analysis revealed that the plates were composed of crystalline zinc carboxylates, as opposed to the more frequently encountered free fatty acids. The silicone-based gel Shin-Etsu KSG-350Z was found to be an effective solution for reducing the efflorescence layer from the highly soluble and underbound paint layers.
... Well-crystallized metal carboxylate salts give sharp, characteristic peaks in the mid-IR between ca. 1500-1600 cm −1 depending on the specific metal cation and fatty acid [2,9]. Ionomeric-type soaps, such as those often encountered in paint layers, give rise to broad absorption bands with obscured features that can make their definitive identification challenging. ...
... Analysis of unmounted fragments of the sample determined that the protrusion (1) is composed primarily of zinc soaps with small quantities of free fatty acids and drying oil, and that the upper blue paint (3) contains lead white, Prussian blue, cobalt blue, and vermilion in a drying oil medium. Paint layers above the ground layers (not shown in the cross-section in Fig. 1) were found to contain a high concentration of zinc white and zinc soaps, and are undoubtedly the source of zinc in the protrusion [9]. ...
... The peak splitting is due to a change in symmetry around the zinc cation compared to the bridging bidentate tetrahedral structure observed in synthesized zinc salts of saturated fatty acids. This could be caused by either a different coordination geometry or by a distortion of the tetrahedral structure around the zinc cation [9]. It has been hypothesized that the presence of mixed or hydrated soaps could prevent a compact packing configuration and lead to peak splitting [32]. ...
Abstract Paintings and painted objects are quite susceptible to degradation, as paint layers are usually composed of complex mixtures of materials that can participate in chemical degradation processes. The identification of the constituent materials in paint (including binders, pigments, and fillers) and the degradation products within paint layers is of particular importance to ensuring the conservation of paintings, by providing important information both about their material history as well as their state of conservation. Metal fatty acid salts (metal soaps) are degradation products that can form in situ from interactions between inorganic pigments and free fatty acids in oil-based binding media, and can cause significant condition issues in paintings. Fourier transform infrared (FTIR) spectroscopy is one of the leading analytical techniques for the study of metal soaps. In this article, the materials analysis of several cross-sections from paintings and painted objects from works in Canadian collections is presented. Recent results on the use of external reflection FTIR (R-FTIR) spectroscopy to identify and map the distribution of paint components and metal soap degradation products is presented. In particular, zinc, lead, calcium, and copper fatty acid salts were all readily identified in paint cross-sections by R-FTIR spectroscopy, along with several pigments and the oil binding medium. The results shown here are among the first detailed examinations of these metal soaps in paint cross-sections using R-FTIR spectroscopy. The use of highly polished samples in which specular reflection is dominant allowed for spectral transformations to be applied to generate transmission/absorption-like spectra which facilitated identification of these species. The distribution of these species across the cross-sections was mapped by integrating characteristic absorption features in the R-FTIR spectra. Attenuated total internal reflection (ATR) FTIR spectroscopy was also performed on several samples, which provided additional compositional details at the interface of paint layers and degradation products.
... Metal soaps may additionally lead to an increased transparency of the paint, thereby revealing the colour of the underlying paint layers [4]. Metal soaps do not only influence the visual appearance of oil paintings, but may also promote cracking, delamination and ultimately flaking of the paint [7,8,9]. Two striking examples of metal soap-induced degradation phenomena can be observed in Figure 1. Figure 1(a) illustrates a ceiling painting located in the Room of Trustees of the Burgerweeshuis in Zierikzee, The Netherlands, in which multiple metal soap crystals have formed inside the orange-red bottom paint film, causing a substantial deformation of the above pictorial layers. ...
... In [18], it has been reported that metal soap formation can contribute to an increase of oil paint stiffness and brittleness. Moreover, the presence of metal soap aggregates has been associated to micro-cracks and delamination phenomena [7,8,9,38], which are triggered by the mechanical strain (and thus stress) generated in the painting system under metal soap growth. Recently, a computational chemo-mechanical model has been proposed that predicts damage development in paint layers as a result of metal soap formation and growth [39]. ...
Metal soap formation is recognised as a critical degradation mechanism in historical oil paintings, which threatens the preservation of museum collections worldwide. Metal soaps form via a complex sequence of chemical reactions between metal ions released by the pigments and saturated fatty acids originating from the drying oil. The latest advances in chemistry research suggest that metal ions and saturated fatty acids may initially react by means of a reversible reaction, which leads to the formation of metal soaps in an amorphous state. Metal soaps may subsequently crystallise via an irreversible reaction into large aggregates that deform the paint layers, potentially triggering delamination, cracking, and ultimately flaking of the paint. This paper proposes a chemo-mechanical model to predict metal soap formation and the consequent mechanical damage in historical oil paintings. The chemical process is described in terms of a set of diffusion-reaction equations, which account for both the reversible reaction between free saturated fatty acids and metal ions forming amorphous metal soap, and the subsequent irreversible reaction to crystalline metal soap. The chemical model is two-way coupled with a mechanical model that effectively describes the cracking processes caused by metal soap formation and growth. The coupling is generated from the mechanical model by accounting for the development of a chemically-induced growth strain in the crystalline metal soap. In addition, the presence of cracks locally hampers the diffusion of chemical species, which is taken into account in the chemical model through a dependency of the diffusion parameter at the crack faces on the amount of mechanical damage generated. The spatial development of the crystalline metal soap phase is simulated by using a tailor-made scanning algorithm that identifies the reaction zone in which metal soap formation takes place. The proposed model is calibrated on experimental data presented in the literature. The model is subsequently applied to analyse two numerical examples that are representative of typical metal soap-related degradation processes observed in historical oil paintings, revealing that the growth process of crystalline metal soap, the deformation of the paint surface, and the consequent cracking and delamination patterns are predicted in a realistic fashion.
... After 10 months, the 80+RH and 80+RH+H+ samples show intense signal for the broad band of zinc carboxylate (POL-Zn) and extremely intense absorption for the crystalline zinc soaps (FA-Zn), with the split peak at 1537-1546 cm −1 displaying different ratios of relative intensity for the two samples. It is interesting to note that the mixtures prepared for this work, even after a short time, developed the type of peak splitting of the ν as COO − absorption in two main bands (centered at 1550/1530 cm −1 ) that has been observed by other researchers in older paints (Helwig et al. 2014;Osmond 2014. Though zinc oleate shows the ν as COO − bands at 1547 and 1527 cm −1 (Robinet and Corbeil 2003;Otero et al. 2014) and zinc azelate at 1556 and 1535 cm −1 , it's been suggested that in fact the doublet at 1550 and 1530 cm −1 may be related to a distortion of the symmetry around the zinc ion (Helwig et al. 2014). ...
... It is interesting to note that the mixtures prepared for this work, even after a short time, developed the type of peak splitting of the ν as COO − absorption in two main bands (centered at 1550/1530 cm −1 ) that has been observed by other researchers in older paints (Helwig et al. 2014;Osmond 2014. Though zinc oleate shows the ν as COO − bands at 1547 and 1527 cm −1 (Robinet and Corbeil 2003;Otero et al. 2014) and zinc azelate at 1556 and 1535 cm −1 , it's been suggested that in fact the doublet at 1550 and 1530 cm −1 may be related to a distortion of the symmetry around the zinc ion (Helwig et al. 2014). Further research on pure, synthetic compounds is needed to understand this aspect and completely exclude a convolution of bands of zinc palmitate with zinc azelate or oleate to explain peak splitting. ...
This study focuses on the short-term reactivity of zinc oxide with different types of drying oils. Chemical grade zinc oxide (with diameter <100 nm) was mixed with oils with different proportions and types of fatty acids. Specifically, reactivity of zinc oxide with alkali-refined linseed oil, raw linseed oil, boiled linseed oil, polymerized linseed oil, stand oil, tung oil, and alkyd was studied between 4 days and 10 months. Effects of short-term exposure to temperature, humidity, and pH were also evaluated. The type of metal soaps formed and the kinetics of formation were monitored with attenuated total reflection Fourier transform infrared (ATR- FTIR) spectroscopy. The results confirm that humidity and heat combined favor the formation of both network-coordinated zinc carboxylates (POL-Zn) and zinc complexes of free fatty acids (FA-Zn) irrespective of the type of oil; however, the phenomenon is particularly severe for alkali-refined, raw, and boiled linseed oils, while stand oil and tung oil show the lowest amounts of zinc soap (FA-Zn) formation. The findings advance our knowledge of the reactivity of zinc oxide with an expanded range of drying oils that are commonly encountered in many nineteenth- and twentieth-century paints and reinforce the notion that even short-term exposure to heat with humidity can have irreversible effects on zinc carboxylate formation.
... Most paintings show a presence of metal soaps to some degree. In the last few years, the identification of metal soaps, that is, the determination of their molecular composition and crystal structures (as well as physical and chemical properties), has increased significantly (Catalano et al. 2014(Catalano et al. , 2015Corbeil and Robinet 2002;Cotte et al. 2016;Helwig et al. 2014;Hermans et al. 2014Hermans et al. , 2015Hermans et al. , 2016Martínez-Casado et al. 2014;Robinet and Corbeil 2003). This increased interest is principally due to conservation issues as these substances may produce efflorescence, spots, and protrusions and may cause the paint layers to separate or occasionally detachment, in particular for zinc and, in a lesser extent, lead soaps (Cotte et al. 2007(Cotte et al. , 2016Higgitt et al. 2003; Keune and Boon 2007;Keune et al. 2011Keune et al. , 2016Osmond et al. 2012;Sawicka et al. 2014;Salvadó et al. 2009). ...
... The reflections corresponding to lead soaps show d-spacings that lie between those of lead stearate and lead palmitate, which is in good agreement with the IR data. This is also observed in other samples where the binding medium is drying oil (Salvadó et al. 2009) and found in the literature for other cations (changes in CH 2 progression bands in mixtures of binary zinc palmitate-stearate salts in oil) (Helwig et al. 2014;Hermans et al. 2015). Since the paint layers are heterogeneous, it is of great importance to measure more than one series (both cross section lines and maps) to obtain data which is as representative as possible of the sample. ...
The formation and distribution of metal soaps produced as a result of the reactivity and aging of the materials in 15th Century egg tempera and oil paintings on wood is presented. The painting technique involves the application of several paint layers over a ground using, sometimes in the same paint layer sequence, drying oil and egg yolk binders. We show, with a selection of examples, how the use of thin cross sections and a combination of various micro-sensitive analytical techniques is adequate to obtain the high quality data necessary for the unambiguous identification of metal soaps and metal oxalates and of their distribution in the paint layers. The techniques include Micro Infrared Spectroscopy (μSR‐FTIR) and X‐Ray diffraction (μSR‐XRD) with synchrotron radiation, Optical Microscopy (OM) and Scanning Electron Microscopy (SEM/EDS). The data obtained sheds light about the underlying reaction and aging mechanisms happening in each paint layer and among them and help to define the state of conservation of the artworks.
... 2,3 Analysis of the pustules indicates that they are primarily composed of salts of palmitic and stearic acids and a smaller proportion of oleic acid. 4,5 Delamination of paint layers, often associated with zinc soap formation, may occur. 6,7 Recent work has shown that a variety of zinc alkanoates with sheet structures can form in oil, which might account for this phenomenon. ...
... The values of ν as COO − cm −1 for Zn−EL are significant in light of the observations made that zinc soap aggregates in real paint films and models prepared using linseed oil have a broad peak at 1570−1590 cm −1 , a value much more similar to that for Zn−EL than that for saturated fatty acid zinc soaps. 5,18 Hermans et al. 18 have discussed the differences in the IR spectra of simple soap salts, mixed metal salts, model Zn-paint films, and real paint films and the possible reasons for them. They dismissed the hypothesis that broadening of the ν as COO − stretch in real paint films is owing to the interaction of fatty acids or triglycerides with ZnO particles. ...
To investigate soap formation in drying oils in historic paints, the reaction between metal acetates (K⁺, Zn²⁺, Pb²⁺) and ethyl linoleate (EL) was studied using optical microscopy, X-ray powder diffraction, and electron microscopy. Pb(II) and Zn(II) react rapidly with EL to form highly structured, spherulitic, luminescent crystallites that aggregate. Evidence from Fourier transform infrared (FTIR) and scanning electron microscopy/energy dispersive X-ray analysis and high-resolution synchrotron powder X-ray diffraction indicates that these are organic–inorganic hybrid complexes or coordination polymers. FTIR absorbance peaks at ca. 1540 cm–1 for Pb(II) and ca. 1580 cm–1 for Zn(II) are consistent with the formation of carboxylate complexes. The complexes formed offer insight into the degradation processes observed in oil paint films, suggesting that soap formation is rapid when metal ions are solubilized and can occur with unsaturated fatty acids that are present in fresh oils. These complexes may account for the atypical luminescence observed in lead-containing cured oil paint films.
... Various degradation phenomena are indeed related to the use of zinc white and numerous studies have been devoted to the formation of zinc soaps. The amorphous or crystalline zinc carboxylates may cause delamination [35,36,42,43]. ...
This research provides new insights into the composition of zinc white paints used by Piet Mondrian during his neoplastic period. Nine paintings, dated between 1921 and 1935, were studied, with a focus on three works in the collection of the Fondation Beyeler (Basel, Switzerland)—Tableau I (1921–1925), Composition with yellow and blue (1932) and Composition with double line and blue (1935)—and on Lozenge composition with yellow lines (1933) in Kunstmuseum Den Haag (The Netherlands). Cross sections from other paintings, most of which previously studied by Van Asperen de Boer in the early 1990’s, were reexamined as well. The analyses revealed a zinc white paint with aluminum phosphate inclusions (ZW-Al/P). In two of the works, a zinc white paint with aluminum sulphate was also found. The occurrence of aluminum phosphate (or aluminum sulphate) in 1920–1930’s paint formulations, and in paintings by Mondrian, has never been reported so far. Likely, the use of this zinc white paint in Mondrian’s and other artworks is more widespread than currently known, but it may have been overlooked in similar case studies, since its identification can only be accomplished with detailed scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM–EDX) analysis of cross sections. Mondrian’s use of pure ‘bright’ ZW-Al/P paint is limited to the later paintings (1932–1935) of this study, whereas in the earlier works (1921–1929) it was mostly mixed with lead white, likely conveying a different hue. Both the ZW-Al/P and Zn-Al/S ‘marker paints’ were also added to yellow, blue or black paint in specific paint layers of the planes and lines, respectively. It was also shown that Mondrian used the same paint on the frames as in the white planes and that the frames were painted while still working on the composition, and not only when he considered the work completed. Indeed, the detection of the zinc white marker paints might help to delineate the chronology of Mondrian’s working process in other paintings and may also be important in the interpretation of overpaints in the course of a treatment when removal of certain paint layers is considered.
... It is worth noticing that the zinc carboxylates in sample 483 have a single peak centred at 1539 cm −1 , which is different to the zinc carboxylates in 482 consisting of a doublet at 1527 and 1539 cm −1 (P3 in Fig. 5 i). This may be due to the different symmetry of the components between the zinc carboxylates [64] in 482 and 483. ...
... 7 In extreme cases, zinc soap formation can lead to delamination and fracture of paint layers, or the appearance of globular protrusions on the paint surface. [8][9][10][11][12][13] These zinc soaps are insoluble crystalline complexes of zinc ions and saturated fatty acids (FA, usually palmitic and stearic acid), formed after partial hydrolysis of ester bonds in the triglyceride oil binder and dissolution of ZnO in the oil by reaction with carboxylic acid groups of the oxidized oil. 5,14 Recent research elucidated the structural evolution of zinc carboxylate species in oil paint systems from ZnO through ionomeric zinc carboxylates to crystalline zinc soaps. ...
The crystallization of metal soaps in polymer matrices is a complex process that affects the stability of oil paintings, as well as the properties of commercial ionomer materials. In the context of conservation of paintings, it is crucial to investigate the influence of solvent exposure on such detrimental chemical processes. Using Fourier transform infrared spectroscopy and a polymer model system that contains metastable amorphous zinc soaps, it is shown that water induces zinc soap crystallization, while solvent swelling alone has no effect. In particular fast-diffusing polar organic solvents with water impurities are able to induce extensive crystallization, delivering high concentrations of water quickly deep into paint layers. Finally, it is demonstrated, both with the model system and real oil paint samples, that even with very short solvent exposure times, significant quantities of crystalline zinc soaps are formed. This strong effect of water impurities in common solvents gives reason to be cautious when conservation treatments are being considered for oil paints that contain zinc white or other water-sensitive chemicals.
... They could either have been intentionally added to modify the mechanical properties of the paint or be the consequence of the reaction between the pigments' metal ions and the binder [40]. Zinc soaps are known to be more likely associated with degradation phenomena than lead soaps, affecting paint layers' delamination and flaking [43][44][45][46]. Thus, it is reasonable to assume that the cyclic and persistent occurrence of liftings and gaps of the paint layer, which required frequent fixing intervention during the years, are caused by of the presence of zinc soaps in all the stratigraphy. ...
In this study, the investigation of the oil painting on canvas I Tesori del Mare made by Plinio Nomellini in 1901 is presented. The aim of the research was threefold: the examination of the state of conservation in view of the restoration treatment, together with the identification of the causes of degradation and the study of the artistic technique. During the years, the artwork underwent several cleaning and fixing interventions, resulting in a patchy appearance of the surface. Nevertheless, the presence of consistent liftings persists, while the protective coating shows uneven chromatic alteration, both requiring further analysis. Multispectral imaging allowed for better visualization of the figuration’s structure and the restored areas. The combined use of Raman spectroscopy, Fourier Transform Infrared spectroscopy in the Attenuated Total Reflection mode (FT-IR ATR), and Scanning Electron Microscopy coupled with an Energy Dispersive Spectroscopy (SEM/EDS) enabled better understanding of the stratigraphy through the identification of some pigments, the binder, and the aged varnish layer on the top. SEM/EDS highlighted the presence of zinc in both the ground layer and the paint layers. Furthermore, FT-IR ATR spectroscopy showed peaks related to metal soaps such as zinc stearate, which are known to cause severe delamination of the paint layers, explaining the recurring lifting issues. Eventually, the varnish layer was found to be acrylic resin, presumably mixed with varnishes applied in past restoration treatments.
... Uniformly, palmitic acid is recovered in larger extent compared to stearic acid. Recently, palmitic to stearic acid ratios showed their relevance, especially for identifying the nature of drying oils in paintings van Dam et al., 2017) and the degradation advancement (Helwig et al., 2014;La Nasa et al., 2018;van den Berg, Burnstock and Schilling, 2019). Generating structural distortion within the polymerized network formed during the drying of oil, the palmitate over stereate ratio can be an indicator of the damage undergone. ...
The work presented in this thesis focused on the analysis of archaeological vessels. Through the search for molecular markers, identified by Gas Chromatography – Mass Spectrometry and the observation of archaeobotanical remains, this study aims to identify the original content of the studied vessels. The analysis of organic residues, both contained in the ceramic sherd and in the waterproofing layer inside the amphora, offers a first reading of the functionality of the object and its content. Particular importance is given to the botanical identification and formulation techniques used to produce a waterproofing matrix that was affixed to the inside of the amphora. The paleobotanical investigation that mainly focused on the search for pollen, brings a new angle of analysis by concentrating on the one hand on the characterization of environmental and/or economic fossil species, and on the other hand on the botanical origin of the identified pollens. In addition to the optimization of existing protocols for the extraction of molecules considered as biomarkers, this study focuses on the benefits of a multi-analytical archaeometric approach through the analysis of different archaeological artifacts from heterogeneous periods and contexts. Focusing on the Roman period, this thesis focuses on the analysis of wine and/or oil amphorae from the Planier 3 shipwreck (France) and the ancient anchorage of San Felice Circeo (Italy) before extending the methodology and the results to a "pouring" vase of singular typology dating from the Bronze Age (West Bank).
... The formation of zinc ions with fatty acids in paint layers [2,[13][14][15][16][17][18]] is a severe threat to the stability and appearance of many oil paintings of the late 19th and early 20th centuries. Modern methods of scanning and transmission electron microscopy, developed primarily for biological samples, allowed visualizing the earliest stages of growth of crystalline zinc soap in a reconstructed sample of zinc white oil paint (ZnO) [19,20]. ...
The paper describes the results of using Raman spectroscopy to study the canvases of Russian avant-garde artists. The study aimed to identify stable signs of paints for the recognition of counterfeits. The subject of the research was zinc white as a component of paints used by the 20th-century Russian avant-garde. Temporal changes occurring with them are due to chemical reactions of organic paint components with whites and with each other. The study shows that Raman spectroscopy is the most sensitive method for detecting ZnO time markers. Its results are compared with the results of pyrolysis–gas chromatography, mass spectrometry, and X-ray analysis of paints from paintings from the Russian avant-garde of the early, middle, and late twentieth century. The study results demonstrate that the paint compositions of the early and late 20th Century have significant similarities. The result of their aging is the appearance of products of thermal desorption and pyrolysis in the form of a homologous series of alcohols and aromatic hydrocarbons formed during the destruction and oxidation of oleic, linoleic, linolenic, palmitic, stearic acids, and/or azelaic acid. These acids are found in flaxseed oil. Zinc white of the early 20th century contains significant additions of gypsum. The composition of the components of mid-century paints is much less diverse. Genuine paintings contain homologous series of fatty acids sorbed on zinc oxide. The paper analyzes historical reasons for this phenomenon. The research shows that one of the stable signs of falsification is the use of titanium white and organic catalysis products based on them. The results of gas chromatography and mass spectrometry and Raman spectroscopy are supplemented by X-ray phase and X-ray fluorescence analysis. The paper provides detailed reference data on the spectra of Raman light scattering, thermal desorption, and pyrolysis of paints of the studied canvases and shows the differences in the mass spectra of genuine paintings and falsifications.
... 3 Accumulation of zinc soaps at the lower part of a paint layer has also been found in zinc white paint reconstructions in a recent study (Osmond et al. 2012). A similar trend is apparent in other case studies of paint delamination and paint loss associated with zinc soaps in modern oil paintings, including previously published studies (Rogala et al. 2010;Helwig et al. 2014) and in other contributions to this volume (Raven et al. 2019;Osmond 2019). ...
The Evolution triptych by Piet Mondrian (1911, oil on canvas, Gemeentemuseum Den Haag) presents a case study of a painting that is seriously affected by zinc soap formation, which has resulted in paint delamination and paint loss, particularly in the cadmium yellow paint areas. The paint is extremely fragile, which makes the paintings vulnerable with regard to handling and treatment. This paper focuses on the analytical research of the painting using various state-of-the-art and novel macro- and micro-imaging techniques. Macro X-ray fluorescence scanning (MA-XRF) revealed the presence of cadmium (Cd) and zinc (Zn) in the affected yellow paints. Paint cross sections of both affected and intact paint areas were investigated using light microscopy, scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM-EDX), attenuated total reflection Fourier transform infrared (ATR-FTIR) micro-imaging, and synchrotron photoluminescence (PL) micro-imaging. With the help of these techniques, the cadmium yellow pigment could be identified as a mixture of cadmium sulfide and cadmium oxalate. The presence of zinc white was established in areas where the yellow paint film is degraded, while the intact areas of yellow paint do not contain any zinc white. In samples of the degraded paints, it was demonstrated that high concentrations of zinc soaps have formed, accumulating at interfaces. This has caused local chemical and physical changes of the paint resulting in delamination between paint layers.
... The fatty acids in these metal soap phases derive from the drying oil binder or paint additives (for instance, stearate salts used as dispersing agents in modern paints). They are almost exclusively the saturated fatty acids palmitic, stearic, or azelaic acid (Plater et al., 2003), with the occasional presence of oleic acid (Helwig et al., 2014). In terms of morphology, it has been observed that lead soaps tend to form large metal soap aggregates like in Fig. 3.1. ...
An overview is presented of the current state of understanding of the chemical pathways that lead to the formation of crystalline metal soap phases in oil paints, based on recent experimental work by the authors and supported by relevant literature. Improved (quantitative) interpretation of Fourier-transform infrared (FTIR) spectra has revealed that metal ions are bound to carboxylate functionalities of the oil polymer during oil paint aging, a state similar to ionomeric polymers. Tailored ionomer-like systems based on linseed oil were synthesized to study the structure of the mature oil paint binding medium, and such systems were used as the starting point for studies on metal soap crystallization. Additionally, series of differential scanning calorimetry (DSC) studies shed light on the driving forces and kinetics of metal soap crystallization, and electron microscopy studies have been used to image the initial stages of metal soap crystallization. The results have been used to construct a model of the chemical reactions leading to metal soaps from a mixture of pigment and oil. Additionally, the model provides insight into diffusion mechanisms for metal ions and fatty acids and potential physical transitions in the structure of metal soaps. The mechanisms described are helpful in explaining the different morphologies of lead and zinc soaps observed in actual samples from historic paintings and the locations within paint films where these are typically found.
... This is a recurrent issue in twentieth century oil paintings (K. Helwig et al., 2014). Micrograph (Fig. 6), at 100 µm scale, indicates high particle-binder ratio, with zinc and calcium as small particles present throughout the paint layer. ...
Contemporary art has employed almost every single synthetic material created by man. Nonetheless, the appeal of oil paint is still present at most public and private collections. Oil paint manufacture has evolved over time along with its formulation and conservation concerns. A reported problem is the formation of exudates on contemporary oil paintings. This work aims to characterize exudates from the Brazilian oil painting "Três pessoas" (1999), by Marina Saleme. Characterization was carried out by Fourier transform infrared spectroscopy, pyrolysis coupled to gas chromatography and mass spectrometry and scanning electron microscopy coupled to energy dispersive x-ray fluorescence spectrometry. Results indicate that exudates are formed by the oxidation of linseed oil and formation of dicarboxylic acids. This process is aided by the formation of zinc stearate in the paint film. Copyright © 2020, Supatra Sen. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
... It is likely that much of the original zinc-containing pigment has reacted to form soaps and that any unreacted pigment is below the detection limit of the methods used. The extensive zinc soap formation in the pink underlayer is undoubtedly the cause of the cracking and lifting in the red passages; crystalline zinc soaps have been associated with similar deterioration phenomena in a number of studies of modern oil paints (Rogala et al. 2010;Helwig et al. 2014;Raven et al. 2019). ...
This paper describes an investigation into the causes of specific degradation phenomena in three oil paintings from the 1960s by eminent Indigenous Canadian painter Rita Letendre. A comprehensive technical study of the paintings was undertaken using a combination of SEM-EDS, FTIR spectroscopy, Py-GC-MS, Raman spectroscopy, XRD, and polarised light microscopy to determine pigments, fillers, additives, paint medium, and degradation products. Compositional factors, including high abundances of manufacturers’ additives such as aluminum stearate and castor wax, have likely contributed to the formation of oil exudates and fatty acid efflorescence. Cross sections illustrated an inhomogeneous distribution of additives and incomplete mixing of components in some colours. This appears to be due to the migration of components over time and also to Letendre’s technique, which included application of thick paint directly from the tube and the addition of extra medium to some impastos. Cracking, lifting, and protrusions in several areas were caused by the crystallization of zinc soaps below the paint surface. FTIR mapping allowed the spatial distribution of crystalline and ionomeric-type zinc soaps to be determined within a problematic paint layer.
... At this new address, Soulages continued to buy his materials from the same colormen in Montparnasse as he had done before: with some exceptions, his usual suppliers were Lucien Lefebvre-Foinet, established in rue Vavin (de La Grandière 2005;Hélou-de La Grandière 2006;Hélou-de La Grandière et al. 2007;Adam 2010;Helwig et al. 2014), and Edouard Adam, situated in Boulevard Edgar Quinet (Adam 2010). ...
Pierre Soulages (b.1919) is the most renowned French painter alive today,
well-known for his use of the color black. During his long and prolific career (around
1500 paintings made to date), he has worked with various oil paint techniques and
later on acrylics, making continuous use of the color black. This paper deals with
paintings from the end of the 1950s, where black paint was layered on and then
scraped off with tools to reveal colored underpainting (Fig. 23.1). The creation
process of Pierre Soulages has often compelled him to adapt his tools (Ragon 1990;
McEnroe 1991; Encrevé 1995) and to innovate in his use of materials. Yet he has
always remained interested in understanding the physical phenomena of the drying
process, discussing the subject with friends such as Marc Havel, chief engineer at
Bourgeois and connoisseur of painting techniques (Havel 1974), or Pierre-Gilles
de Gennes, winner of the Nobel Prize in Physics. Paradoxically, in spite of this
constant interest, some of his paintings from an important period in the 1950s–
1960s are deteriorating. This paper focuses on those paintings, their specific patterns
of degradation, and the roles that technique and material composition have played
in the process.
... This absence of zinc soaps was detected in Male Figure from Couple with Clouds in Their Heads by SalvadorDalí, 1936. 3 Examples are:[1,5,7,[11][12][13][14]. ...
The early twentieth century oil painting Beach Scene had severe delamination problems. The painting consisted of an oak panel primed with three white, oil-based ground layers with several oil paint layers on top. The reverse of the panel was varnished. The delamination occurred at the interface between the first ground layer and the panel. A brownish, semi-translucent zone was visible at the bottom of the first preparation layer. Material analyses with SEM-EDX, FTIR-ATR and THM-Py-GC/MS offered insight into the materials present and their degree of deterioration. The brownish zone was explained as dissolution of the zinc white pigment by (fatty) acids from the panel and the oil binder. It was hypothesised that the glazed framing system of the painting restricted natural expansion and contraction of the tangentially cut panel in fluctuating climate. Compression shrinkage then caused tenting of the paint film. This study improved our understanding of Akkeringa’s painting technique and the current condition of the painting. The study also provided support for future decisions on treatment and preventive conservation measures not only for Beach Scene, but also for Akkeringa’s other works on panel.
... Low concentrations of sulfur (S) were also detected in the gel material (Supplemental Table 1). Previous studies have also detected Cl inside lead and zinc soap aggregates (Osmond, Keune, and Boon 2005;Keune and Boon 2007;Helwig et al. 2014). Keune and Boon (2007) suggest lead chloride mineral phases may be present in lead soap protrusions, possibly from contaminants in the pigment. ...
Zinc (carboxylate) soaps, formed by reactions between zinc oxide (ZnO) and fatty acids in a drying oil, are known to cause deterioration in the paint layers of modern and contemporary oil paintings. This study investigates zinc carboxylates that developed in an oil painting test panel designed to mimic the aging and degradation encountered in actual works of art. Following accelerated and natural aging, protrusions were noted on the surface of the test panel. A large protrusion with erupted gel features was extracted from the test panel, mounted in top view, and then cut to reveal the sample's cross section. The gel features, which resulted from the unreacted oil binder's separation from the paint matrix, facilitated zinc carboxylate formation. Using reflectance µ-FTIR and SEM-EDX analysis, the morphologies and spatial distributions of zinc carboxylates within the gel regions of the protrusion were studied. A concentration gradient of zinc within the gel material was observed in the cross-sectional view, indicating patterns of zinc carboxylate formation and migration.
... This happens through a diffusion-reaction process that is governed by the diffusion of free saturated fatty acids and metal ions through the paint. The irreversible crystallization depletes these from the surface of the crystal, creating a concentration gradient that drives further diffusion and therefore the progression of crystallization ( Helwig et al., 2014;Hermans et al., 2016 ). The reactions governing metal soap formation can be summarized as (Hermans, 2017) (1) in which M + n R denotes a metal ion M bound to n fatty acid chains R that are part of the polymer network, M(RCOO) n is the metal soap complex, k + , k − and k crys are the rate constants for each reaction, and prefixes a and c refer to the amorphous or crystalline state of metal soap. ...
... Zinc soaps were either identified as zinc stearate or palmitate characterised by the well-defined bands at 1539, 1462, 1396 cm −1 (A1, M3) [22], or as a mixture of zinc carboxylate characterized by a broad band at 1590-1630 and 1394 cm −1 (A2, A4, A5, M2). The latter zinc carboxylate spectrum also showed the presence of wax, characterized by a splitting doublet at 719/729 cm −1 and 1461/1471 cm −1 (M2). ...
Abstract Two Burmese oil paintings on zinc supports depicting portraits of royal families, dated to the early 20th century by Saya Aye and Saya Mya, showed interesting corrosion, efflorescence and soapy globules for analytical investigation prior to conservation for exhibition. The features and colours on the two paintings, one of which was in worse condition than the other, were obscured by a whitish haze. Paint rupture and losses due to corrosion of the zinc support were apparent. Using a combination of 3D digital microscopy, FTIR microscopy and SEM–EDS, the degradation, as well as the paint materials and artists’ techniques were analysed and discussed. Zinc soaps, oxalates, silicates, hydroxychlorides, carbonates, wax and mould characterized the haze. Interestingly, brown outlines in Aye’s painting appeared unusually dark. Upon analysis, this degradation phenomenon was attributed to micro-sized greenish soap globules that have developed around copper/zinc (brass) particles, causing the brown outlines to lose their intended luminosity. The other painting by Mya, in which the same brown outline was applied, did not show such formation of globules. Detection of oxalates in the corrosion of zinc support and brown outlines are discussed. This study shows the importance in understanding the artists’ materials and techniques, which have consequential effects to the degradation observed.
... Based on appearance, which conforms to images in the literature (e.g. Keune (2005), Osmond (2005), Helwig (2014)) this is thought to be a zinc soap aggregate. In Fig. 4 bottom right, the SEM backscatter image (BSE) shows a corresponding area of what appears to be predominantly organic material which may represent zinc carboxylate. ...
Landscape with apple trees in blossom and peasant woman (c.1881-93) and Beach at Póvoa de Varzim (1881) are two oil paintings on panel by the Portuguese artist Silva Porto (1850-1893). Both paintings exhibit paint losses revealing the underlying wood where crumbled ground residue surrounds worm-like raised lines of zinc soap following the wood grain. In addition, the Landscape painting exhibits raised paint in horizontal ridges aligned with the panel’s wood grain. These deformations are associated with 2-12 mm paint losses which show no relation to the paint’s colour or thickness. Both paintings have the same ground construction consisting of a first layer of zinc white directly on top of the panel, and a second layer consisting of a mixture of lead white, barium sulphate and calcium carbonate. The panels are thought to be commercially prepared. Materials were identified with a combination of optical microscopy, micro-Raman, and scanning electron microscopy with energy dispersive x-ray spectroscopy on embedded cross sections, and with micro-Fourier transform infrared spectroscopy on micro-samples. The presence of zinc soaps concentrated along the wood grain is discussed in relation to a previous study identifying lead soap concentration aligned with the wood grain on a much earlier panel. The zinc soap containing material in the two 19th century paintings appears to be contributing to paint losses due to a volume change associated with the concentrated zinc soap. We aim to provide analytical evidence that the paint losses and associated surface deformations are being caused by zinc soap formation within the ground layers and disseminate this form of metal soaps degradation. A scheme illustrating possible steps leading to deformation and paint loss is provided. This problem is causing extreme instability and risk of further loss. Currently, treatment options are unclear.
... Based on appearance, which conforms to images in the literature (e.g. Keune (2005), Osmond (2005), Helwig (2014)) this is thought to be a zinc soap aggregate. In Fig. 4 bottom right, the SEM backscatter image (BSE) shows a corresponding area of what appears to be predominantly organic material which may represent zinc carboxylate. ...
Landscape with apple trees in blossom and peasant woman (c.1881-93) and Beach at Póvoa de Varzim (1881) are two oil paintings on panel by the Portuguese artist Silva Porto (1850-1893). Both paintings exhibit paint losses revealing the underlying wood where crumbled ground residue surrounds worm-like raised lines of zinc soap following the wood grain. In addition, the Landscape painting exhibits raised paint in horizontal ridges aligned with the panel’s wood grain. These deformations are associated with 2-12 mm paint losses which show no relation to the paint’s colour or thickness. Both paintings have the same ground construction consisting of a first layer of zinc white directly on top of the panel, and a second layer consisting of a mixture of lead white, barium sulphate and calcium carbonate. The panels are thought to be commercially prepared. Materials were identified with a combination of optical microscopy, micro-Raman, and scanning electron microscopy with energy dispersive x-ray spectroscopy on embedded cross sections, and with micro-Fourier transform infrared spectroscopy on micro-samples. The presence of zinc soaps concentrated along the wood grain is discussed in relation to a previous study identifying lead soap concentration aligned with the wood grain on a much earlier panel. The zinc soap containing material in the two 19th century paintings appears to be contributing to paint losses due to a volume change associated with the concentrated zinc soap. We aim to provide analytical evidence that the paint losses and associated surface deformations are being caused by zinc soap formation within the ground layers and disseminate this form of metal soaps degradation. A scheme illustrating possible steps leading to deformation and paint loss is provided. This problem is causing extreme instability and risk of further loss. Currently, treatment options are unclear.
... Based on a survey initiated by the Georgia O'Keeffe Museum in 2009, a subset of O'Keeffe's paintings produced between 1920 and 1950 were identified as having disfiguring micro-protrusions scattered across their surfaces. These protrusions exhibit a strong UV-induced fluorescence response, range in size from 10 to greater than 200 µm, and occasionally appear erupted with a caldera-like shape consistent with soap aggregates found in other modern and early modern paintings (Osmond et al. 2005;O'Donoghue et al. 2006;Faubel et al. 2011;Ferreira et al. 2011;Duffy et al. 2014;Helwig et al. 2014). The protrusion formation process must have started at an early stage in these artworks history: in a 1947 correspondence between O'Keeffe and conservator Caroline Keck (Keck 1947), the artist mentioned that she noticed opaque, granular textures and pinpoint losses appearing in several of her oil paintings created between 1928-1936, some of which today exhibits particularly large protrusions. ...
A significant number of oil paintings produced by Georgia O'Keeffe (1887-1986) show surface protrusions of varying width, up to several hundreds of microns. These protrusions are similar to those described in the art conservation literature as metallic soaps. Since the presence of these protrusions raises questions about the state of conservation and long-term prospects for deterioration of these artworks, a 3D-imaging technique, photometric stereo using ultraviolet illumination, was developed for the long-term monitoring of the surface-shape of the protrusions and the surrounding paint. Because the UV fluorescence response of painting materials is isotropic, errors typically caused by non-Lambertian (anisotropic) specularities when using visible reflected light can be avoided providing a more accurate estimation of shape. As an added benefit, fluorescence provides additional contrast information contributing to materials characterization. The developed methodology aims to detect, characterize, and quantify the distribution of micro-protrusions and their development over the surface of entire artworks. Combined with a set of analytical in-situ techniques, and computational tools, this approach constitutes a novel methodology to investigate the selective distribution of protrusions in correlation with the composition of painting materials at the macro-scale. While focused on O'Keeffe's paintings as a case study, we expect the proposed approach to have broader significance by providing a non-invasive protocol to the conservation community to probe topological changes for any relatively flat painted surface of an artwork, and more specifically to monitor the dynamic formation of protrusions, in relation to paint composition and modifications of environmental conditions, loans, exhibitions and storage over the long-term.
A series of medium- and long-chain zinc carboxylates (zinc octanoate, zinc nonanoate, zinc decanoate, zinc undecanoate, zinc dodecanoate, zinc pivalate, zinc stearate, zinc palmitate, zinc oleate, and zinc azelate) was analyzed by ultra-high-field 67Zn NMR spectroscopy up to 35.2 T, as well as 13C NMR and FTIR spectroscopy. We also report the single-crystal X-ray diffraction structures of zinc nonanoate, zinc decanoate, and zinc oleate-the first long-chain carboxylate single-crystals to be reported for zinc. The NMR and X-ray diffraction data suggest that the carboxylates exist in three distinct geometric groups, based on structural and spectroscopic parameters. The ssNMR results presented here present a future for dynamic nuclear polarization (DNP)-NMR-based minimally invasive methods for testing artwork for the presence of zinc carboxylates.
ATR–FTIR (attenuated total reflection–Fourier-transform infrared) microscopy with imaging is widely used in the heritage field to characterise complex compositions of paint cross-sections. However, some limitations include the need for ATR crystal contact with the sample and the inability to resolve particle size below the IR diffraction limit. Recently, a novel O-PTIR (optical-photothermal infrared) spectroscopy technique claimed to open a new avenue for non-invasive, efficient, and reliable analysis at sub-micron resolution. O-PTIR produces transmission-like FTIR spectra for interpretation, without the need to touch the sample, which are highly favourable attributes for analysing heritage samples. This paper reports the comparison of O-PTIR and ATR–FTIR techniques applied to a cross-section embedding a thin paint fragment that delaminated from a late 19th to early 20th-century oil portrait. The hazy paint fragment consisted of zinc soaps (both crystalline and amorphous), gordaite (NaZn4Cl(OH)6SO4·6H2O), and zinc lactate, that could not all be well-resolved with ATR–FTIR imaging. With O-PTIR analysis, the degradation compounds could be resolved at sub-micron resolution with an equivalent or better signal-to-noise ratio. This case study shows how the two techniques can be used to obtain comprehensive information at a broad level with ATR–FTIR and a detailed level with O-PTIR
Oil paint is a dynamic system that undergoes chemical alteration on several time and length scales. At the short term, curing reactions are necessary for oil to dry properly. At longer time scales, a wide variety of other chemical processes can negatively affect the visual appearance or mechanical properties of historical artistic paint systems. The development of chemical imaging methods capable of covering length scales continuously from the millimetric to micro- or even nanoscale is key in understanding the chemical composition of a painting and the historical changes thereof. Such imaging methods can help in assessing to which extent the original painting’s composition may have been modified by chemical degradation processes. Processes that occur in the highly heterogeneous mixtures of binders, pigments, additives, alteration products and possibly later repainting and restoration treatments. Establishing the precise biography of the painting contributes to evaluate its authenticity. New modalities and novel methods of microchemical imaging provide access to previously unexplored length scales, are capable of better differentiation between the various oil paint components (original composition or later addition), and allow performing faster analysis to produce higher definition images. In this review, we report on recent methodological developments and future prospects to determine oil paints composition using microchemical imaging at the micro- and nanoscale.KeywordsImagingSpectroscopyMicroscopyPaintPigments
Formation and aggregation of metal carboxylates (metal soaps) can degrade the appearance and integrity of oil paints, challenging efforts to conserve painted works of art. Endeavors to understand the root cause of metal soap formation have been hampered by the limited spatial resolution of Fourier transform infrared microscopy (μ-FTIR). We overcome this limitation using optical photothermal infrared spectroscopy (O-PTIR) and photothermal-induced resonance (PTIR), two novel methods that provide IR spectra with ≈500 and ≈10 nm spatial resolutions, respectively. The distribution of chemical phases in thin sections from the top layer of a 19th-century painting is investigated at multiple scales (μ-FTIR ≈ 102 μm3, O-PTIR ≈ 10-1 μm3, PTIR ≈ 10-5 μm3). The paint samples analyzed here are found to be mixtures of pigments (cobalt green, lead white), cured oil, and a rich array of intermixed, small (often ≪ 0.1 μm3) zinc soap domains. We identify Zn stearate and Zn oleate crystalline soaps with characteristic narrow IR peaks (≈1530-1558 cm-1) and a heterogeneous, disordered, water-permeable, tetrahedral zinc soap phase, with a characteristic broad peak centered at ≈1596 cm-1. We show that the high signal-to-noise ratio and spatial resolution afforded by O-PTIR are ideal for identifying phase-separated (or locally concentrated) species with low average concentration, while PTIR provides an unprecedented nanoscale view of distributions and associations of species in paint. This newly accessible nanocompositional information will advance our knowledge of chemical processes in oil paint and will stimulate new art conservation practices.
Up to 70 % of the oil paintings conserved in collections present metal soaps, which result from the chemical reaction between metal ions present in the painted layers and free fatty acids from the lipidic binders. In recent decades, conservators and conservation scientists have been systematically identifying various and frequent conservation problems that can be linked to the formation of metal soaps. It is also increasingly recognized that metal soap formation may not compromise the integrity of paint so there is a need for careful assessment of the implications of metal soaps for conservation. This review aims to critically assess scientific literature related to commonly adopted analytical techniques for the analysis of metal soaps in oil paintings. A comparison of different analytical methods is provided, highlighting advantages associated with each, as well as limitations identified through the analysis of reference materials and applications to the analysis of samples from historical paintings. Conservation and Heritage scientists frequently face challenging issues related to metal soaps phenomena in oil paint systems. This review provides a critical overview of analytical techniques available for the identification, localisation and study of the presence of metal soaps. Advantages and disadvantages of each technique are discussed and the comparison between data collected from ad hoc and real painting samples is reported.
Oil paint binding media are complex polymer networks that harbor populations of metal ions. Understanding of the reactivity of these metal ions, often closely linked to paint degradation, is crucial to support paintings conservation efforts. By developing a spectrum fitting approach for the analysis of Fourier transform infrared spectra, we have studied in detail how the molecular structures of zinc carboxylate species in oil paint ionomers change in the lifetime of a painting. It was found that high ZnO pigment content, humidity, and low paint viscosity all stimulate the formation of ionomeric zinc carboxylate species, while the structures (chain or oxo) adopted by those species depend on carboxylate concentration and humidity. Moreover, we found evidence for a difference in reactivity between the two structures for ionomeric zinc carboxylates toward the formation of crystalline zinc soaps. The results have yielded an abundance of information about the internal chemistry of oil paint layers and metal-containing polymers in general.
Zinc white – zinc oxide – is a prevalent industrial age pigment which readily reacts with fatty acids in oil-based paints to form zinc carboxylates (zinc soaps). Pigment properties, oil composition, paint additives and environmental conditions are all significant factors in this process. Release of fatty acids via hydrolysis is particularly implicated. Formation and crystallization of zinc soaps in paintings has serious visual and structural consequences. Fourier transform infrared spectroscopy (FTIR) is a highly sensitive technique for detection of metal carboxylates. Progression of soap formation and distributions are distinguishable in paint cross sections through correlation of microscopic images, elemental analysis and FTIR mapping. Aluminum stearate was found to strongly influence zinc stearate formation during investigations of naturally aged oil-based artists’ commercial and custom-produced reference paints containing zinc oxide in various oil and pigment combinations. Zinc soaps are implicated in deterioration of increasing numbers of paintings dating from the nineteenth century onward. They may manifest as disfiguring lumps or surface blooms resistant to removal. In other paintings, paint cleavage is associated with high concentrations of crystalline zinc soaps at unstable interfaces. The condition of paintings affected by zinc soaps is challenging conventional approaches to cleaning and consolidation and requires continuing research.
A significant number of oil paintings produced by Georgia O’Keeffe (1887–1986) show surface protrusions of varying width, up to several hundreds of microns. These protrusions are similar to those described in the art conservation literature as metallic soaps. Since the presence of these protrusions raises questions about the state of conservation and long-term prospects for deterioration of these artworks, a 3D-imaging technique, photometric stereo using ultraviolet illumination, was developed for the long-term monitoring of the surface shape of the protrusions and the surrounding paint. Because the UV fluorescence response of painting materials is isotropic, errors typically caused by non-Lambertian (anisotropic) specularities when using visible reflected light can be avoided providing a more accurate estimation of shape. As an added benefit, fluorescence provides additional contrast information contributing to material characterization. The developed methodology aims to detect, characterize, and quantify the distribution of micro-protrusions and their development over the surface of entire artworks. Combined with a set of analytical in situ techniques, and computational tools, this approach constitutes a novel methodology to investigate the selective distribution of protrusions in correlation with the composition of painting materials at the macroscale. While focused on O’Keeffe’s paintings as a case study, we expect the proposed approach to have broader significance by providing a non-invasive protocol to the conservation community to probe topological changes for any relatively flat painted surface of an artwork, and more specifically to monitor the dynamic formation of protrusions, in relation to paint composition and modifications of environmental conditions, loans, exhibitions, and storage over the long term.
To understand the mechanisms and factors that trigger soap formation and the dynamics of the reactive compounds in paints, advanced nuclear magnetic resonance (NMR) and X-ray analyses, complemented by analysis with FTIR spectroscopy, were performed on a series of lead carboxylates and model paint samples. Similar spectroscopy and lead coordination were observed for lead carboxylates of carbon chain length of 9 to 11 and 16 (palmitic acid) and 18 (stearic acid). Experiments as a function of temperature and humidity provided insight into the factors that increase soap formation. The local dynamics of palmitic acid and lead palmitate, in a linseed oil matrix at different temperatures (T) were measured by ²H NMR spectroscopy. The results show the extent of mobility of palmitic acid and lead palmitate in the paint matrix, how they differ, and how they depend on T. The kinetics of soap formation in model paint films subjected to different relative humidities was monitored by ¹³C NMR spectroscopy; the rate of soap formation increases with relative humidity. The results are discussed in the context of their implications for the conservation and preservation of works of art affected by lead soap formation.
The painting C’est grace à nous by Asger Jorn shows unusually soft black paint with substantial delamination. To provide insight into the condition and conservation issues, samples were analysed using Fourier-transform Infrared Spectroscopy (FTIR), Direct temperature Mass Spectrometry (DTMS) and Scanning Electron Microscopy Energy Dispersive X-ray Spectroscopy (SEM-EDX). The results show that the oil medium of the black paint layer in contact with the zinc containing ground has contributed to the development of zinc fatty acid salts (zinc soaps). Examination of the painting confirmed that the paint delamination was mainly found where the black paint was unusually soft. The implications for delamination and considerations for a possible consolidation treatment of C’est grace à nous are discussed in the context of the analytical results.
The privately owned oil painting Composition with Color Planes 4 (1917) by Piet Mondrian (1872–1944) has been the subject of ongoing investigation since 2011. The painting consists of color planes in a field of differing whites. Some of these white areas suffer from delamination issues, in combination with flaking. Previous research demonstrated a link between the presence of zinc oxide and the delamination phenomena. More recently, the formation of zinc soaps was found to play a role. In this study, cross sections from both delaminating and relatively intact white areas were investigated with light microscopy, SEM-EDX, and ATR-FTIR imaging to obtain more information about the stratigraphy and condition of the paint layers. Two stages in metal soap formation were identified in the delaminating areas. The first stage consists of noncrystalline zinc soaps or zinc ions bound to carboxylate functional groups in the polymerized oil network. Crystalline zinc soaps, which represent the second, final stage of metal soap formation, are generally linked to the development of zinc soap related deterioration phenomena. In this case, they were found at the interface between the delaminating paint layers. Possible implications for treatment and factors that might trigger further delamination will be discussed.
Using the recently developed techniques of electron tomography, we have explored the first stages of disfiguring formation of zinc soaps in modern oil paintings. The formation of complexes of zinc ions with fatty acids in paint layers is a major threat to the stability and appearance of many late 19th and early 20th century oil paintings. Moreover, the occurrence of zinc soaps in oil paintings leading to defects is disturbingly common, but the chemical reactions and migration mechanisms leading to large zinc soap aggregates or zones remain poorly understood. State-of-the-art scanning (SEM) and transmission (TEM) electron microscopy techniques, primarily developed for biological specimens, have enabled us to visualize the earliest stages of crystalline zinc soap growth in a reconstructed zinc white (ZnO) oil paint sample. In situ sectioning techniques and sequential imaging within the SEM allowed three-dimensional tomographic reconstruction of sample morphology. Improvements in the detection and discrimination of backscattered electrons enabled us to identify local precipitation processes with small atomic number contrast. The SEM images were correlated to low-dose and high-sensitivity TEM images, with high-resolution tomography providing unprecedented insight into the structure of nucleating zinc soaps at the molecular level. The correlative approach applied here to study phase separation, and crystallization processes specific to a problem in art conservation creates possibilities for visualization of phase formation in a wide range of soft materials.
The William Robinson Gallery is the only gallery in Australia dedicated to the work of a living artist. The significance of this collection in conjunction with the artist's co-operation has provided impetus for a research project dedicated to compiling a technical art history of Robinson's materials and techniques for posterity, and to inform long-term conservation of the artist's works. Artist's interviews, technical analysis and an online conservation survey reveal that Robinson's materials and techniques evolved in response to the changes in his subject matter and also highlighted discrepancies where the presence of certain materials, or lack thereof, was unintended by the artist. Some of these materials, such as zinc stearate, are associated with deterioration in the artists’ work. Samples from 16 paintings were investigated using a combination of analytical techniques including optical microscopy, ultraviolet fluorescence photography, Fourier Transform Infrared microspectroscopy, Scanning Electron Microscopy with Energy Dispersive X-ray analysis and Synchrotron source Fourier Transform Infrared microspectroscopy.
Zinc oxide is a prevalent pigment in twentieth century oil based paints. Its reactivity with fatty acids has consequences for paint film properties and stability. Resulting zinc carboxylates are implicated in a variety of deterioration phenomena affecting late nineteenth and twentieth century paintings. Naturally aged artists’ oil paints and reference paint film draw downs are investigated using scanning electron microscopy, and conventional attenuated total reflectance and synchrotron transmission Fourier transform infrared spectroscopy. This study reports results for soybean oil paints and Naples yellow hue pigment mixtures with zinc oxide in addition to previously reported findings for linseed and safflower oil based paints involving zinc oxide alone and in combination with lead and titanium whites. A solvent model examining interactions between zinc oxide and stearic acid or aluminium stearate supports observations made in paint films. The significance of aluminium stearate as a stearic acid source in paint formulations is corroborated and is more significant than oil type or pigment combination for formation of high concentrations and more pronounced separation of saturated C16 and C18 chain zinc carboxylates.
Human skin emits a variety of volatile metabolites, many of them odorous. Much previous work has focused upon chemical structure and biogenesis of metabolites produced in the axillae (underarms), which are a primary source of human body odour. Nonaxillary skin also harbours volatile metabolites, possibly with different biological origins than axillary odorants.
To take inventory of the volatile organic compounds (VOCs) from the upper back and forearm skin, and assess their relative quantitative variation across 25 healthy subjects.
Two complementary sampling techniques were used to obtain comprehensive VOC profiles, viz., solid-phase microextraction and solvent extraction. Analyses were performed using both gas chromatography/mass spectrometry and gas chromatography with flame photometric detection.
Nearly 100 compounds were identified, some of which varied with age. The VOC profiles of the upper back and forearm within a subject were, for the most part, similar, although there were notable differences.
The natural variation in nonaxillary skin odorants described in this study provides a baseline of compounds we have identified from both endogenous and exogenous sources. Although complex, the profiles of volatile constituents suggest that the two body locations share a considerable number of compounds, but both quantitative and qualitative differences are present. In addition, quantitative changes due to ageing are also present. These data may provide future investigators of skin VOCs with a baseline against which any abnormalities can be viewed in searching for biomarkers of skin diseases.
This study describes the changes in the infrared spectra of oil paint as a result of aging. The focus is on the influence of pigments on the long-term changes in the oil binding medium. Several naturally aged paints made with different pigments were analysed using Fourier-transform infrared spectroscopy (FTIR). One of the most pronounced effects observed in the infrared spectra of aging paint is the shifting and broadening of the carbonyl band due to the formation of carboxylic acids. Another effect of pigments on the oil binding medium is the catalysis of the hydrolysis of triglycerides, as indicated by the decreasing intensity of the ester absorption. Finally, the nature of the pigment included has a profound effect on the CH stretch absorptions. From these results it is clear that pigments can significantly alter the infrared spectra of drying oil, and should therefore be identified to ensure the correct assessment of the infrared spectra in drying oil paint.
RESUME : Les carboxylates de zinc Zn(C n H 2n-1 O 2) 2 (nommés ZnCn) et de magnésium Mg(C n H 2n-1 O 2) 2 (H 2 O) 3 (nommés MgCn) protégeant la surface du métal correspondant ont été étudiés par diffraction des rayons X sur monocristal et/ou par rayonnement synchrotron sur des poudres. Les structures ont été déterminées pour n = 9-14 pour les carboxylates de zinc et pour n = 7 et 10 pour les carboxylates de magnésium. Ce sont toutes des structures lamellaires avec des feuillets reliés par des liaisons de Van der Waals. Le zinc a la coordination tétraédrique. Une revue des données structurales incluant des données de la littérature pour les composés au zinc permet de présenter la cristallochimie de tous ces composés. Dans le cas des composés au magnésium, la coordination du métal est octaédrique avec trois atomes d'oxygène de fonctions carboxylates et trois atomes d'oxygène de molécules d'eau. Des modèles structuraux de deux composés mixtes Zn(C10-14) et Zn(C10-16) sont proposés. Des polytypes sont observée pour les composés au zinc. MOTS-CLES : Savons métalliques, structures cristallines, inhibiteurs de corrosion, diffraction des rayons X, polytypes.
Groupe de Personnages (1938), a canvas painting created in Paris during the Spanish Civil War, required stabilization prior to exhibition. Upon examination, flaking paint and tiny spherical protrusions within those paint layers were identified. A project was initiated to determine materials that could have contributed to the current state of this painting. Because Groupe de Personnages had not undergone harsh conservation treatments this was a unique opportunity to study Miró's techniques and materials from a period of great artistic experimentation. Scientific analysis revealed the identity of pigments and the presence of zinc soaps. In addition, because Miró was a prolific writer, evidence was found in his letters, studio notes and interviews to corroborate analytical results – contributing to a more complete understanding of his artistic intentions and the conservation-related consequences of his selected materials.
Extended abstract of a paper presented at Microscopy and Microanalysis 2005 in Honolulu, Hawaii, USA, July 31--August 4, 2005.
Translucent white lumps or inclusions in red lead and lead-tin yellow type I paint layers are discussed in detail. These lumps are described as varying in size. Most often visible under the microscope and sometimes to the naked eye, the inclusions impart a gritty texture to paint films. Inclusions are found to be ubiquitous in European oil paintings with red lead and lead-tin yellow type I pigments. The earliest example of a lead soap inclusion has been identified in the 13th-century Westminster retable. Inclusions commonly are made up of lead carboxylates and lead carbonate and are formed by the interaction of the lead-containing pigments and oil-binding medium. Certain characteristics of the inclusions, such as the fluorescence of the particles in ultraviolet light and the reduced azelaic acid content, are proposed to be responsible for the misinterpretation of aged oil paint layers as being bound in protein. Techniques of analysis of inclusions presented in this article include energy dispersive x-ray analysis in the scanning electron microscope, x-ray diffraction, Fourier transform infrared spectroscopy, and thermally assisted transmethylation gas chromatography-mass spectrometry. The analysis of inclusions in paint layers is presented in tables that identify the paintings the samples are taken from, describe the paint layers, and list the content of the inclusions. An appendix details the preparation of lead fatty acid soaps and presents characteristic infrared frequencies for lead soaps of palmitic, stearic, and azelaic acids.
The structure of the title compound, poly[zinc(II)-bis(μ-octanoato-O:O′)], [Zn(C8H15O2)2]n, consists of polymeric sheets parallel to (100) in which tetrahedrally coordinated Zn²⁺ cations are connected by carboxylate bridges in a syn–anti arrangement.
Jean Paul Riopelle (1923–2002) was one of the most important Canadian artists of the twentieth century, yet he is relatively unknown in the U.S. He began his career in Montreal in the 1940s, where he played a role in the influential Automatist movement, and established his reputation in the burgeoning art scene of postwar Paris, where his circle included André Breton, Samuel Beckett, and Sam Francis. During his career, Riopelle produced over six thousand works, including more than two thousand paintings.
This volume, the second in the Artist's Materials series, grew out of a research project of the Canadian Conservation Institute. Initial chapters present an overview of Riopelle's life and situate his work within the context of twentieth-century art. Subsequent chapters address Riopelle's materials and techniques, focusing on his oil paintings and mixed media works, and on conservation issues. The preface is by Yseult Riopelle, the artist's eldest daughter and editor of his catalogue raisonné. This first book-length study of the artist in English will interest curators, conservators, conservation scientists, and general readers.
Marie-Claude Corbeil is manager of the Conservation Science Division at the Canadian Conservation Institute (CCI) in Ottawa. Kate Helwig and Jennifer Poulin are conservation scientists at the CCI.
Where did the crystalline particles found on art works come from?
To characterize more fully the metal soaps found in paint films or on metal surfaces, several metal soaps were synthesized and their X-ray diffraction pattern and Fourier transform infrared (FTIR) and Raman spectra measured. Metal soaps were obtained from four different fatty acids found in drying oils - two saturated (palmitic and stearic acids) and two unsaturated (oleic and linoleic acids) - and from copper, zinc and lead, three metals that are typically found in metal alloys and paint systems. Data are reported for the following compounds: palmitic acid, stearic acid, oleic acid, linoleic acid, zinc palmitate, zinc stearate, zinc oleate, zinc linoleate, copper palmitate, copper stearate, copper oleate, lead palmitate, lead stearate and lead oleate. Features that are characteristics of specific compounds were observed. Soaps obtained from different fatty acids with the same metal ion show differences, as do soaps obtained with the same fatty acid but with different metal ions. Identification is key to understanding how and why metal soaps form on actual objects, and this may lead to preventive measures.
The painting Couple with Clouds in their Heads by Salvador Dalí (1936) from the Museum Boijmans van Beuningen in Rotterdam was seriously disfigured by ring formations on the edges of the painting and a matt, white-looking semi-transparent material on the surface. Paint samples, taken from affected and unaffected paint areas, investigated with light, scanning electron and ATR-FTIR microscopy and X-ray diffraction showed that the zinc white-containing oil paint was converted into zinc soaps, zinc formate dihydrate, zinc acetate dihydrate and zinc sulfide. The conversion of the zinc white started at the surface of the painting. Zinc soaps are formed by a reaction of zinc white and fatty acids from the oil. Formate and acetate, most likely emitted from the wooden frame, reacted with the zinc white pigment. Finally, the zinc sulfide is suggested to derive from zinc white reacting with hydrogen sulfide. The high degree of degradation of the zinc white-containing paint and the ring formation is postulated to be triggered by the exposures of the panels to high temperatures which occurred during a 1-day photo shoot in 1936 shortly after completion of the paintings by Dali. Furthermore, it seems likely that the glazed frame created a (semi-)closed system and subsequently acid gasses diffused into the system via “leakages” due to a poor match between the frame and the panel, and may have initiated a reaction with the surface of the painting. As the glazed and framed panels were a non-ventilated system, the acid gasses did not distribute evenly over the surface, but rather were most intense at the edges of the painting. The ring formation caused by the gasses is compared with the phenomena of Liesegang rings, a reaction-diffusion process.
Zinc oxide is a prevalent pigment in twentieth century oil based paints. Its reactivity with fatty acids has consequences for paint film properties and stability. Resulting zinc carboxylates are implicated in a variety of deterioration phenomena affecting late nineteenth and twentieth century paintings. Naturally aged artists’ oil paints and reference paint film draw downs are investigated using scanning electron microscopy, and conventional attenuated total reflectance and synchrotron transmission Fourier transform infrared spectroscopy. This study reports results for soybean oil paints and Naples yellow hue pigment mixtures with zinc oxide in addition to previously reported findings for linseed and safflower oil based paints involving zinc oxide alone and in combination with lead and titanium whites. A solvent model examining interactions between zinc oxide and stearic acid or aluminium stearate supports observations made in paint films. The significance of aluminium stearate as a stearic acid source in paint formulations is corroborated and is more significant than oil type or pigment combination for formation of high concentrations and more pronounced separation of saturated C16 and C18 chain zinc carboxylates.
A recent visual survey of Abstract Expressionist-era paintings in the collection of the Hirshhorn Museum and Sculpture Garden (HMSG), Smithsonian Institution revealed a particular type of paint layer separation. Earlier work by the authors showed that zinc oxide in oil paint is a contributing factor to the problem. Ten samples from five Abstract Expressionist-era paintings as well as twenty-three samples eight years or older from the Smithsonian Institution’s (SI) Materials Study Collection were analyzed by pyrolysis – gas chromatography – mass spectrometry (Py-GC-MS), and unexpectedly significant amounts of oleic (cis-octadecenoic) acid were detected in samples containing high proportions of zinc oxide (25 % or greater by weight). In a typical fully cured oil paint, the oleic acid is oxidized to azelaic (nonanedioic) acid. Although the formation of zinc soaps in oil paints is well-known, the detection of zinc oleate in paints by Py-GC-MS has never been described. The close-packing of the oleate chains in the plate-like structure of zinc oleate prevents the oxidation of the cis-double bond, and therefore prevents the formation of azelaic acid. The detection of zinc oleate in paintings is an indication that the paint layers are at risk for future separation.
This study describes the examination and analysis of four mixed media paintings from the 1960s and 1970s by Canadian artist Jack Chambers (1931–1978). The documentary evidence about his materials and methods is summarized and compared with the results of analysis of multilayer paint samples. The combination of Fourier transform infrared spectroscopy (FTIR) and gas chromatography-mass spectrometry (GC-MS) allowed the components of the paint media to be characterized: ortho-phthalate alkyd resins, iso-phthalate alkyd resins, drying oils, dammar, Pinaceae resin, and turpentine were identified in varying proportions. Many pigments and fillers were identified by FTIR and Raman and are enumerated. The effect that Chambers’ complex technique has had on the aging and degradation of the paintings is discussed. The severe cracking of the paint layers in one of the four paintings may be the result of a high proportion of dammar and turpentine diluent mixed with the alkyd paint and may also be related to the type of alkyd resin medium. Different history and environmental conditions may also be factors.
The presence of zinc oxide oil paint and the condition problems observed in a group of paintings from the collection of the Hirshhorn Museum and Sculpture Garden prompted analytical examination of the museum's mid-20th century holdings. Results reveal a link between upper layer deterioration and underlying zinc oxide paint layers, and suggest that certain visible signs of deterioration may signal the presence of more serious and widespread condition problems. The popularity of zinc oxide house paint among mid-century artists creates a higher probability of this type of deterioration in works from this period.
Zinc-based metal-organic frameworks (MOFs), binary zinc carboxylates ZnCnCn′ with Cn and Cn′ = CH3(CH2)n - 2COO-, have been synthesised and characterised for anti-corrosion applications. The crystallographic structures of ZnC10C14 and ZnC 10C16 were determied from synchrotron powder diffraction data and refined by the Rietveld method. Protective coatings on electrogalvanised steel composed of ZnC12C16 have been developed by formulating a particular "carboxylating" bath. The ZnC12C16 coating exhibits better anti-corrosion behaviour than the pure Zn(C16)2 and Zn(C12)2 coatings, according to electrochemical and non-electrochemical measurements. The crystallographic results and corrosion measurements demonstrate the great flexibility of the zinc carboxylate lamellar structures in modifying the insolubility and hydrophobicity of the protective coatings. In addition, the conditions for the ZnC12C16 coating process fulfil the specifications for the surface treatment of zinc. Finally, these new compounds, which can be easily synthesised in water, provide a new and environmentally friendly anti-corrosion treatment for metals. New zinc-based metal-organic frameworks (MOFs), binary zinc carboxylates ZnCnCn′ with Cn and Cn′ = CH3(CH2)n - 2COO-, have been synthesised and characterised for anti-corrosion applications on zinc. The crystallographic structures demonstrate the great flexibility of these MOFs in modifying the insolubility and hydrophobicity of the protective coatings on metals in an aqueous solvent process. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
An analysis of emission spectra recorded on twelve metal formates under heating showed the existence of two crystal forms of anhydrous salts for some of them. The i.r. absorption spectra of these phases are reported. A comparison between final decomposition products and relative temperatures observed in air and under vacuum showed the effects of oxygen on the reactions involved. We discuss the possible mechanisms by which these reactions take place.
The crystallographic structures of metallic soaps acting as a barrier against aqueous corrosion, Mg(C 10H 19O 2) 2(H 2O) 3 (MgC10), Zn(C 11H 21O 2) 2 (ZnC11), Zn(C 12H 23O 2) 2 (ZnC12) and Zn(C 14H 27O 2) 2 (ZnC14) have been determined ab-initio from synchrotron powder diffraction data and refined by the Rietveld method. The structures are layered, the magnesium and zinc atoms being octahedraly and tetrahedraly O-coordinated in the sheets respectively. The zinc carboxylates present two various structures depending on the n value, the number of carbon in the aliphatic chains. Moreover, the powder patterns, for n constant, are complicated because of the presence of polytypes. Crystallochemistry of these metallic soaps is discussed.
Zinc oxide is a prevalent industrial-age pigment that readily reacts with fatty acids in oil-based paints to form zinc carboxylates. Zinc stearate aggregates are associated with deterioration in late nineteenth and twentieth century paintings. The current study uses both conventional and synchrotron Fourier transform infrared spectroscopy (FT-IR) to investigate metal carboxylate composition in a range of naturally aged artists' oil paints and reference paint film draw-downs. The paints contain zinc oxide alone or in combination with lead white, titanium white, and aluminum stearate and are prepared with linseed and safflower oils. Attenuated total reflectance (ATR)-FT-IR using the conventional source identifies marked differences in carboxylate profiles between exposed and protected surfaces in a large number of samples. Synchrotron FT-IR microspectroscopy of thin paint cross-sections maps metal carboxylate distributions at high spatial resolution and resolves broad concentration gradients and micrometer-scale phase separation of carboxylate species. Aluminum stearate, a common paint additive, is found to influence the distribution of zinc carboxylates more strongly than pigment composition or oil type. The presence of aluminum stearate results in higher concentrations and more pronounced separation of saturated C16 and C18 chain zinc carboxylates in the margin of paint nearest the polyester substrate. The presence of aluminum stearate in association with zinc oxide has a clear influence on zinc carboxylate formation and distribution, with potential implications for long term stability of vulnerable paintings.
Structure and transition behavior of zinc(II) stearate crystal were investigated by infrared and XAFS spectroscopies. Structure of zinc stearate at room temperature was estimated as follows. From XAFS analysis, the coordination number of the carboxylate groups around the zinc atom was evaluated as 4 and the ZnO distance as 1.95 Å. Based on the infrared spectrum and a normal mode analysis, the conformation of the alkyl chain was confirmed as all-trans and the sub-cell packing was considered as parallel type, and also the coordination form of the carboxylate groups was determined as bridging bidentate type. As increasing temperature, zinc stearate has a solid–liquid phase transition at 130°C. At the transition, the alkyl chains goes into liquid like state as reported by Mesubi but the coordination structure was confirmed to be maintained.
Max Beckmann's "Pierrette und Clown" (Pierrette and Clown), 1925, is undoubtedly one of the major works in the collection of the Kunsthalle Mannheim, Germany. In this painting, damages can be seen, described as protrusions. These are blisters and crater-like holes, filled with metallic soap aggregates. Painting samples and cross-sections of the ground layer have been examined by environmental scanning electron microscopy (ESEM) and by confocal synchrotron radiation micro-X-ray fluorescence (CSR[small mu ]XRF) in order to get compositional and spatial distribution information about the components and their behaviour. The analyzed samples of the unpainted ground layer consist of lithopone (ZnS[middle dot]BaSO4), as several bulges resembling small blisters, presumably protrusions in the nascent state. Direct CSR[small mu ]XRF measurements on closed blisters showed that these blisters have a high concentration of zinc in the centre with surrounding layers of barium and strontium. Relatively small amounts of lead have been recorded. In order to determine the composition of the organic compounds (e.g. fatty acids) and of the inorganic pigments (lead white, zinc white, lithopone), Raman spectroscopy has been successfully applied to flakes of an opened protrusion. The obtained Raman spectra could be assigned to derivatives of fatty acids and lithopones. These results allow conclusions to be made about the chemical behaviour by measuring the identical sample spot of the protrusion with CSR[small mu ]XRF as well as Raman spectroscopy
This review surveys analysis of airborne and vehicle emitted low-molecular-weight carboxylic acids. Attention is paid to providing a comprehensive coverage of collection techniques, sample handling, storage, extraction methods followed by a discussion of recent developments in carboxylic acid analysis using chromatographic (gas and ion chromatography) and electrophoretic (capillary electrophoresis) techniques. The occurrence and sources of carboxylic acids in the ambient air are also summarized.
The correlation of the infrared spectra of zinc(II) carboxylates with their structures was investigated in the paper. The complexes with different modes of the carboxylate binding, from chelating, through bridging (syn-syn, syn-anti, monatomic), ionic to monodentate were used for the study, namely [Zn(C6H5CHCHCOO)2(H2O)2] (I) with chelating carboxylate group (C6H5CHCHCOO=cinnamate), [Zn2(C6H5COO)4(pap)2] (II) with syn-syn bridging carboxylate (C6H5COO=benzoate; pap=papaverine), [Zn(C6H5CHCHCOO)2(mpcm)]n (III) with syn-anti carboxylate bridge (mpcm=methyl-3-pyridylcarbamate), [Zn(C5H4NCOO)2(H2O)4] (IV) with ionic carboxylate group (C5H4NCOO=nicotinate), [Zn(C6H5COO)2(pcb)2]n (V) with monodentate carboxylate coordination (pcb=3-pyridylcarbinol) and [Zn3(C6H5COO)6(nia)2] (VI) with syn-syn and monatomic carboxylate bridges (nia=nicotinamide). First, the mode of the carboxylate binding was assigned from the infrared spectra using the magnitude of the separation between the carboxylate stretches, Deltaexp=nuas(COO-)-nus(COO-). Then the values Deltaexp were compared with those calculated from structural data of the carboxylate anion (Deltacalc). The conclusions about the carboxylate binding which resulted from the Delta values, were confronted with the crystal structure of the complexes. The limitations and recommendations were formulated to assign the mode of the carboxylate binding from the infrared spectra. The dependence of the Deltaexp values on the magnitudes of Zn-O-C angles in bidentate carboxylate coordination was observed.
The room temperature structures and lattice arrangements of a homologous series of zinc(II) n-alkanoates from chain length, n(C) = 4-20, inclusive, have been studied using infrared spectroscopy, X-ray diffraction and polarizing light microscopy. Lattice parameters from single crystal and powder diffraction data, for zinc(II) hexanoate, are compared to validate the use of the powder method. Since they are in excellent agreement, the powder data are analyzed by a software programme to determine lattice parameters for all the homologues. These are used, in conjunction with infrared, X-ray, density and molecular model calculations to determine molecular and lattice structures. The compounds are isostructural, in that, each zinc atom is tetrahedrally coordinated to oxygen atoms from four different carboxylate groups and each ligand forms a Z,E-type bidentate bridge with two tetrahedral zinc atoms resulting in a syn-anti arrangement. The hydrocarbon chains are in the fully extended all-trans configuration and are tilted at an average angle of 60 degrees to the zinc basal plane. For the short chain length compounds with n(C) < or =8, a double bilayer in-plane-perpendicular-perpendicular-in-plane arrangement of hydrocarbon chains, with two molecules per unit cell, is indicated. For the others, an interdigitating in-plane-in-plane bilayer with head-to-tail interactions, with one molecule per unit cell, is proposed. A geometric model is presented to account for odd-even chain effects and to explain the differences in melting points and densities between these adducts. All the compounds crystallize in the monoclinic space group with P symmetry and are arranged in a two-dimensional network along the ac plane within the unit cell.
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