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Multivariate Analysis of Μ-Ftir Data
Atr-Ftir Analysis of Unmounted Gel Material
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Characterization of Zinc Carboxylates in an Oil Paint Test Panel

Profile image of Nicole LittleNicole Little

2019, Studies in Conservation

https://doi.org/10.1080/00393630.2019.1666467
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22 pages

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Abstract

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.

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Gillian Osmond

2000

Zinc oxide is a prevalent industrial age pigment which readily reacts with fatty acids in oil-based paints to form zinc carboxylates. It is present in many paintings from the late nineteenth century through until the present day. This thesis examines the reactivity of zinc oxide in the context of serious aesthetic and structural deterioration in paintings linked to formation and aggregation of zinc soaps of C 16 and C 18 straight chain saturated fatty acids. It extends current understanding of how zinc soaps form, why they aggregate and the conditions that promote deterioration. This knowledge is important for effective long term management of contemporary painting collections. Historical aspects of zinc oxide pigment production and paint formulation are reviewed. Pigment particle properties, fatty acid profiles, paint additives and environmental conditions are all significant factors influencing the reactive tendencies of zinc oxide in oil-based paints. Approaches to the characterisation of oil paint are reviewed. Techniques are emphasised which are appropriate for samples from unique paintings with implicit limitations on size and availability, particularly complementary techniques applicable to embedded paint cross-sections. Fourier transform infrared microspectroscopy (FTIR) is a highly sensitive technique for characterisation of metal carboxylates and a key tool in determining the progression of the formation of zinc soaps. Synthesis of zinc carboxylates of the common fatty acids found in mature paints is described and reference spectra are produced. The systematic application of FTIR and scanning electron microscopy (SEM) and elemental analysis (EDX) throughout this thesis identifies trends to assist interpretation of deterioration in historic paint samples. Correlation of backscatter electron (BSE) images, elemental analysis and synchrotron FTIR (SR-µFTIR) mapping of paint cross-sections suggests saponification of zinc oxide is distinguishable within paint cross sections at an early stage. Individual spectra extracted from SR-µFTIR maps allow discrimination between metal carboxylates present in a single sample based on high spatial and spectral resolution. Naturally aged oil-based artists' commercial and custom control reference paints containing zinc oxide are investigated to consider links between compositional factors and zinc carboxylate formation. Paints are prepared with linseed, safflower and soybean oils and pigmented with zinc oxide alone or in combination with lead or titanium whites or within Naples yellow hue formulations. Results establish the significance of aluminium stearate as a component in paint formulations in combination with zinc oxide; its presence is more significant than oil or pigment composition in influencing zinc stearate formation and metal carboxylate distributions in paints of otherwise similar iii composition and exposure history. Paints with zinc oxide as a secondary pigment are also susceptible to zinc stearate formation and segregation in the presence of a stearic acid source. Interactions between zinc oxide and stearic acid are examined within a solvent model designed to reduce and control variables impacting the study of paint films. Rapid reactions in solvent models using nano-sized zinc oxide are comparable to those observed longer term in reference paints with larger artist grade pigment. The products of reactions are characterised. A suspension which forms in the toluene model comprises zinc oxide particles surrounded by zinc stearate in characteristic bilayer structures with comparable morphology to that observed in samples from paintings. Paint cross-sections from zinc oxide-containing paintings by expatriate Australian artist E. Phillips Fox (1865-1915) and Vietnamese artist Nguyễn Trọng Kiệm (1933-1991) are examined in detail. Fox's paintings are typically painted on hygroscopic glue-based ground layers and/or have been subject to heat-based lining procedures or exposed to uncontrolled, sometimes extreme environments and are visibly affected by zinc carboxylate aggregation. The morphology and chemical signature of circular formations rich in zinc, carbon and oxygen common to the paintings are examined in detail using SEM-EDX and SR-µFTIR imaging. Crystallisation of basic zinc carbonate is observed within aggregates. Kiệm's paintings have been exposed to hot, humid, polluted conditions and extensive hydrolysis of the oil medium is indicated. The presence of unusual zinc and sulphur containing lumps in one work has been investigated previously and the proposed mechanism of formation is reassessed. X-ray microdiffraction allows a tentative hypothesis of formation of multifunctional layered basic zinc salts. Preferential dissolution of zinc oxide over lead white, lithopone and a range of coloured pigments is observed in backscatter electron images, interpreted in conjunction with elemental and FTIR analysis. Structural consequences are indicated for affected paints. Zinc lactate is characterised in samples from paintings by both profiled artists; zinc lactate has been reported previously as a disfiguring surface accretion but these are the first known instances of zinc lactate identified within the paint. Unique paintings are chemically complex and infinitely variable. This complexity coupled with interactions occurring over long timescales is challenging to replicate experimentally, and fundamental mechanisms are difficult to establish. This thesis combines results from simple models, reference paints and actual paintings and contributes important new detail to the continuing study of zinc soap formation in paintings. iv Declaration by author This thesis is composed of my original work, and contains no material previously published or written by another person except where due reference has been made in the text. I have clearly stated the contribution by others to jointly-authored works that I have included in my thesis. I have clearly stated the contribution of others to my thesis as a whole, including statistical assistance, survey design, data analysis, significant technical procedures, professional editorial advice, and any other original research work used or reported in my thesis. The content of my thesis is the result of work I have carried out since the commencement of my research higher degree candidature and does not include a substantial part of work that has been submitted to qualify for the award of any other degree or diploma in any university or other tertiary institution. I have clearly stated which parts of my thesis, if any, have been submitted to qualify for another award. I acknowledge that an electronic copy of my thesis must be lodged with the University Library and, subject to the General Award Rules of The University of Queensland, immediately made available for research and study in accordance with the Copyright Act 1968.

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Evolution of Zinc Carboxylate Species in Oil Paint Ionomers
Katrien Keune

ACS Applied Polymer Materials

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.

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An infrared spectroscopic study of the nature of zinc carboxylates in oil paintings
Katrien Keune

J. Anal. At. Spectrom., 2015

A systematic search to explain the COO− vibration band shift for zinc carboxylates formed in a work by van Gogh.

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Electron Microscopy Imaging of Zinc Soaps Nucleation in Oil Paint
Gillian Osmond

Microscopy and microanalysis : the official journal of Microscopy Society of America, Microbeam Analysis Society, Microscopical Society of Canada, 2018

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 d...

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The concentration and origins of carboxylic acid groups in oil paint
Katrien Keune

RSC Advances, 2019

Zinc complexation can be used to quantify the concentration of carboxylic acid groups in an oil painting.

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    Uncovering hidden jewels: an investigation of the pictorial layers of an 18th-century Taskin harpsichord
    Roxana Radvan, Lucian Ratoiu, Ioana Maria Cortea

    Heritage Science, 2020

    The Peleș National Museum in Sinaia, Romania, includes within its collection of musical instruments a unique harpsichord of the Taskin workshop. The instrument is representative of the eighteenth century French harpsichords, finished in 1772 by Taskin Pascal, a master instrument-maker of his time. This paper presents the results of a comprehensive diagnostic investigation carried out on the exterior decoration of the harpsichord, by means of multiple non-invasive and micro-invasive analytical techniques: multispectral imaging, X-ray fluorescence (XRF) spectroscopy, laser-induced breakdown spectroscopy (LIBS), Fourier transform infrared spectroscopy (FTIR), and optical microscopy (OM). Imaging investigation highlighted various past interventions and restorations—fillings, consolidation and chromatic reintegration of the paint layer, as well as some interesting details related to the preparatory layer and on the painting technique. Results of the elemental analysis indicated that a rich color palette was used for the luxurious decorations: vermilion for most of the red areas, lead white, a copper-based pigment for the blue and violet hues, along with Prussian blue, chromium oxides for the green tones and possibly emerald green, and iron oxides for the red, yellow and ochre tones. FTIR analysis highlighted the existence of two-layers ground based on the first layer of chalk and animal glue, covered by a second oil-bound layer of lead white. Metal soaps, gypsum, barium sulfate as well as an intermediate sizing layer based on shellac were also identified. Examination of samples’ cross-sections revealed a rich stratigraphy, up to ten layers being highlighted for samples taken from the case sides’ painting. The presence of two gold foils on the gilded areas, one at the top and the other within the intermediate layers, provides clear evidence of later interventions. The corroborated results offer valuable physico-chemical insights on the complex history of this prestigious harpsichord with remarkable aesthetic and historical value, as well as key information for its most suitable restoration approach.

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    The Identification of Multiple Crystalline Zinc Soap Structures Using Infrared Spectroscopy
    kate helwig

    Applied Spectroscopy

    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, unsaturated, 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, b...

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