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Lomax S.Q.,National Gallery of Art
Journal of Coatings Technology Research | Year: 2010

Over 50 commercial paints were examined by x-ray powder diffraction in order to try and identify the synthetic organic pigments present. The binders included acrylic, oil, gum, and alkyd. Some pigments could be identified, though analysis is often complicated by the presence of large amounts of fillers and extenders in the paints relative to the small quantities of the pigment. A few of the paints did not have reflections due to fillers or extenders but the pigments could still not be identified. The best success in identifying the pigments was with acrylic binders, where the pigments could be identified in more than half of the samples examined, and with alkyds, where the pigment could frequently be identified. However, other binders, especially oil and gum, contain so many fillers that the pigment reflections are obscured. X-ray powder diffraction, therefore, is of limited utility in the identification of synthetic organic pigments in paints. © FSCT and OCCA 2009. Source

Carretti E.,University of Florence | Bonini M.,University of Florence | Dei L.,University of Florence | Berrie B.H.,National Gallery of Art | And 3 more authors.
Accounts of Chemical Research | Year: 2010

The works of art and artifacts that constitute our cultural heritage are subject to deterioration, both from internal and from external factors. Surfaces that interact with the environment are the most prone to aging and decay; accordingly, soiling is a prime factor in the degradation of surfaces and the attendant disfigurement of a piece. Coatings that were originally intended to protect or contribute aesthetically to an artwork should be removed if they begin to have a destructive impact on its appearance or surface chemistry. Since the mid-19th century, organic solvents have been the method of choice for cleaning painted surfaces and removing degraded coatings. Care must be taken to choose a solvent mixture that minimizes swelling of or leaching from the original paint films, which would damage and compromise the physical integrity of all the layers of paint. The use of gels and poultices, first advocated in the 1980s, helps by localizing the solvent and, in some cases, by reducing solvent permeation into underlying paint layers. Unfortunately, it is not always easy to remove gels and their residues from a paint surface. In this Account, we address the removal problem by examining the properties of three classes of innovative gels for use on artwork-rheoreversible gels, magnetic gels, and "peelable" gels. Their rheological properties and efficacies for treating the surfaces of works have been studied, demonstrating uniquely useful characteristics in each class: Rheoreversible gels become free-flowing on application of a chemical or thermal "switch". For art conservation, a chemical trigger is preferred. Stable gels formed by bubbling CO2 through solutions of polyallylamine or polyethylenimines (thereby producing ammonium carbamates, which act as chain cross-links) can be prepared with a wide range of solvent mixtures. After solubilization of varnish and dirt, addition of a weak acid (mineral or organic) displaces the CO2, and the resulting free-flowing liquid can be removed gently. Incorporation of magnetic, coated-ferrite nanoparticles into polyacrylamide gels adds functionality to a versatile system comprising oil-in-water microemulsions, aqueous micellar solutions, or xerogels that act as sponges. The ferrite particles allow the use of magnets both to place the gels precisely on a surface and to lift them from it after cleaning. Novel formulations of poly(vinyl alcohol)-borate gels, which accept a range of organic cosolvents, show promise for swelling and dissolving organic coatings. This family of gels can be quite stiff but can be spread. They are non-sticky and have sufficient strength to be removed by peeling or lifting them from a sensitive surface. These three classes of gels are potentially very important soft materials to augment and improve the range of options available for conserving cultural heritage, and their interesting chemical-physical properties open a rich area for future scientific investigation. © 2010 American Chemical Society. Source

Lomax S.Q.,National Gallery of Art | Lomax J.F.,U.S. Naval Academy | Luca-Westrate A.D.,U.S. Naval Academy
Journal of Raman Spectroscopy | Year: 2014

Synthetic organic pigments are widely used in modern and contemporary works of art. They have been examined by a variety of techniques including spectroscopic methods such as Fourier transform infrared spectroscopy, Raman, and X-ray powder diffraction as well as chromatographic or mass spectrometric techniques such as pyrolysis-gas chromatography/mass spectrometry and laser desorption ionization mass spectrometry (LDI-MS). Often, a combination of techniques has been used to examine these pigments. This paper describes use of Raman spectroscopy to create a database of colorants including two pigments not previously reported, PO1 and PO2. Then, using Raman spectroscopy in combination with LDI-MS, samples from modern works of art by artists including Mark Rothko, Barnett Newman, and José de Rivera were examined in order to identify the pigments present. One finding was that Rothko used a variety of red pigments over the course of his career including PR11 which has not been previously reported in artwork, and PO2 found with its positional isomer PR1. Knowledge of the colorants serves to inform conservators about display and treatment decisions. Published 2014. This article is a U.S. Government work and is in the public domain in the USA. Raman spectroscopy, in combination with laser desorption ionization mass spectrometry, was used to examine 130 standard synthetic organic pigments including several previously unpublished pigment spectra. These spectra were used as references in the examination of samples from works of art, including, among others, red pigments used by Mark Rothko. © Published 2014. This article is a U.S. Government work and is in the public domain in the USA. Source

Simonot L.,CNRS Pprime Institute | Thoury M.,French Natural History Museum | Delaney J.,National Gallery of Art
Journal of the Optical Society of America A: Optics and Image Science, and Vision | Year: 2011

Photoluminescence is one of the processes by which photons are emitted after the absorptionof incoming photons at a higher energy. But the yield and spectral band shape of the emission can be altered by the optical properties of the luminophore environment through scattering and absorption. To understand these effects on a photoluminescent turbid layer, the Kubelka-Munk model, which is a two-flux approximation of the radiative transfer equation, can be used. Compared to previous works, this translucent layer can be applied on a colored opaque background. The model takes into account the absorption, scattering, and luminescent properties of the layer and the reflection by the background, for both the light excitation and the light emission. The competition between these different optical interactions is studied; e.g., the model can predict the presence of an emission maximum by increasing the thickness of the luminescent layer on a light background. Moreover, the model is extended to two important cases: the presence of a photoluminescent background and the effect of a refractive index discontinuity. © 2011 Optical Society of America. Source

Jeong M.-J.,French Natural History Museum | Dupont A.-L.,French Natural History Museum | de la Rie E.R.,National Gallery of Art
Cellulose | Year: 2012

The degradation of cellulose in paper due to the formation of a tideline at the wet-dry interface when paper is suspended in water was explored. SEC/MALS was used to assess the molar mass, while ICP/MS, SEM/EDS and CE/UV provided a qualitative and quantitative analysis of the elements and inorganic ions present in the paper. Immediately after the formation of the tideline, no significant depolymerization was observed at the wet-dry interface, despite the accumulation of water soluble brown and/or fluorescent degradation compounds and salts containing sodium, chlorine, sulfur and calcium. Various artificial aging configurations were applied to the paper with tidelines to evaluate the effect of the material accumulated at the wet-dry interface on the long-term stability of paper. The decrease in the molar mass of cellulose (above, at and below the tideline) differed depending on the type of aging, i. e. whether the entire sheet of paper was aged or whether small amounts of paper from the different areas were sampled and aged, individually or together, which evidences a complex degradation pathway. In the former aging configuration the material accumulated in the tideline affected the degradation of the tideline area to the same extent or more than the other areas. When the different areas of the paper sheet were sampled and aged together, it was found that the presence of the tideline clearly affected the degradation of the other paper areas. Conversely, in that case, cellulose within the tidelines was the least degraded. The area below the tideline, through which the water migrated, showed the most significant degradation. © 2012 Springer Science+Business Media B.V. Source

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