Tom Learner

Analytical mass spectrometry of artists’ acrylic emulsion paints by direct temperature resolved mass spectrometry and laser desorption ionisation mass spectrometry

Abstract Direct temperature resolved mass spectrometry (MS) is a microanalytical technique to analyse modern paints by thermal separation and ionisation of organic pigments and polymeric fractions from a platinum/rhodium filament inside the ionisation chamber of the mass spectrometer. Most organic pigments in modern paints are desorbed at lower temperatures. Ethyl acrylate/methylmethacrylate or butyl acrylate/methylmethacrylate copolymers used in acrylic emulsion paints produce mono and oligomeric subunits released from the copolymers at high temperature by pyrolysis. Characteristic low voltage electron ionisation and ammonia chemical ionisation (CI) mass spectra of these copolymers facilitate their identification. DTMS of three different commercial acrylic emulsion paints showed low and high temperature events that could be related to the presence of organic pigments and the acrylic copolymers. Polyethylene glycols with molecular weight up to 2000 Da were identified as additives under ammonia CI conditions. The azo pigments PY3, PY73 and PY74, and the phthalocyanine pigment PG7 reported show molecular ions and a few characteristic fragment ions under direct temperature resolved mass spectrometry (DTMS) analytical conditions. Yellow azo pigments were identified under DTMSMS conditions by their high energy collisionally induced fragment patterns of their parent ions. Laser desorption ionisation mass spectrometry (LDIMS) using a nitrogen laser (337 nm) of an acrylic emulsion paint with four different organic pigments produced radical cations, protonated or sodiated ions of the pigments. The amide bond in the azo pigment PY3 was photolytically cleaved and produced a specific fragment ion. The trace additive polyethylene glycol was observed preferentially while the acrylic copolymers were transparent.

A Review of the Classes, Structures, and Methods of Analysis of Synthetic Organic Pigments

Abstract Synthetic organic pigments have been manufactured since the latter part of the 19th century. Thousands of these pigments have been introduced for a variety of applications, including paints, printing inks, and the coloration of plastics and textiles. Many synthetic organic pigments are azos, containing the azo linkage (−N=N−). Newer classes of synthetic organic pigments have been introduced to overcome limitations of poor lightfastness or solvent fastness(bleed resistance) exhibited by some of the earlier pigments. This paper reviews the major classes of synthetic organic pigments in terms of their structures, physical properties, and specific applications. These classes include arylide yellows, diarylide yellows, β-naphthols, naphthols, benzimidazolones, disazo condensation pigments, pyrazolones, nickel azo yellow, phthalocyanines, quinacridones, perylenes and perinones, isoindolinone and isoindoline pigments, triarylcarbonium pigments, diketopyrrolo-pyrrole pigments, thioindigoids, and several miscellaneous pigments. Current methods of analysis of these pigments are discussed.

A review of synthetic binding media in twentieth‐century paints

Abstract Numerous synthetic resins have been developed for use as binding media in twentieth‐century paint. These synthetic paints, including those intended for household or industrial markets as well as those designed specifically for artists’ use, have been widely used on works of art. However, most painting conservators are still relatively unfamiliar with the types of synthetic resin used. Although the subject is vast and complex, the binding media used in this field can be simplified into four principal classes: acrylic, alkyd, polyvinyl acetate and nitrocellulose. This paper outlines the nature, use and salient properties of these four classes and includes sections on their probable additives. Despite the difficulty in identifying additives, an awareness of which types are likely to be present will greatly assist the understanding of each binders properties.

Conservation concerns for acrylic emulsion paints

Abstract Acrylic emulsion (or more accurately dispersion) paints present major challenges to conservators, yet remarkably few studies of these materials have been published. Although frequently associated with paintings, these paints are also encountered on other supports, such as paper, textiles, objects and walls. The intent of this paper is to present much of the conservation information that does exist in a concise format to expedite much needed further discussion and research by conservators, paint manufacturers and artists. Brief descriptions of the development and analysis of acrylic emulsion paints are given, but the focus of this review is on conservation concerns, in particular issues surrounding the properties of paints, aging and cleaning.

A cause of water-sensitivity in modern oil paint films: The formation of magnesium sulphate

Abstract Unvarnished twentieth-century oil paintings are often sensitive to aqueous swabbing, a method routinely employed by conservators for surface cleaning. This study proposes a connection between sensitivity and the presence of magnesium sulphate heptahydrate which has been identified on the surface of some of water-sensitive paintings. The probable source of magnesium is magnesium carbonate, an additive in some twentieth-century oil paints, which has reacted with atmospheric sulphur dioxide (SO2). Films made using modern manufactured paints and formulations made in the laboratory were exposed to gaseous SO2 and raised relative humidity and examined using scanning electron microscopy with energy dispersive X-ray spectroscopy and X-ray diffraction to characterize the crystalline entities. Films containing magnesium carbonate formed magnesium sulphite and sulphate hydrates. Films containing zinc oxide were also investigated. These formed zinc and sulphur containing salts. Sensitivity to swabbing with water before and after exposure was evaluated. Films that developed salts, demonstrated increased sensitivity to aqueous swabbing after exposure to SO2. Findings suggest that increased water sensitivity may be due to a combination of the formation of hygroscopic degradation products and to weakening of the paint film due to salt-induced disruption of the surface.

Issues in Contemporary Oil Paint

This chapter introduces recent research related to aspects of the deterioration of modern paintings in oil media. The research was informed by collaboration between conservators and scientists and utilises phenomenological and experimental methodologies to investigate the causes of optical changes in paint, such as efflorescence and salt formation, and the sensitivity to water that occurs on surface cleaning some unvarnished oil paintings. Examples are given of results based on case studies of paintings and the results of systematic experimental investigation of paint samples, including those supplied by manufacturers and reconstructions based on paint formulations. Results of this research are given including the cause of water sensitivity related to the formation of magnesium sulphate hydrate in selected manufactured oil paints, and criteria for further investigation of the phenomena in other paints. Contributions in this volume that address other classes of material deterioration including the formation of metal soap crusts and treatment approaches are introduced.

A Preliminary Evaluation of the Surfaces of Acrylic Emulsion Paint Films and the Effects of Wet-Cleaning Treatment by Atomic Force Microscopy (AFM)

Abstract The surfaces of acrylic emulsion (dispersion) paint films were investigated by atomic force microscopy (AFM), with supporting chemical information provided via infrared spectroscopy. The morphological and chemical features of the surfaces of a series of recently cast reference paint films were explored, including characterization of changes induced by exposure to water and mineral spirits. A portable in situ AFM system was also used to document the surfaces of acrylic emulsion paintings in Tates collection. The complete and partial removal of migrated surfactant was successfully imaged alongside features arising from mechanical action. Differences in paint surfaces and migrated surfactant layers were observed in relation to pigment type, paint brand and exposure to accelerated ageing, in addition to applied wet surface cleaning treatments. The findings contribute to further understanding of the subtle changes occurring at acrylic emulsion paint film surfaces and are relevant to current debates on the removal/disturbance of original surface surfactant from these films through conservation treatment.

The evaluation of Laropal A81: Paraloid B‐72 polymer blend varnishes for painted and decorative surfaces – appearance and practical considerations

Abstract The search for stable varnishes which will saturate and provide a desired finish for a variety of painted and decorative surfaces is ongoing. Varnish blends were investigated, composed of ratios from 2% to 98%, of Laropal A81 (a highly stable and commercially available low molecular weight urea‐aldehyde resin) in Paraloid B‐72, in order to capitalise on the positive properties of the individual resins and allow tailoring for specific surface problems in a single application. Aspects relevant to appearance and practice are reported in this paper. The structure of dried films was microscopically examined and their refractive index (RI) measured. Their effect on painted surfaces was evaluated through both instrumental measurement of specular gloss and saturation, and through subjective evaluation, which was influenced by diffuse as well as specular reflectance. Dried films of the entire range of blends were found to exhibit useful optical and aesthetic properties on a variety of paint surfaces, although it was felt that blends at either end of the range produced the most promising varnishes. Although the blends produced films that were phase separated, this did not cause any detrimental optical effects and may even contribute to the range of achievable surface effects. Subjective evaluation of appearance and working properties is shown to be important in any such investigation.