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CLARIFYING THE HAZE Efflorescence on Works of Art Paintings, sculptures, and dorks on paper dating from the late 19th through 20th centuries from The Museum of Modern Art in New York (MoMA) and the Los Angeles County Museum of Art (LACMA) with a wide range of conservation and exhibition histories were studied when hazy, whitish, obscuring patches were observed on their surfaces. The art conservation literature mentions various phenomena that have produced these white patches, for example, opacification of a surface varnish deposition of foreign matter growth of microorganisms emergence of light scattered from subsurface voids in a paint layer or light scattered from fine particles on the surface Although we had not planned to restrict our investigation to any of these phenomena, it turned out that all 20 analyzed works (by 16 artists) had fine, clearto-translucent particles on their surfaces. Mold, microfissures, or other phenomena did not appear to play any significant role in producing the haziness. This investigation became an opportunity to pursue a specific aspect of the problem in more detail, namely, the presence of concenot Iight-scattering particles on the surface of fine art
Eugena Ordonez The Museum of Modern Art, New York
John Twilley Los Angeles County Museum of Art 416 A
Where did the crystalline particles found on art works come from? Background In the 1950s, it was belleved that this "crystalline bloom" was caused primarily by ammonium sulfate crystals on the surface (i). Recent work on museum artifacts and ethnographic objects (2,3) has shown that other compounds can scatter the light as well. Only a few papers in the conservation literature address this problem on paintings or works on paper (e.g. 4—6) and dometimes the findings are contradictory. In art conservation, the terms "blanching", "chalking", and "bloom" have been used inconsistently to describe the various phenomena that result in irregular hazy patches on surfaces. With the hindsight of our findings, we have chosen the term "efflorescence". In architectural conservation, the building industry, and geology, efflorescence refers to salts found on the surfaces of walls or rocks. In these cases, water-soluble compounds, which are at least partly derived from the substrate materials migrate through the substrate to the surface There they might react with compounds in the environment or change their state of hydration to form
Analytical Chemistry News & Features, July 1, 1997
visible clusters. We have broadened the term to include compounds other than inorganic salts and to refer to the migratory process as well as the end product. Because the works of art are of significant aesthetic and cultural importance, only very small samples, barely perceptible to the naked eye (~ 50 to 75 um)) can be extracted. Analysis of these minute samples has been made possible only by advances in microanalytical methods such as FT-IR microspectroscopy which we used extensively Other methods include X-ray diffraction (XRD) with a Gandolfi camera transmitted polarized light microsconv and scanning electron microscoDV (SEM) with energy-dispersive X-ray spectrometry GC/MS was used in only a few cases because of limited access to the instrument Some elemental analysis was done without sampling by using X rav-exciterl enerov rlispersi e X ra tr try (¥T)^) Fatty acid efflorescence Approximately two-thirds of the studied works had free fatty acid deposits on their surfaces. These works were created from various paints, including oil, egg tempera (egg yolk), alkyd (ester modified with drying oil), wax crayon, and oilstick (drying oil mixed with wax). Whereas other research had found free fatty acids to be only a minor component in blemishes on paintings (4), we found it to be the primary component. A dramatic example of fatty acid efflorescence was found on a group of sculp0003-2700/97/0369-416A/$14.00/0 © 1997 American Chemical Society
Andrew Wyeth, Christina's World, 1948. Tempera on gessoed panel, 81.9 x 121.3 cm. The Museum of Modern Art, New York. Purchase. Photograph copyright 1997, The Museum of Modern Art, New York.
tures from the 1960s by Claes Oldenburg in the Museum of Contemporary Art, Los Angeles. These works, consisting of cast plaster painted with alkyd paint, had a velvety coat of white crystals on certain colored areas. Analysis of three sculptures by FT-IR yielded spectra consistent with those of long-chain fatty acids, but XRD yielded a pattern unmatched by any phase in the International Center for Diffraction Data (ICDD) reference set. The melting point was 56 "C. Similar analyses of a commercial sample of United States Pharmacopeia (USP)-grade stearic-palmitic acid clarified the problem by matching the diffraction pattern IR spectrum and melting point This product consists of 40%
stearic acid, 40% palmitic acid, and 10% oleic acid, with the balance unspecified (Figure 1). The palmitic(p)-stearic(s)-oleic system included discrete phases incorporating p-p, p-s, and s-s dimers with no solid-solution behavior in between the stoichiometric ratios (7). Furthermore, oleic acid and other monounsaturates do not form crystalline phases with either palmitic (8) or stearic acid (9), and binary systems containing two unsaturated acids do not form mixed-acid compounds, being simple eutectic mixtures in each case (10). Hence the non-involvement of the oleic acid in the diffraction results is understandable.
An oil painting on linen canvas at MoMA by Pierre Clerk, Painting II done in 1955, also exhibited efflorescence in certain colored areas, especially black and deep burgundy. The IR spectra of this sample were very similar to that of the Oldenburg sculpture except for the presence of a small amount of fatty acid salt as indicated by a small peak at 1564 cm"1. The fine surface particles differed throughout the work, appearing dendritic in one erea and blocky in another, but the IR spectra were consistent An extensive case of efflorescence was found on the cover of a leather-bound volume of Death of a Salesman by Arthur Miller at MoMA. The surface was covered
Analytical Chemistry News & Features, July 1, 1997 4 1 7 A
Analytical
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Figure 1 . FT-IR spectra of (top) efflorescent crystals from Red Table and (bottom) USP stearic-palmitic acid.
with afluffywhite accumulation resembling mold, located predominantly along the recesses of the leather grain (Figure 2). Again, the IR spectrum showed that the efflorescence was primarily fatty acids. The ubiquitous presence of fatty acid efflorescence in works of art that varied considerably in their material composition, construction, and exhibition and treatment history prompted us to investigate the possible sources of fatty acid. Free fatty acids have been incorporated into paint systems by several routes, including direct addition during formulation and later as additions by artists attempting to achieve certain visual effects. Materials used in conservation treatments can also be a source of fatty acids. C o m m e r c i a l s o u r c e s of f a t t y acids
Raw linseed oil naturally contains a small percentage of free fatty acids. Stand oils are made by heating oil in the absence of air, resulting in the isomerization of unsaturated fatty acids and other distinctive changes without oxidation. Linseed stand oils contain higher levels of free fatty acids because of thermally induced triglyceride cleavage. Roller and Burmester deduced that a stand oil had been used in the extensively efflorescing areas on a painting by Serge Poliakoff because of the presence of the high amount of free fatty acids and isomerized linoleic acids found 418 A
in the paint film (5). An area exhibiting no efflorescence in the same painting contained no isomerized linoleic acids and only a small amount of free fatty acids, suggesting to Roller and Burmester that the artist had used a cold-pressed oil. Because of the lack of unsaturated bonds, certain fatty acids such as palmitic (P) and stearic (S) acid remain uninvolved in the cross-linking reactions during film formation. The ratio of these fatty acids found in dried paint films, determined by derivatization GC/MS, has been widely used to infer the oil source for paints (1113). Linseed oil has a P/S ranging between 1.1 and 2.3, with the most frequent ratio at 1.7; walnut and safflower oils have their most frequent ratios near 2.6; and poppyseed oil, near 5. In some cases, the ratios of these fatty acids in the efflorescence are so close to those in the oils that it suggests that they have arisen exclusively by release from the paint medium itself (6). The very high P/S of 6.5 in the efflorescence from the Oldenburg alkyd example strongly suggests that he used a "long oil" alkyd based on soya oil, because the P/S in soybean is frequently near 6. However, with the advent of modern, finely divided pigments, especially the organic pigments, fatty acid salts were added to compensate for the loss of wetting ability. Surface active agents such as aluminum stearate, ammonium stearate, and zinc stearate were added during the
Analytical Chemistry News & Features, July 1, 1997
grinding of the pigment with oil. Aliphatic amines (e.g., stearyl monoamine) have also been used for several decades to coat organic pigments (14). The charged end of the molecule adsorbs directly onto the pigment surface with the long-chain hydrocarbon oriented outward, giving the pigment particles an oleophilic nature. The amount of surface active agent added varies. According to some sources in the paint industry, quantities of aluminum stearate on the order of 2% by weight of pigment are used, although others say that the amount is notfixed.It should be noted though that because aluminum stearate is so light, with a specific gravity of around 1, it can be present in a much higher volume percentage than the weight percent would suggest (15). In assessing product information, it is important to be aware that the product names in common usage for these compounds may be misleading. For example, aluminum stearate implies a trialkanoate; yet synthesis of the trialkanoate requires anhydrous procedures, and the product spontaneously undergoes hydrolysis in the presence of moisture (16). Therefore, manufacturers offer grades that are differentiated by melting point and are actually mixtures of the distearate and free fatty acids Also because the feedstock from which these stetirtite salts are iria.de is virtually never nnre stearic acid but rather a shishy mixture of stearic palmitic oleic and other minor SDecj„„
0f(-en
as little as
9Rty stearic acid anrl npnrlv SfW nalmitir
acid are present Therefore the "aluminum stearate" added to piernents during
grinding may include a considerable amount of free stearic acid as well la other fatty acids. In the 19th century, beeswax was often added to improve paint consistency, eliminate the segregation of pigment and oil during storage, and reduce yellowing of oils (17). By the 1890s, as much as 30% beeswax had been added to paints, causing serious defects such as darkening and cracking paint films (18). The composition of the beeswax (a complex mixture of predominantly fatty acid monoesters up to C and free fatty acids and paraffmic hydrocarbons up to C ) is such that efflorescence of wax components could be a problem when added in these high concentrations
Artists' recipes
Commercially manufactured art materials have frequently been altered by artists for practical and aesthetic reasons. Artists' manuals recommended "sweetening" slow-drying oils to avoid rancidity caused by free fatty acids in the oils. Reaction with a base such as sodium bicarbonate or quicklime (calcium oxide) or by standing with chalk (calcium carbonate) or lead white (basic lead carbonate) would precipitate the fatty acid salts (17) The oii could then be used for grinding pigments with less concern that the paint would turn rancid or harden while stored in paint tubes Artists have also gone to great lengths to obtain specific properties in their paint. For example, Arthur Dove added wax to his resin oil tempera and resin oil colors and also worked with wax emulsion paints (19). In one recipe, melted beeswax is combined with ammonium carbonate to form a wax-soap/water emulsion to which pigments are added (17). Because ammonium salts of fatty acids undergo dissociation with the loss of ammonia vapor, they represent an intrinsic of free fatty acids (16). In a recent artists' manual a recipe consisting of 4 parts beeswax and 1 part each dammar resin (a triterpenoid resin) and sun-thickened (oxidized) linseed oil was noted for its "thick buttery but lio"ht rnn
