January 1998 Volume 20 Number 1
January 1998 Volume 20 Number 1
This article first appeared in Analytical Chemistry and was written for chemists. The authors intend, at some future date, to publish a version dealing with the various conservation issues not addressed herein. It is the opinion of the Editor, however, that this information is too interesting to let slip by until that time.
Paintings, sculptures and works on paper dating from the late 19th through the 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, the opacification of a surface varnish, deposition of foreign matter, growth of micro-organisms, 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 different artists) had fine, clear to translucent particles on the surface. 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 concentrated areas of light scattering particles on the surface of fine art.
In the 1950s it was believed that this "crystalline bloom" was caused primarily by ammonium sulfate crystals on the surface. (1) 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 (4,5,6) and sometimes 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 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 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 (ca. 50 to 75 microns), 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. Other methods include X-ray diffraction (XRD) with a Gandolfi camera, transmitted polarized light microscopy, and scanning electron microscopy (SEM) with energy dispersive x-ray spectrometry. Gas chromatography-mass 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-ray excited, energy dispersive X-ray spectrometry (EDS).
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 oil stick (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 sculptures 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 color 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 reference set. The melting point was 56°C. Similar analyses of a commercial sample of U.S.P. 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)
Figure 1. FT-IR spectra of (top) efflorescence crystals from Red Table and (bottom) USP Stearic-palmitic acid.
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 any 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 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 were very similar to that of the Oldenburg piece 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 area 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 with a fluffy white 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.
Figure 2. Fatty acid accumulations on the cover of Death of a Salesman.
Free fatty acids have been incorporated into paint systems by several routes including direct addition during formulation and as later additions by artists attempting to achieve certain visual effects. Materials used in conservation treatments can also be a source of fatty acids.
Raw linseed oil naturally contains a small percentage of free fatty acids. Stand oils are made by heating the 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. Koller 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 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 Koller 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 crosslinking 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 (11, 12, 13). Linseed oil has a P/S ratio 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 ratio of 6.5 in the efflorescence from the Oldenburg alkyd example strongly suggests that he used a "long oil" alkyd based upon 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 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 not fixed. 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 feed stock from which these stearate salts are made is virtually never pure stearic acid-but rather a slushy mixture of stearic, palmitic, oleic and other minor species-often as little as 28% stearic acid and nearly 50% palmitic acid are present. Therefore, the "aluminum stearate" added to pigments during grinding may include a considerable amount of free stearic acid as well as 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) But 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 C48, and free fatty acids and paraffinic hydrocarbons up to C27) is such that efflorescence of wax components could be a problem when added in these high concentrations.
Commercially manufactured artists' materials have frequently been altered by artists for practical and aesthetic reasons. Artists' manuals recommended "sweetening" slow drying oils to avoid rancidity caused by the free fatty acids in the oils. Reaction with a base such as baking soda (sodium bicarbonate) or quicklime (calcium oxide) or by standing with chalk (calcium carbonate) or white lead (basic lead carbonate) would precipitate the fatty acid salts. (17) The oil 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 the fatty acids undergo dissociation with the loss of ammonia vapor, they represent an intrinsic source of free fatty acids (16). In a recent artists' manual, a recipe consisting of 4 parts beeswax and 1 part each of dammar resin (a triterpenoid resin) and sun thickened (oxidized) linseed oil was noted for its "thick, buttery but light consistency" (20).
Many conservation treatments used beeswax for consolidating flaking paint. The heated wax was often combined with a natural resin such as dammar and introduced into the paint layer either locally in the areas of damage or overall if the flaking was extensive. Paintings were also impregnated with the wax-resin mixture to decrease their expansion and contraction during environmental moisture fluctuations. Wax and wax-resin mixtures have also been used as decorative or protective coatings on paintings and sculptures.
Organic solvents used alone and in complex combinations with other organic materials have been used for many different purposes in conservation. For example, emulsified cleaners are often considered effective for cleaning paint surfaces. One such recipe included carnauba wax, ceresin wax, bleached beeswax, two types of petroleum benzine, water, triethanolamine, and stearic acid. It is plausible that fatty acid components in these mixtures could be left in the paint film to later become residues on the surface. The reabsorption of solvated efflorescent materials back into the paint film, during an attempt at their removal, could also lead to the recurrence of surface deposits. Gradual loss of the residual solvent may be responsible for the reemergence of well-crystallized fatty acids (2).
It is difficult to determine the exact source of fatty acids in an artwork exhibiting efflorescence. Conservators seldom have access to the paints used by the artist or detailed technical information about how the artist painted. Conservation treatment records are frequently missing or have abbreviated descriptions of the treatments that the artwork has undergone. However, in the cases of the three works discussed above, we ascertained with some degree of certainty what the sources of efflorescence were.
The fortuitous discovery of old paint cans from the same time period and of the same type as used by Oldenburg, which contained residual paint, allowed us to determine that the efflorescence on the sculptures arose from the artist's paint. Large clear crystals were observed on the surface of the paint in the cans. The FT-IR spectra of the crystals were similar to those of the crystals on the sculpture.
In the case of the Clerk painting, the artist told us that, at the time the painting was done, he had been experimenting with wax media such as encaustic. He mentioned that it was possible he had added beeswax and other materials to his tubed paints. Clerk also noted that two other paintings of this period also had the same hazy patches, which he initially thought were mold. The material on the leather book cover appears to be the byproduct of a leather dressing treatment used in an attempt to keep the leather supple. The label on the jar of leather dressing thought to have been used stated that it contained neatsfoot oil, anhydrous lanolin and carnauba wax.
Related compounds such as fatty acid salts, fatty acid esters and paraffinic hydrocarbons were also found as efflorescent materials on several works studied in this research. The fatty acid esters or salts were usually minor components in the efflorescence which also contained free fatty acids. Interestingly, infrared analysis showed that a clear soft layer on one of the paint cross sections taken from an Oldenburg sculpture was a zinc alkanoate, probably derived from the interaction of free stearic and palmitic acids with zinc white pigment. Williams (4) mentions that he found the fatty acid salts, lead stearate and lead palmitate, as the predominant blemishing material on an oil painting; in general he found free fatty acids to be minor components in the efflorescence on paintings but common on ethnographic objects.
Paraffinic wax efflorescence was present on several Charles Russell wax sculptures depicting animals and scenes typical of the old West, from the collection of the Amon Carter Museum and the Buffalo Bill Historical Center. In many cases these small sculptures were the prototypes from which serial casts in bronze were created (after the artist's death) by foundry mold-making and replication. These wax models are the actual manual work of the artist and are unique.
Russell worked with an eclectic group of materials for these sculptures, usually building the figures out of dental wax around a wire armature. The sculptures had localized white efflorescent deposits on their surfaces, which were identified by XRD and FT-IR. Examination revealed that the affected areas lacked a thin, virtually invisible shellac coating, which covered the rest of the surface and apparently inhibited the emergence of wax crystals.
A painting's history of conservation treatment and the formulation of the paint's binding medium are important considerations in assessing the possible sources of the fatty acid efflorescence, but other factors such as the pigment composition and the physical structure of the work (e.g., the number and thickness of the paint layers) can assist in explaining the distribution of efflorescence on the surface.
An oil painting on canvas at MoMA by Meret Oppenheim entitled Red Head, Blue Body had an unfinished painting on its reverse. Extensive crystalline deposits were present on the unfinished painting but only on specific colors-yellow areas had no deposits, but the green areas were so heavily coated as to appear white. In another MoMA painting, Andrew Wyeth's Christina's World, large fatty acid crystals were found in very localized areas such as along a fine strand of hair painted with a green pigment. (Figure 3) Because Wyeth traditionally used egg tempera paint that he made himself, this finding suggests that the pigment is playing a critical role.
Figure 3. Detail of hair strands in Christina's World. The central strand is covered with fatty acid crystals.
At LACMA, reference panels dating from 1972 to 1979 of various commercial artists' oil colors were studied (21). They showed that colors containing certain pigments were more likely to be afflicted by fatty acid efflorescence. For example, carbon black and cobalt blue (cobalt aluminate) were commonly affected, however lead white (basic lead carbonate), zinc white (zinc oxide) and pigment mixtures containing them usually were not. The formation of fatty acid soaps at the pigment/medium interface may have an immobilizing effect on the fatty acids. Paints lacking these basic pigments with ionizable cations cannot form stable fatty acid soaps and so cannot prevent fatty acids from migrating to the paint surface. Whether a pigment element such as lead will ionize to form a soap depends on the pigment's composition. For example, in the heavily affected red paint of the Oldenburg sample, the red pigment was identified by XRD as lead molybdate. Unlike basic lead carbonate it does not appear to possess lead in an available form. Additionally, the oil absorption properties of any pigment and the extent to which the pigment enhances the drying of the linseed oil are factors that affect the physical nature of the paint film and help determine if efflorescence occurs.
Of the artworks studied, the role that pigments play was most clearly demonstrated in a set of woodcut prints at MoMA done with oil paint in 1988 by Donald Judd. The set consists of thirty monochromatic prints, 10 red, 10 blue, and 10 black. Close examination with a stereomicroscope showed that all ten black prints had fine crystals in the crevices of the impasto; the twenty red and blue prints were not affected. IR analysis showed that these crystals were composed of fatty acids. According to documentation, the artist used "cadmium red light, ultramarine blue and ivory black" that is, cadmium sulfoselenide, complex sodium aluminum sulfosilicate and carbon/calcium phosphate. This finding was confirmed with polarized light microscopy and SEM-EDS, which also showed that, except for the barium sulfate particles mixed in with the cadmium red, the pigments were relatively pure. The artist's printer told us that oil paints were used straight out of the tube without the addition of other media.
In general, however, caution must be exercised when attempting to correlate pigment type with the suppression or enhancement of fatty acid efflorescence because of the many variables. For example, the grade of a particular pigment can considerably change its properties in the paint film, as can the inorganic extenders and inerts and the type of binder and binder additives. The environmental history of the work must also be considered. It is possible that for these reasons there are contradictory findings about the roles of certain pigments and their relationship to efflorescence. For example, in contrast to the findings on the reference panels where cobalt blue was commonly observed to be affected by efflorescence, Singer has cited a group of English works from 1893 in which cobalt blue is associated with decreased fatty acid efflorescence.(6) Likewise, although efflorescence wasn't noted on Judd's cadmium red prints, it was observed on another museum's set of cadmium red prints by Judd, as well as on the cadmium red reference panels at LACMA.
Several explanations of possible mechanisms have been given for the migration of fatty acids in oil paintings. For example, a "liquid" mixture of free fatty acids might move through the paint to the surface where oxidation and volatilization of certain fatty acids occur, leaving the saturated fatty acids as crystal deposits. (5) Williams has suggested that physical and chemical incompatibilities caused by film shrinkage, syneresis and changes of polarity from oxidation could cause certain components to separate. (4)
A recent article on the development of turbidity in acrylic paint films has interesting implications. (22) The authors found that the turbidity caused by crystallization of poly(ethylene glycol) (PEG) within the acrylic polymer matrix increased when the paint was exposed to temperatures above the glass transition temperature of the acrylic (tg=12° C) and below the melting point of PEG (50° C). In this range, the PEG chains can move through the rubbery acrylic matrix until they encounter a nucleation site where they crystallize in a preferred morphology.
Published values for the Tg of aged oil paint films (31° to 45° C) show a marked dependence on pigment type, oil to pigment ratio, and moisture content. (22) With the addition of other materials to the oil paint that might lower the Tg further, it is quite likely that oil paintings are often exposed to temperature and humidity sufficient to raise the polymerized oil matrix into the region above its Tg value but below that of the minimum melting point in the stearic-palmitic system (55° C). In this condition, crystallization would be encouraged in a manner analogous to the acrylic example. The dependence of Tg on the type and proportion of pigment may account for certain cases of localized efflorescence that are difficult to explain in terms solely of the chemical interactions between pigment and fatty acid. Future research that includes the determination of paint film Tg in cases of fatty acid efflorescence may be very enlightening.
A 1918 oil portrait at LACMA by Pierre A. Renoir of his son Jean Renoir as a huntsman had a thin white deposit over several different colored regions, most noticeably the blue of the subject's trouser leg and the brown of his boots. Polarized light microscopy of a sample revealed a birefringent compound of high refractive index. XRD demonstrated that it was cotunnite, (lead chloride). No reference to this material on oil paintings was found in the literature.
Interestingly, a second occurrence of this compound was found on an oil on canvas at LACMA painted in 1948 by the American artist John Storrs. The Renoir and the Storrs paintings differed radically in their manner of execution; the Renoir had profuse interblending of wet paints, whereas the latter had carefully delineated applications of single colors. The Storrs was further complicated by the fact that the artist had extensively revised his work with new applications of paint in 1949. A thorough study made it clear that the cotunnite was not consistently associated with any one color application in either version of the Storrs painting.
A third occurrence of this compound shed light upon one mechanism for its formation. Recent seismic stabilization of UCLA's Powell Library required the removal and partial replication of hundreds of square meters of painted ceilings. To document the materials and methods of the original 1920s decoration, a structural and compositional study intended to guide the replication was conducted. The original ceiling consisted of oil-painted cast plaster of Paris units made to resemble wooden beams and panels. Only narrow plaster fills bridging the junctions between them were painted in-situ on the ceiling. Cotunnite was found as a white haze on the paint covering these transitions and was probably formed through the interaction of soluble chloride present in the plaster transitions (which perhaps were not fully dry at the time they were painted) with the basic lead carbonate in the paint.
In the case of the two canvas paintings, because the solubility of the lead chloride is too low for it to have been transported to the surface without consequent evidence of water damage, it seems likely that a more readily soluble chloride migrated through the paint layers and reacted with the lead white in the pigment mixture nearer the surface.
Another kind of inorganic efflorescence was found on The Orator, a LACMA painting by the German Expressionist Magnus Zeller, which had a few, widely spaced single crystals protruding from the oil paint layer. The painting had not been treated since leaving the artist's studio, so the condition did not appear to be an inadvertent result of any conservation treatment. XRD analysis yielded a pattern similar to that of the mineral wattevilleite, Na2Ca(SO4)24H2O) perhaps mixed with a small amount of epsomite (MgSO47H2O). The fact that this material was not found in a finely divided form over the surface suggests that it may have become concentrated in localized deposits by gradual recrystallization from the painting support because of moisture. Because the canvas was primed with gesso containing gypsum, the presence of small amounts of more soluble sulfates in the priming layer would not be surprising. The conservator noted that the painting had an unusual surface coating, which appeared to be original and which was water-soluble. Quite likely, the application of a water-based coating to the painting introduced the moisture that led to the recrystallization of the sulfate on the surface.
The examples recall the typical efflorescence of water-soluble salts found on masonry materials. These compounds are often carbonates resulting from the carbonation of excess alkali in Portland cement or sulfates derived from ground water and building materials such as brick. If works of art are connected to structural elements of buildings that exchange moisture with the air, the salts extracted from the building material eventually migrate to the work of art. Water loss by evaporation leads to the saturation of salt solutions in the pore spaces just beneath the surface. Additional water loss leads to the growth of salts containing water of crystallization on the surface and, upon further loss of water, to recrystallization in anhydrous forms.
Because of fluctuations in the ambient temperature and humidity, phase transformations accompanied by further crystal growth may frequently recur as cycles of hydration and dehydration ensue. Some compounds possess a multiplicity of hydrated crystal phases that greatly complicates their identification, even when a good diffraction pattern can be obtained. Such is often the case with magnesium sulfate and its binary salts.
Considerable work remains to be done to determine the mechanisms by which these materials migrate out of paint films and the consequences of their loss from the paint. Likewise the conservation practices used to remove these surface particles, which in many cases can not be brushed off, need to be studied to determine if the use of heat, solvents, or of penetrating coatings and varnishes are sound treatments. These measures mask the problem temporarily, but in the long run may do harm by enhancing efflorescence or exacerbating later treatment. They also usually involve further alteration of the paint surface.
For example, the oil paintings of Stanley Spencer were studied because of a recurring history of surface deterioration. (23) Spencer's paintings, done in 1932, were unvarnished. However as part of conservation treatments begun in the 1950s, the paintings were varnished to treat the appearance of uneven surfaces, caused in part by crystalline deposits. When the paintings were examined in 1992, the patchy appearance and surface deposits had recurred. Many works of art will continue to suffer a similar history of deterioration if efflorescence is not better understood.
ACKNOWLEDGMENTS
The authors wish to thank Zora Pinney for bringing the problem of crystalline deposits on contemporary artist's oils to our attention some 15 years ago and for having the foresight to set aside examples for the day when their investigation would be possible. Michael Schilling and Harant Khanjian of the Getty Conservation Institute kindly included samples of these and of the alkyd example in their GC-MS methods development project and made an advance copy of their 1996 ICOM publication available. Glenn Wharton brought the condition of paints on Claes Oldenburg's sculptures to our attention and provided samples. Patricia Houlihan of MoMA generously made available examples of the Loc-Lin alkyd enamel which had been out of production for some years. In her role as conservation consultant Tatyana Thompson recommended scientific analysis of the original decorative colorants of UCLA's Powell Library during which our identification of cotunnite efflorescence occurred. We wish to thank Bob Gamble for his perspectives on the use of aluminum stearates in the manufacturing of artist's oils.
REFERENCES
1) Bromelle, N. Museums Journal, 1956, 55, 263-267.
2) Pearlstein, E. Studies in Conservation, 1986, 31, 83-91.
3) Harley, C. Studies in Conservation, 1993, 38, 63-66.
6) Singer, B.; Devenport, J.; Wise, D. The Conservator 1995, 19, 3-9.
7) Saguchi, M.; Asada, E. Nippon Kagaku Zasshi, 1961, 82, 958-962.
9) Smith, J.C. J. Chem. Soc.,1939, 974-980.
10) Stewart, H.W.; Wheeler, D.H. J. Am. Oil Chem. Soc. 1941, 18.
11) Challinor, J.M. Journal of Analytical and Applied Pyrolysis, 1991, 18, 233-244.
12) Mills, J.S.; White, R. The Organic Chemistry of Museum Objects, Butterworths, London, 1987.
20) Massey, R. Formulas for Painters; Watson-Guptill Publications: New York, 1967, 118.
22) Whitmore, P.M.; Colaluca, V.G.; Farrell, E. Studies in Conservation 1996, 41, 250-255.
23) Odlyha, M.G.; Hedley, G.; Flint, C.D.; Simpson, C.F. Analytical Proceedings, 1989, 26, 399-401.
I can't say I was ever lost,
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Daniel Boone
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