Identification of Binding Media and Pigments in the Paintings of Jacob Lawrence
By Michael R. Schilling, Narayan Khandekar, Joy Keeney, and Herant P. Khanjian, The Getty Conservation Institute

A comprehensive analytical survey was undertaken to identify the painting materials used by Jacob Lawrence throughout his long career. Because paint consists of pigments, extenders, and binding media (materials that make the pigments stick to the painted surface), it was necessary to identify all of these materials in his paintings. It is clear that Lawrence would have had access to nearly the full range of twentieth-century commercial pigments and binding media.

One important goal of the survey was to identify the aqueous media that Lawrence used to distinguish between the various types of temperas found in his paintings and, furthermore, to differentiate them from gouache. This information contributes to a better understanding of his painting techniques and, ultimately, may aid conservators in their efforts to preserve his works. In light of the fact that modern tempera paints have never before been fully investigated, this study also reveals important information about the mixtures prepared by artists' colormen in the twentieth century.

From the previous article in this volume (1), it is clear that Jacob Lawrence favored working in various water-based organic binding media. Interestingly, many water-soluble paint media contain common foodstuffs (2). For example, plant gums, sugar, and starch are media based on carbohydrates. Casein (found in milk), glue (often obtained from animal hides), and egg are proteinaceous media. Moreover, Lawrence used commercially available tube colors in many of his later works. Manufacturers often add materials to tube colors, in addition to the binding media, to modify the working properties of the paints, stabilize the mixtures, and also for commercial reasons. These include glycerol, seed oils (linseed, poppy, and walnut), natural resins (dammar, rosin), plasticizers (dimethyl phthalate), and water (3). From these lists, it is quite clear that Lawrence's paint media can be complicated mixtures of many substances.

The present study builds on the work of Steele and Hal-pine, who examined paintings in Lawrence's The Migration of the Negro series (4). Twenty-one paintings that date from 1938 to 1976 were included in this survey. Samples of paint, most of which were smaller than a period on a printed page, were removed from each painting by scalpel and analyzed using extremely sophisticated scientific instrumentation (5). Pigments were identified using polarized-light microscopy (6). Organic binding media were identified with gas chromatography coupled with mass spectrometry (GC-MS)(7) following one experimental procedure for the analysis of proteins (8), a second for natural resins (9), and a third for plant gums (10). For a few extremely small samples, Fourier-transform infrared microspectrometry (FTIR-microscopy) was used to preserve paint material (11). It was seldom possible to use every scientific test on each paint sample because of the limited quantities of sample that were available. Fortunately, the appearance of the paint and its solubility in water gave indications as to the type of medium that was present, and so the tests for each sample were selected with that in mind. It should be noted that GC-MS tests occasionally yield inconclusive results due to interference by pigments and other components in paint.

The test results for pigment and binding medium analysis, listed in Table 1, show that Lawrence employed numerous paint formulations during his career. His pigments covered the full range of natural minerals, synthetic inorganic, and organic color-ants. Moreover, he employed various mixtures of water-soluble binding media: gum and glue; casein; egg; egg and oil; egg, oil, and rosin; starch or dextrin (glucose); egg and starch; gum arabic and other plant gums.

Interpretation of the binding medium results should always be done carefully, and two examples serve to illustrate this point. On the painting Beggars No. 1, the red paint was tested for protein and carbohydrates, whereas the blue paint was tested solely for carbohydrates because the sample quantities were insufficient to perform both types of tests. Casein, glue, and gum arabic were detected in the red paint, and gum arabic was found in the blue paint. Assuming that the casein originated from the paper sizing, it is presumed that both paints were made with a traditional formulation of glue and gum arabic (12). And in The Checker Players, both egg and casein were detected in both the ground and paint samples, although the ground sample tested much more strongly for casein than for egg. The difficulty of completely separating the thin paint film from the very thick ground layer in the sample preparation accounts for the presence of both proteins in the analytical results. Interpretation of analysis leads to the conclusion that egg is the binder for the paint, and casein is the binder for the ground. This conclusion is also consistent with both the artist's and art supply manufacturers' practices of the time.

Because the test procedures made it possible to detect various additives in the paint samples, this permits additional conjecture about the origin of the paints. For example, one may speculate that when a paint sample does not contain additives, Lawrence may have formulated it himself from traditional artists' recipes. Such is the case for the dark paint of The Checker Players, which contains the protein and fats characteristic of egg tempera and which Lawrence himself states that he made. In contrast, the brown egg tempera paint from Struggle… From the History of the American People, panel No. 11, also contains gallic acid preservative, in addition to rosin, oil, and glycerol modifiers, which is evidence for a tube color.

Oxalic acid, often noted as a marker for biological activity, was detected prominently in two paintings: the dark paint of The Checker Players and the yellow of Street to Mbari. Therefore, these paintings may have suffered more from microbial activity than the other paintings with similar media. This indicates that Lawrence could have formulated them himself, or, if they are tube colors, the preservatives were less effective than the other formulations Lawrence employed. It is also possible that the storage or display conditions over the lifetime of these two paintings were somewhat more favorable for microbial activity.

As mentioned earlier, a great deal of information about the composition of a painting may be obtained from a careful examination of the appearance of a paint and its water solu-bility, which can aid in interpreting the analytical results. For instance, the black paint of Harriet and the Promised Land, panel No. 10, showed no carbohydrates or oils; unfortunately, the protein test could not be performed on this paint. However, the appearance of the paint suggests that glue was used instead of gouache. For instance, the paint is more opaque and has more body than gouache, it is slowly soluble in water, and it did not test positively for gum. Also, clay is present among the pigments, which is a filler frequently used in temperas to give them more opacity.

This study amply illustrates the wealth of information that can be obtained by applying modern analytical procedures to the study of painted works of art. The scientific results were found to support much of the historical and anecdotal infor-mation, but in many instances it shed new light on the water-soluble media that Lawrence used so masterfully through his career.


Notes
  1. See Elizabeth Steele's article in this volume, "The Materials and Techniques of Jacob Lawrence."
  2. John S. Mills and Raymond White, The Organic Chemistry of Museum Objects, 2nd ed. (London: Butterworths, 1994).
  3. Ralph Mayer, The Artist's Handbook of Materials and Techniques (New York: The Viking Press, 1940).
  4. Elizabeth Steele and Susana M. Halpine, "Precision and Spontaneity: Jacob Lawrence's Materials and Techniques," in Elizabeth Hutton Turner, ed., Jacob Lawrence: The Migration Series (Washington, D.C.: Rappahannock Press in association with The Phillips Collection, 1993).
  5. The authors are grateful for the assistance of Elizabeth Steele, who assisted with many of the binding medium analyses and who contributed to the interpretation of the findings.
  6. Pigment samples were ground and dispersed in Meltmount (from Cargille Laboratories, refractive index of 1.66) beneath a size 1 coverslip. Pigments were identified using polarized-light microscopy by comparing the optical properties of the samples to those of a set of pigment reference standards. For an excellent summary of the use of polarized-light microscopy, see Walter C. McCrone, "The Microscopical Identification of Artists' Pigments," Journal of the International Institute for Conservation—Canadian Group 7, 1‚2 (1982), pp. 11‚34.
  7. Gas chromatography with mass spectrometry (GC-MS) is a technique that can be used to identify carbohydrates, proteins, and many paint additives. The samples must be treated with reactive chemicals prior to analysis, and each type of medium may require a specific pretreatment procedure.
  8. In the GC-MS analysis of proteins and glycerol, samples are first degraded with 6N hydrochloric acid for 24 hours at 105ƒ C to form free amino acids and glycerol. Afterward, the acid is evaporated and the residue is reacted with MTBSTFA (Pierce Chemical Company) in pyridine for five hours at 105ƒ C to form the TBDMS de-rivatives, which are then separated on a 30M DB-5MS capillary column (J & W Scientific). Selected ion monitoring is used to obtain improved detection limits. The amino acid composition of the sample was compared to the compositions of standard casein, glue, and egg by correlation coefficients and other mathematical procedures to identify the protein. Data interpretation is discussed in detail in M. Schilling and H. Khanjian, "Gas Chromatographic Analysis of Amino Acids as Ethyl Chloroformate Derivatives III: Identification of Proteinaceous Binding Media by Interpretation of Amino Acid Composition Data," in ICOM Committee for Con-servation Preprints, 11th Triennial Meeting, Edinburgh, Scotland, 1‚6 September 1996, ed. J. Bridgland (London, 1996), pp. 220‚7. The use of MTBSTFA is described in P. Simek, A. Heydov·, and A. Jegorov, "High Resolution Capillary Gas Chromatography and Gas Chromatography‚Mass Spectrometry of Protein and Non-Protein Amino Acids, Amino Alcohols, and Hydroxycarboxylic Acids as Their Tert-butyldimethylsilyl Derivatives," Journal of High Resolution Chromatography 17 (1994), pp. 145‚52. A manuscript that describes the complete GC-MS analysis procedure is currently in preparation.
  9. Natural resins (such as rosin), additives (DMP, gallic acid, beeswax), fats and oils (egg fats, seed oils) were prepared for GC-MS analysis by treatment with a 40% solution of Meth Prep II (Alltech Associates, Inc.) in benzene at 60ƒ C for one hour and allowed to react overnight. The methyl ester derivatives and other volatile components were separated on the same column used for protein analysis and identified by computer-aided matching to mass spectral reference data. Michael Schilling described the sample preparation procedure in a presentation at the annual meeting of the Western Association of Art Conservators held in Catalina, Calif., in 1990. A similar procedure is described in R. White and J. Pilc, "Media Analyses," National Gallery Technical Bulletin 17 (1996), pp. 91‚103.
  10. Carbohydrates were prepared for GC-MS analysis by degrading the paint samples in 1N trifluroacetic acid for one hour at 125ƒ C. After evaporating the acid, the residue was reacted stepwise with methoxyamine hydrochloride and acetic anhydride in pyridine. The derivatives were dissolved in chloroform and separated on a 15M DB-WAX column (J & W Scientific), then analyzed by selected ion monitoring. Sample carbohydrate compositions were matched to data for standard plant gums, starch, glucose, and sugar by correlation coefficients. It should be noted that glucose is present in both starch and dextrin, so it is not possible to differentiate them. A manuscript that describes the procedure is in preparation.
  11. Fourier-transform infrared microspectroscopy (FTIR) is a technique that has a number of advantages over GC-MS. FTIR can be used directly on a paint sample without requiring aggressive chemical pretreatment. Because samples may be recovered for subsequent analysis by other methods, it is termed a nondestructive technique. Furthermore, it can also identify pigments and other nonvolatile species (such as acrylic media) that cannot be detected by GC-MS. However, it can only detect components present in concentrations above five to ten weight percent, although many substances are well below that level in commercial paints. A representative sample particle was placed on a BaF2 window, flattened by a stainless steel roller, and analyzed by transmitted infrared beam with an aperture of approximately 100 _ 100 microns, using a 15X objective. Each spectrum was the sum of 200 scans at a resolution of 4 cm-1. Based on the initial analysis results of bulk material, extraction was made by placing a microdroplet of solvent on the sample, and analysis was performed on the resultant extracted dry solvent ring. Infrared spectra of the samples contain bands that correspond to the paint components. To identify materials in a paint sample, the infrared spectrum may be matched to spectra for reference materials using a computer algorithm. Of course, other components may be present in the samples at concentrations below the 5% detection limit. For more details, see M. R. Derrick, Practical Guide to Infrared Microspectroscopy, ed. Howard J. Humecki (New York: Marcel Dekker, 1995).
  12. Glue and gum mixtures were described in numerous recipes from the F. Weber & Company Archives, acc. no. 950018, Getty Research Institute, Research Library.

Table 1 Results of Pigment and Binding Medium Analysis

Beggar No. 1, 1938, tempera on composition board. The Metropolitan Museum of Art.
Red: Iron red, minerals.a Casein, glue, glycerolb
Blue: Ultramarine,c titanium white, iron red. Gum arabic

Painting the Bilges, 1944, gouache on paper. Hirshhorn Museum and Sculpture Garden.
Blue: Ultramarine, clay, minerals. Gum arabic, glucose
Red: Red lake. Medium not tested

African Gold Miners, 1946, gouache on paper. Hirshhorn Museum and Sculpture Garden.
Blue: Ultramarine, cobalt blue, zinc white. Gum arabic
Black: Lamp black.d Gum arabic, glucose, sugar

New Jersey, 1946, gouache and watercolor on paper. National Museum of American Art.
Red: Pigments not tested. Gum arabic, glucose

The Checker Players,* 1947, egg tempera on hardboard. Worcester Art Museum.
Dark color: Titanium white, iron red, umber. Egg, casein
Ground: Titanium white, minerals. Egg, casein, beeswax
*Please refer to the discussion of these results, p. 267.

Sedation, 1950, casein tempera on paper. The Museum of Modern Art.
Brown: Red lake, brown lake, charcoal. Protein results erratice
Green: Pigments not tested. Casein, sugar,f trace of glue

Vaudeville, 1951, egg tempera on hardboard. Hirshhorn Museum and Sculpture Garden.
Blue: Pigments not tested. Egg, oil, DMPg
Ground: Titanium white. Casein
Verso ground: Zinc white, titanium white, iron red, gypsum, clay, minerals. Casein
Coating: Beeswax

Struggle… From the History of the American People, No. 11: 120.9.14.286.9.33-ton 290.9.27 be at 153.9.28.110.8.17.255.9.29 evening 178.9.8…—an informer's coded message, 1955, egg tempera on hardboard. Private collection, New York.
Brown: Bone black, titanium white, iron red, minerals. Egg, oil, glycerol, gallic acid, rosin

The Cue and the Ball, 1956, casein temperah on paper. Hirshhorn Museum and Sculpture Garden.
Green: Chromium oxide. Glucose. Protein results erratic. Water-insoluble

Magic Man, 1958, egg tempera on hardboard. Hirshhorn Museum and Sculpture Garden.
Black: Charcoal, yellow ocher, burnt umber, raw umber, iron red. Egg, oil, glycerol
Light black: Charcoal, titanium white, gypsum. Egg, oil, glycerol
Ground: Pigments not tested. No carbohydrates
Verso ground: Titanium white, clay, gypsum, minerals. Protein results inconclusivei

Men exist for the sake of one another…, 1958, egg tempera on hardboard. National Museum of American Art.
Blue: Ultramarine, charcoal, burnt umber, titanium white, iron red. Egg, glycerol
Ground: Titanium white, charcoal, gypsum, minerals. Glycerol, no protein. Tests inconclusive

The Library, 1960, tempera on hardboard. National Museum of American Art.
Ocher: Iron yellow, iron red, titanium white, gypsum, charcoal. Egg
Brown: Raw umber, titanium white, burnt umber. Egg, oil, DMP

Parade, 1960, tempera on hardboard. Hirshhorn Museum and Sculpture Garden.
Blue: Cobalt blue, ultramarine, burnt umber, titanium white, gypsum. Egg, glycerol
Ground: Titanium white, gypsum, iron red, charcoal. Egg, glucose

Playing Card (Joker) and Playing Card (King), 1962, tempera on paper. Hirshhorn Museum and Sculpture Garden.
King—Red: Iron red, titanium white, clay, minerals. Glue, no carbohydrates
King—Black: Bone black, ultramarine, minerals, iron red. Glue
Joker—Blue: Cobalt blue, clay, minerals. Glue
Joker—Yellow: Protein results erratic

Ordeal of Alice, 1963, egg tempera on hardboard. Private collection, New York.
Light brown: Titanium white, burnt umber. Egg, oil, glycerol

Street to Mbari, 1964, tempera, gouache, and graphite on paper. National Gallery of Art.
Yellow: Yellow ocher, ultramarine. Glue, no carbohydrates
Blue: Ultramarine, red lake, charcoal, raw umber. Glue, glucose

Students with Books, 1966, egg tempera on hardboard. Mr. Merril C. Berman, New York.
Yellow: Yellow ocher, iron red, umber, minerals. Egg, oil, no carbohydrates
Brown: Raw umber, gypsum, minerals. Egg, oil, rosin, DMP
Purple: Iron red, cobalt blue, titanium white, charcoal. Egg, oil, rosin

Daybreak—A Time to Rest, 1967, egg tempera on hardboard. National Gallery of Art.
Blue: Ultramarine, cerulean blue. Egg, oil, DMP, protein results inconclusive
Light blue: Ultramarine, titanium white, iron red. Egg, oil, rosin, DMP
Yellow: Yellow ocher. Egg, oil
Ground: Titanium white. Protein results inconclusive
Varnish: Acrylic resin

Harriet and the Promised Land, No. 10: Through Forests, Through Rivers, Up Mountains, 1967, gouache and tempera on paper. Hirshhorn Museum and Sculpture Garden.
Black: Lamp black, clay. Glucose, but no oil or fats. Presumed to be glue due to appearance and slow solubility in water

Other Rooms, 1975, gouache and tempera on paper. Collection of Jacob and Gwendolyn Knight Lawrence.
Glossy gray: Pigments not tested. Plant gum

In a free government… , 1976, gouache on paper. National Museum of American Art.
Brown: Bone black, burnt umber. Plant gum, glucose, sugar

Notes
  1. "Minerals" in the pigment listing denotes an unknown transparent mineral material.
  2. Glycerol is listed only when its concentration exceeded 1% by weight in the sample.
  3. Synthetic ultramarine was the form of the pigment identified in all of the paints.
  4. It was not possible to differentiate between bone black and ivory black in this study because they possess identical optical properties. Hence, the term bone black refers to either pigment.
  5. For three paint samples, the protein results showed no evidence for any amino acid or the internal standard. Accordingly, they were termed protein results erratic, and no conclusions could be drawn.
  6. Sugar refers to cane sugar, which is composed of glucose and fructose.
  7. DMP refers to dimethyl phthalate, a commonly used plasticizer.
  8. Binder is assumed to be casein because of surface appearance, palette, and complete insolubility in water.
  9. In samples denoted with protein results inconclusive, the amino acid composition did not match any reference material.