The Spectroscopic Analysis of Paints Removed ... - ACS Publications

Chapter 17

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The Spectroscopic Analysis of Paints Removed from a Polychrome Wood Sculpture of Male Saint Patricia L. Lang,* Shawn P. Leary, Rebecca F. Carey, Melissa N. Coffer, Rick E. Hamilton, Amber L. Klein, Randall T. Short, and Philip A. Kovac Department of Chemistry, Ball State University, 2000 West University Avenue, Muncie, IN 47306 *E-mail: [email protected]

The examination of paints removed from a late 15th century South German sculpture known as Male Saint from the circle of Hans Multscher was performed utilizing both infrared spectroscopic and energy dispersive x-ray analysis. Pigments and paint components identified that are consistent with the sculpture’s date include red ochre, azurite, gold, calcium carbonate, gypsum, China clay, hide glue, protein/oil binder, and linen fibers. The presence of a copper acetoarsenite in the green paint on the base is indicative of an application of paint after 1800.

Introduction Unfamiliar to many is the genre of limewood sculptures that were made in Germany starting in the 15th century. Amongst the most distinguished and prodigious of the early craftsmen was Hans Multscher (1400-1467) of the German region of Swabia (1). Multscher’s work spanned a period that marked a transition in this region from Gothic to more realistic forms of art (2). By 1430, his sculptures and those from his large workshop were characterized by a sense of movement exhibited under the naturalistic drape of the figure’s cloak. Such features were, in part, facilitated by the use of the wood, Tilia platphyllos, a broad-leafed limewood species indigenous to southern Germany. The wood’s

© 2012 American Chemical Society In Collaborative Endeavors in the Chemical Analysis of Art and Cultural Heritage Materials; Lang, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


uniform cellular structure gives rise to its elasticity, lightness, and tractability, traits important for the carver (1). Additionally, until the end of the 15th century, it was traditional for the limewood sculptures to be painted. Although the polychrome palette was typically a limited one of blue, green, red, black, gold, and white pigments, a range of textures and patterns could be created by modifying the contour of the gesso ground by the use of textiles, and/or patterning tools (1). The current work under study is a polychrome wood sculpture from the circle of Hans Multscher housed in the David Owsley Museum of Art at Ball State University. Male Saint, 2007.004.002, stands over 5 feet tall and is sculpted from the thick “C” of a limewood trunk which remains after the heartwood core is removed. See Figure 1. The dark beige paint on his neck, face, hands, and hair shows little deterioration. Substantial paint remains on the saint’s gold cloak, the blue inner lining of the cloak, and the green octagonal base, although there are scattered areas of loss. The binding of the red book that hangs from his belt shows substantial paint loss. Least intact is the yellow ochre-colored paint on the gown where there is uniform paint and ground loss. Male Saint was acquired by the museum in 2007.

Figure 1. Male Saint, Circle of Hans Multscher, 1450/1499. Gift of David Owsley via the Alconda-Owsley Foundation, 2007.004.002. Photo courtesy of the David Owsley Museum of Art at Ball State University. (see color insert)

286 In Collaborative Endeavors in the Chemical Analysis of Art and Cultural Heritage Materials; Lang, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


The paint analysis was performed as a diagnostic prerequisite to a future conservation process. Although there are many chemical instrumental techniques available for analysis of pigments and paint components, including non-destructive techniques such as Raman microscopy, X-radiography, and X-ray fluorescence (3), the complementary techniques of infrared spectroscopy and scanning electron microscopy with energy dispersive x-ray detection (SEM-EDS) are a powerful combination of methods that are easy-to-use provided that a small sample can removed. Many organic and inorganic colorants, binders, additives, grounds, and sizing agents are easily identified (4, 5) using the selective, molecular information provided in their infrared spectra, while those inorganics that have absorptions below the detector cut-off range or which are infrared inactive can often be identified with the characteristic elemental information provided in the EDS spectrum (6). Further, attenuated total reflection infrared microspectroscopy has been used to identify pigments (7); however, the universal ATR accessory available on most instruments is sensitive enough to provide high quality spectra on extremely small paint samples as small as 500 μm (8) with no sample preparation. While the resolution or detection limit may not be as good as the infrared microscope, the information gained can be quite valuable as the paper describes, and the fact that it is a surface technique is useful as different spectral information can be obtained on each side of a paint sample. The ease of analysis was an important consideration, given that the work was done as part of an immersive learning experience for undergraduate students in the Chemistry of Artists’ Pigments course. Consequently, the analysis described is not an exhaustive study of the sculpture, but an initial investigation that yielded interesting results.

Experimental Section Using a scalpel, samples approximately 250-750 μm in diameter, were obtained from areas that were unobtrusive or already deteriorating. (See Figure 2.) Removal of samples this size did not result in visible damage to the art. Multiple paint samples and one fiber bundle were obtained from the locations (L) shown in Figure 3. The samples included a blue paint (L1 and L3), green paint (L2 and L4), red paint (L6), gold/red paint (L5), and a fiber bundle (L7). After removal, the samples were examined under a stereomicroscope to note the color, homogeneity, and general appearance of the samples. A Perkin-Elmer 1600 infrared spectrometer fitted with a universal attenuated total reflectance accessory with a diamond element was used to acquire spectra at 4 cm-1 spectral resolution with four signal-averaged scans per spectrum. The infrared spectra were compared with those in Artists and Artisans Materials Infrared Spectral Library (9) or to the principal author’s reference library for identification purposes. Multiple spectra were obtained on each paint sample.



Figure 2. Close-up of bottom right of blue cloak on Male Saint showing localized areas of deterioration prior to sampling.

Figure 3. Sample locations.

SEM/EDS spectra were obtained using the electron beam source from a JEOL Scanning Electron Microscope as an excitation source and the emitted radiation was collected with a Noran 666B Energy Dispersive X-ray detector. The accelerating voltage was set to 15 keV, and the working distance was 30 mm. Paint samples were placed on carbon tab with no other preparation. Microscopic analysis of the pigments was performed when additional information was needed for confirmation. 288 In Collaborative Endeavors in the Chemical Analysis of Art and Cultural Heritage Materials; Lang, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

Results and Discussion


White on Back of Most All Samples One of the pigments found on the back of the paints removed from all sampling locations was calcium carbonate, CaCO3, which was used in the ground. The infrared spectrum of a representative sample is shown in Figure 4. Characteristic CO32- bands at 1420 cm -1 (asymmetric C-O stretching), 875 cm -1 (out-of-phase bending) and 728 cm -1 (OCO in-plane deformation) are marked in Figure 4 and assigned based on literature (10). Additionally, the spectrum matches reference spectra of its most common form, calcite (9). Calcium carbonate has been an important artist’s material since the classical times. CaCO3 is found mainly in sedimentary rocks such as chalk and limestone but is also found in skeletal material of marine life. In German paintings on wood panels, the calcium carbonate used was typically from quarried chalk that had been ground and washed (11). The presence of the substance on the back of most of the paint samples, in a layer of about 500 μm thick, is consistent with the known preparation of the wood surface as discussed in the introduction (1).

Figure 4. Infrared spectrum of white on back of all paints.

Blue Paint from Cloak Bottom Samples were taken from the bottom left lining of the cloak (L1) outside of the fold (Figure 3) and the bottom right lining of the cloak (L3) as shown in Figures 2 and 3. When viewed under a polarizing microscope, a birefrigent structure was clearly visible under cross-polars. A representative infrared 289 In Collaborative Endeavors in the Chemical Analysis of Art and Cultural Heritage Materials; Lang, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


spectrum of the blue from that location is shown in Figure 5. The spectrum matches those of the blue basic form of copper, 2 CuCO3 ⋅ Cu(OH)2, or azurite (11). Characteristic absorptions at 3426 cm-1 (O-H stretching), 1401 cm-1 (asymmetric CO32- stretching), 949 cm-1 (C-O-H bending), and 834/814 cm-1 (out-of-phase bending) are marked (12). Azurite is one of the most important blue pigments in European painting during the middle ages and Renaissance, since its stable color is more economically convenient than the preferred ultramarine blue. It was prepared by grinding, washing, and levigating the natural mineral. Whilst the artificial basic copper carbonate, blue verditer, has an almost identical infrared spectrum (9), the blue particles in these samples from Male Saint have an irregular, broken fractured appearance characteristic of natural azurite (11).

Figure 5. Infrared spectrum of blue paint from cloak identified as azurite. Blue side against the ATR element.

Green Paint from Base A representative infrared spectrum of the green paint from both sides of the back of the base (L2 and L4) of the sculpture is shown in Figure 6. The presence of distinctive bands at 1555 cm-1 and 1451 cm-1 are indicative of the coupled carbonyl stretches of an acetate, the higher being the out-of-phase COO- stretch and the lower, the COO- in-phase stretch. This is strongly suggestive of verdigris, a hydrated basic copper acetate that can take on various forms, most of which are blue (11). However, the out-of-phase acetate stretching frequency does not match that found in verdigris which is at 1600 cm-1 (9, 13). There was no presence of 290 In Collaborative Endeavors in the Chemical Analysis of Art and Cultural Heritage Materials; Lang, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


either blue or yellow pigments in our samples upon close examination under the microscope, which would indicate a mixture was used. SEM/EDS data obtained on the green particles, indicated the presence of both copper and arsenic. This suggests Emerald Green, Cu(C2H3O2)3·3Cu(AsO2)2 , a copper acetoarsenite, and the acetate stretching frequencies match up with the reference spectrum (9) of Emerald Green. The identification of this pigment indicates that the green paint was applied later, since Emerald Green was not used until the early 19th century.

Figure 6. Infrared spectrum of green paint from base. Green side against ATR element.

The sample is much more complicated, however, as one observes additional bands in the spectrum shown in Figure 6. Under microscopic examination the green paint samples have a transparent “glaze” on top of the green paint, and a representative spectrum obtained from the glaze is shown in Figure 7. The spectrum matches that of a natural protein, which could be a gelatin, egg white, or casein protein used as a binder or glaze (9). Characteristic broad bands due to natural polyamides are marked at 3290 cm-1 (N-H stretching), 3073 cm-1 (overtone of the amide II band), 1629 cm-1 (amide I, interaction of C=O stretch with NH2 deformation), and 1540 cm- 1 (amide II, involving the NH deformation and CN stretch) (14). An attempt to identify hydroxyproline present in the sample, which would indicate hide glue, using Erlich’s reagent was inconclusive due to the minimum sample size (15). Although the identification of a protein in this sample allows the absorptions at 3300 and 1630 cm-1 to be assigned in the spectrum shown in Figure 6, still 291 In Collaborative Endeavors in the Chemical Analysis of Art and Cultural Heritage Materials; Lang, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


unexplained are the absorptions at 2927, 2854, 1737, and 1160 cm-1. These can be assigned to methylene asymmetric and symmetric stretching, respectively, and the C=O stretching and C-O stretching, respectively of an ester; all of which are consistent with the frequencies in linseed oil (9).

Figure 7. Infrared spectrum of the transparent glaze on green paint from base. Glaze side against ATR element.

Gold and Red Paint from Cloak Samples taken from the Saint’s cloak on his left (L5) were gold and red colored. Under microscopic examination of the samples’ cross sections, a gold layer appeared to be the last applied, but in other samples, the gold looks intimately burnished into the red pigment. Under the gold/red layer was a white layer (500 μm thick), followed by an opaque, striated glaze layer (about 150 μm thick). The pigment layers could not be physically separated as they were too thin and tightly bound. A representative spectrum of the paint, however, is shown in Figure 8. Absorptions at 3695, 3620, 1090, 1032, 998, and 907 cm-1 are most consistent with those of kaolin, in both frequency and relative intensity (9), although the frequencies are not an exact match. The sharp 3695 and 3620 cm-1 bands are non-hydrogen-bonded O-H stretching absorptions, while the lower frequency absorptions correspond to Si-O stretching modes. Kaolin, or China clay, is an aluminum silicate with the typical formula Al2O3⋅ 2SiO2⋅2H2O; it is a natural clay that is used to provide a paint with improved consistency. Additionally, the absorptions at 3532 and 3392 cm-1 are characteristic of the 292 In Collaborative Endeavors in the Chemical Analysis of Art and Cultural Heritage Materials; Lang, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


asymmetric and symmetric O-H stretches found in gypsum, CaSO4⋅2H2O (9, 16) along with a 1619 cm-1 band and a shoulder at 1135 cm-1 that can be assigned to gypsum (9, 15). Additional spectra of these samples show more clearly the presence of the intense 1135 cm-1 absorption in gypsum, due to SO42- asymmetric stretching (9, 16). Unassigned absorptions in the region between 1400 and 3000 cm-1 are indicative of an organic substance(s) presence, perhaps an oil and a protein.

Figure 8. Infrared spectrum of gold and red paint from cloak. Gold/red side was against the ATR element.

However, none of these findings indicate a pigment responsible for either the gold or red color present. The SEM/EDS data show the presence of gold, clearly indicating a gilding applied on top of the red. Elemental iron is also present, suggesting a red ochre, Fe2O3, as a source of the red color in the second layer. Other elements identified (C, O, Al, Si, S, Ca) were consistent with the presence of China clay and gypsum, which are typically substances used with ochre pigments. Mg was also present, indicating the presence of another unidentified mineral, perhaps a silicate of calcium and magnesium. Overlapping bands in the Si-O stretching region would explain the apparent shifts in the kaolin absorptions in that region. The findings suggest a water gilding technique was used where the thin gold leaf was applied to an adhesive layer of a colored clay mixed with hide glue. The red bole warms the color of the gilding and provides cushion against which to burnish (17). 293 In Collaborative Endeavors in the Chemical Analysis of Art and Cultural Heritage Materials; Lang, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

Red Paint from Book Binding


A sample of red paint removed from the book binding (L6) consisted of a red colored layer on top, a white layer, followed by a striated glaze layer next to the wood. These appeared very similar under the microscope to those observed in the three layers found underneath the gold and red paint from the cloak (L5). A representative infrared spectrum is shown in Figure 9. The spectrum shows previously discussed bands due to kaolin (stars), gypsum (circles), protein (triangles), and calcium carbonate (rectangles). The 2919 and 2850 cm-1 bands and the shoulder at about 1730 cm-1 (not marked), can be due to the presence of an esterified oil, such as linseed.

Figure 9. Infrared spectrum of red paint from book binding.

As in the case with gold and red paint removed from the cloak, there is no indication of a red pigment in the spectrum in Figure 9 or any taken from this sample. However, iron is present in the SEM/EDS spectrum, suggesting a red ochre as the compound responsible for the color as found in the outer cloak. Elements consistent with kaolin, gypsum, and calcium carbonate are also present (C, O, Al, Si, S, Ca). Mg was present, as in the paint of the outer cloak, suggesting that an additional silicate was also present. Although no gold was detected in 294 In Collaborative Endeavors in the Chemical Analysis of Art and Cultural Heritage Materials; Lang, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

the elemental analysis, scattered metallic particles are apparent in some samples viewed under the microscope, and perhaps there was not enough gold present to be adequately detected. The book binding from which the red paint was removed was in an area with little paint remaining. An infrared spectrum of the white layer confirmed the presence of calcium carbonate, and that of the striated, opaque glazed bottom layer confirmed the presence of protein. An Erlich’s test on the latter was inconclusive.


Fiber Bundle Removed from Base of Cloak A fiber bundle was removed from Saint’s left side at the bottom of the cloak (L7). The sampling area is shown more clearly in Figure 10. The bundle was white and gradually turned very dark at the end that had been attached to the sculpture. The spectrum at the white end of the bundle (Figure 11) showed bands consistent in frequency and intensity to the C-O stretching region of cellulose at 1107, 1152, 1027, 999, and 981 cm-1. Under microscopic examination, the presence of fine lumen and cross-hatchings are visible in the fibers (18, 19), an indication that the cellulose fibers are from a linen textile. The spectrum in Figure 11 also shows the presence of protein, as indicated by the amide I and II bands. As infrared spectra are taken of the fibers toward the darker end, the protein bands become more intense relative to the cellulose absorptions. Erlich’s tests were positive for the fibers at the dark end indicating the presence of hydroxyproline, an amino acid present in hide glue. Thus, one can conclude that a linen was attached to the sculpture with a gelatin or an animal glue which has discolored over time. The linen was perhaps used for the purpose of joining two pieces of the sculpture together in this location. A publication on a similar sculpture of this genre reports that linen was applied to the entire wood surface before ground was applied (6).

Figure 10. Area on bottom left of cloak where textile fibers were sampled.



Figure 11. Infrared spectrum of white end of fibers.

Summary and Conclusions Pigments and Components With the exception of the Emerald Green identified on the base, all materials are consistent with the purported date of the sculpture. This finding, of course, does not verify that they are original paints, only that they could be. A summary is shown in Table I. with the layers listed from the last applied to those nearest to the wood. The only evidence of possible unoriginal paint is that of the Emerald Green. Not only was that pigment not synthesized until well after the sculpture’s date, the green base was the only area where there was a clear indication of a transparent protein glaze applied on top of the paint. On close examination of the sculpture’s base, one can observe a very thin layer of a slightly different shade of green paint underneath the Emerald Green, which is most likely the original paint. Resampling of that area has not resulted in identification of this paint, as it adheres tightly to the sculpture and samples were not obtained. The detection of protein and oil absorptions in all of the samples except for the blue azurite might be an indication that the artist used a combination of oil and protein as the binding medium. These bands might have been obscured in the spectra of the blue paint because the intense azurite absorption at 1400 cm-1 may 296 In Collaborative Endeavors in the Chemical Analysis of Art and Cultural Heritage Materials; Lang, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


mask the amide II band, and the azurite particles were easily physically isolated from the rest of the paint. A band at 1650 cm-1 in Figure 5 can be observed which could be the amide I band. There are also some indications of gypsum (1619 and 1130 cm-1) and silicates (region around 1100 cm-1) present, also common in the red paints. Another conclusion is that the red on the outer cloak and the red on the book binding are the same paint formulations, consistent with the simple palette of the day. Of particular interest is the striated, opaque, glazed bottom layer of the red paints. Identified as protein, most likely it is a hide-glue used as a sizing agent before the ground is applied, and the striations show the roughening of the wood’s surface as preparation to accept the size.

“Macroscopic” ATR Analysis The use of a universal ATR accessory for initial investigations of art and cultural objects has some obvious advantages, its ease of use being one. In this study, it allowed the analysts to quickly and easily gain surface information on samples as small as 500 x 500 μm. Infrared microspectroscopy has higher spatial resolution, but sample preparation and acquisition of spectra are more time-consuming. Thus, the use of a macroscopic universal ATR accessory served well as an efficient screening method for material identification, the results of which point to particular areas for future study.

Further Work As discussed in the introduction and elaborated on in another publication in this monograph (20), the examination of Male Saint was undertaken as part of 15-week immersive learning course. To help increase the chance of finding an interesting result (for the students) using primarily infrared spectroscopy in the time period allowed, sampling was avoided in locations where there the paints might be composed of earth pigments, oxides, or non-infrared absorbing species. This was the reason the ochre-colored robe was not sampled in this study. The areas of the sculpture’s hands, face, neck, and hair show paint that appears relatively undamaged and firmly attached and were not part of this study. However, before conservation proceeds, those areas will be sampled. The green paint on the sculpture’s base will be re-sampled in the areas where one might find applications of the original paint. Infrared and EDS techniques will be used to identify the materials. Additionally, the layering and chronology of paint and gilding application will be analyzed using microscopic examination of carefully prepared cross-sections. When a higher spatial resolution of samples is necessary, infrared microspectroscopic analysis will be used.


Table I. Summary of paint layering and pigments identified Area

Layering: appearance and color

Substances

Cloak lining L1, L3

Top (1) Blue

Azurite

(2) White

Calcite

(3) striated, opaque, glaze

protein

Top (1) Transparent glaze

protein

(2) green

Emerald green with protein, oil

(3) White

Calcite

Top (1) Metallic gold, fluoresces with UV

Elemental gold, with protein, oil

(2) Red-brown

Red ochre with protein, gypsum, clay, other silicates, calcite

(3) White

Calcite

(4) Striated, opaque glaze

Protein

Top (1) Red

Red ochre with protein, gypsum, clay, other silicates

(2) White

Calcite

(3) Striated, opaque, glaze

Protein


Base L2, L4

Outer Cloak L5

Book binding L6

Acknowledgments The authors gratefully acknowledge Mr. Peter F. Blume, Director of the David Owsley Museum of Art at Ball State University for allowing us the privilege to sample Male Saint, Mr. Robert Galyen for the SEM/EDS analysis at TAWAS, Inc., and Michael Kutis, Department of Geology at Ball State, for assistance in microscopic examination.

References 1. 2. 3. 4.

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Lang, P. L.; Keefer, C. D.; Juenemann, J. C.; Tran, K. V.; Peters, S. M.; Huth, N. M.; Joyaux, A. G. Microchem. J. 2003, 74, 33–46. Website of Art Conservator, Nina Owczarek http://ninasue.com/ Research_files/Mourning%20St%20John%20Examination.pdf (accessed Jan. 2, 2012). Rizzo, A. Anal. Bioanal. Chem. 2008, 392, 47–55. Coombs, D. Int. J. Vibr. Spec. 2 [online] 1998, 2, 3−13. http:// www.ijvs.com/volume2/edition2/section1.html (accessed Jan. 2, 2012). Infrared and Raman Users Group Spectral Database; Price, B., Pretzel, B., Eds.; The Infrared and Raman Users Group: Philadelphia, PA, 2007; Volumes 1-2. Andersen, F. A.; Brecevic, L. Acta Chem. Scand. 1991, 45, 1018–1024. In Artist Pigments, A Handbook of Their History and Characteristics; Roy, A. Ed.; Oxford Press: New York, 1993; Vol. 2. Frost, R. L.; Martens, W. N.; Mahmutagic, R. E.; Kloprogge, J. T. J. Raman Spectrosc. 2002, 33 (4), 252–259. Kuhn, H. Stud. Conserv. 1970, 15 (1), 12–36. The Coblentz Soceity Desk Book of Infrared Spectra, 2nd ed.; Craver, C. D., Ed.; The Coblentz Society: Kirkwood, MO, 1982. Neuman, R. E.; Logan, M. A. J. Biol. Chem. 1950, 184, 299–306. Derrick, M. R., Stulik, D. Landry, J. M. Infrared Spectroscopy in Conservation Science; Oxford University Press: New York, 1999. Sandu, I. C. A.; Afonso, L. U.; Murta, E.; De Sa, M. H. Int. J. Conserv. Sci. 1 [online] 2010, 1, 47−62. http://www.ijcs.uaic.ro/pub/IJCS-10-06_Sandu.pdf (accessed Jan. 2, 2012). Goodway, M. JAIC [online] 1987, 26 (1), 27−44. http://cool.conservationus.org/jaic/articles/jaic26-01-003.html (accessed Jan. 2, 2012). McCrone, W. C.; Draftz, R. G.; Delly, J. G. The Particle Atlas; Ann Arbor Science Publishers: Ann Arbor, MI, 1967. Lang, P. L. The Chemistry of Artists’ Pigments: An Immersive Learning Course. In Collaborative Endeavors in the Chemical Analysis of Art and Cultural Heritage Materials; Lang P. L.; Armitage, R. A., Eds.; ACS Symposium Series 1103; American Chemical Society: Washington, DC, 2012; Chapter 14.


The Spectroscopic Analysis of Paints Removed ... - ACS Publications

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