BIERSTADT'S LATE PAINTINGS: METHODS, MATERIALS, AND MADNESS
DARE MYERS HARTWELL
1. Diane Kotowski of the Department of Art, University of Denver, dates Weeping Oaks, Clear Creek, California, to 1880
National Gallery of Art, August 29, 1989 Science Department Analysis Report
Analysis of ground layers from five paintings
Material: oil paintings on canvas
Ground layers from five paintings by A. Bierstadt (table 1) were analyzed using light microscopy and powder x-ray diffraction spectroscopy. In each case, the black ground was found to be a mixture of graphite, a clay material (possibly montmorillonite), and iron oxides.
TABLE 1. GROUND LAYERS FROM FIVE PAINTINGS BY A. BIERSTADT
Samples from The Last of the Buffalo (Buffalo Bill Historical Center) and Rocky Mountain Sheep (private collection) were delivered to the department in the form of prepared cross sections. Each possessed intact grounds and image layers. Small pieces of tacking margins from the remaining paintings were submitted for analysis. Although these samples possessed intact ground layers, image layers were absent. Scrapings of the ground layers were taken using a scalpel. The ground samples were mounted as dispersions in Cargille Melt-Mount (nD = 1.662). Optical properties and particle morphologies were documented using polarized light microscopy.
Each of the five dispersions was found to consist of a mixture of three different components and/or particle types. Gray-black flakes or plates represented the predominant constituent. Although completely opaque, the gray-black flakes appeared to be coarsely crystalline and were compacted into numerous thin layers. Unmounted gray-black flakes were easily crushed with the tip of a stirring rod and left a dark smudge on the paper substrate. The flakes ranged from individual particles 2–3 μm in size to agglomerates 1–3 mm across. Optical and morphological properties exhibited by the unknown flakes are consistent with those for graphite (Gettens and Stout 1966; McCrone et al. 1979). X-ray powder diffraction spectroscopy yielded results consistent with JCPDS powder diffraction standards for graphite (e.g., JCPDS standard 13-0148 and 01-0640).
Light yellow-white through yellow-brown particles (size range = 1–114 μm) made up the second most abundant ground component. The particles were either isotropic or exhibited very low birefringence (nD = 1.662). Shape ranged from amorphous through angular, somewhat glassy fragments. Although Gettens and Stout (1966) state clay was frequently combined with graphite to serve as a binder, observable microscopic characteristics of the particles were insufficient for identifying the unknown as clay. Additionally, McCrone et al. (1979) states that clays are generally unidentifiable on microscopic bases alone. Therefore, powder x-ray diffraction (XRD) was chosen as an alternate analytical method.
Samples from two Bierstadt paintings were selected for analysis: 1. The Last of the Buffalo (Buffalo Bill Historical Center); and 2. Rocky Mountain Sheep. Results of the XRD analyses revealed the unknown to be montmorillonite. The identification was confirmed with a match to JCPDS standard 12–131. Montmorillonite is a member of the smectite group of clays. The basic chemical formula of the group is Al4Si8O20(OH)4 • nH20 (Deer et al. 1966). Chemical variation yields the clays in the smectite group. Montmorillonite has the formula (Na)0.7(Al3.3Mg0.7)Si8O20(OH)4 • nH2O.
The least abundant particle type in the ground layer samples was represented by highly birefringent (nD > 1.662) red through orange and yellow particles. These particles occurred in minor to trace amounts. Particle morphology was heterogeneous and varied from elongate and splintery through finely divided, somewhat opaque spherulites. Characteristics exhibited by the unknown particles are consistent with those for iron oxides and compared favorably to laboratory standards.
In conclusion, ground samples from the five Bierstadt paintings examined were composed of a mixture of graphite, clay, and iron oxides. The clay, for at least The Last of the Buffalo and Rocky Mountain Sheep, was demonstrated to be most closely allied to montmorillonite, a member of the smectite group of clays. For the remaining paintings, a more specific identity of the clay component was not assigned. It should be noted that Deer et al. (1966) describe smectite clays as being useful in the formulation of paints, paper, rubber, ceramics, and a variety of other materials.
Although the graphite ground (as it exists on the canvas support) appears to be discrete layer, it is noteworthy that the individual flakes/plates of graphite are arranged layer upon layer. Furthermore, these layers, as seen in cross section, are in varying states of adhesion to one another. It is difficult to determine how many applications were necessary to achieve the ground thickness observed.
Finally, the iron oxides present in the ground cannot necessarily be classified as an additive. Toch (1907) states that iron oxides, silica, or calcium carbonate were often added to graphite paint in order to provide a more suitable surface on which succeeding films could adhere. Gettens and Stout (1966), however, describe iron oxides as naturally occurring in clays. The reason for the presence of iron oxides in the grounds examined is unknown.
MICHAEL PALMER, Conservation Scientist RENE DE LA RIE, Head, Scientific Research Department
. A Leitz Orthoplan with Npl objectives was used. Several of the black particles were mounted in Cargille Melt-Mount (nD = 1.662) and examined at magnifications of 250× to 630×.Powder diffraction was carried out on all samples using a Phillips 3100 x-ray generator equipped with a copper anode and nickel target. Current settings were 45 kV and 25 mA. A 114-mm Gandolfi camera was used. Runtimes ranged from 5 to 12 hours in air.
Deer, W. A., R. A.Howie, and J.Zussman. 1966. An introduction to the rock-forming minerals. London: Longman Group.
Gettens, R. J., and G. L.Stout. 1966Painting materials: A short encyclopedia. New York: Dover Publications.
McCrone, W. C., J. G., Delly, and S. J.Palenik. 1979. The particle atlas, Vol. 5. 2d ed.AnnArbor, Mich.: Ann Arbor Science.
Toch, M.1907. The chemistry and technology of mixed paints. New York: D. Van Nostrand Company.
REFERENCESNote: All unpublished reports are available upon request from the author.
Berrie, B. H., and Palmer, M. R.1986. Analysis report/Bierstadt, A.The Last of the Buffalo/9.12 Corcoran Gallery. Corcoran Gallery of Art, Washington, D.C.
Berrie, B. H., and Palmer, M. R.1988a. Analysis report/Bierstadt, A./The Last of the Buffalo/Owner: Buffalo Bill Historical Center (Cody, WY). Corcoran Gallery of Art, Washington, D.C.
Berrie, B. H., and Palmer, M. R.1988b. Analysis report/Bierstadt, A./The Last of the Buffalo/9.12/Owner: Corcoran Gallery. Corcoran Gallery of Art, Washington, D.C.
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Palmer, M. R.1989. Analysis report/Bierstadt, A./Analysis of ground layers from five paintings. Corcoran Gallery of Art, Washington, D.C. This report is published in the appendix.
Parkin, H. M., and P. C.Huston. 1985. Treatment report for The Last of the Buffalo (Buffalo Bill Historical Center). Perry Huston & Associates, Inc., Fort Worth, Tex.
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Wolbers, R. C.1991a. Analytical reports on samples from both versions of The Last of the Buffalo. Corcoran Gallery of Art, Washington, D.C.
Wolbers, R. C.1991b. Syllabus for Cleaning of paintings: A refresher course with Richard C. Wolbers, “Slides” section. University of Delaware, Newark, Del.
DARE MYERS HARTWELL has been the chief conservator at the Corcoran Gallery of Art, Washington, D.C., since 1983. She has a master's degree in art history from the University of Minnesota and received her conservation training at the Minneapolis Institute of Arts and the Institut Royal du Patrimoine Artistique, Brussels, Belgium. Her study in Brussels was funded by a Fellowships for Museum Professionals grant from the National Endowment for the Arts. She was previously the assistant painting conservator at the Upper Midwest Conservation Association in Minneapolis and the associate conservator at the North Carolina Museum of Art in Raleigh.