Cross Section Analysis of Sample from Detroit Industry by Diego Rivera

Contributed by LEON P. STODULSKI, Detroit Institute of Arts JERRY JOURDAN, BASF, Inc.

Buon fresco technique requires applying very finely ground natural and synthetic alkali stable pigments suspended in water onto wet plaster. As the plaster "sets" by chemical reaction with atmospheric carbon dioxide, the pigments become an integral part of the wall surface. This produces a very tough, colored surface. The extent to which this technique produces pigment embedded in the wall's surface and not merely on its surface was investigated in the laboratories of the Detroit Institute of Arts using a sample taken from the Detroit Industry fresco painted by Diego Rivera in 1932-33.

A small fragment of the dark green background from Rivera's south wall's central panel was mounted in polyester resin and sectioned using an ultramicrotome, each slice being between 0.1 to 1 fxm thick. The final 0.27-mm thick cross section obtained is shown in reflected visible light at 160 X magnification (see Figure J-1a) and in reflected ultraviolet light at approximately 320 X magnification (Figure J-1b). In both these photomicrographs, the thin layer of dark green pigment—previously shown by x-ray diffraction analysis1 to be virid-ian (Cr2O3 • 2H2O)—actually appears to be on top of the white underlying plaster. Even when viewed under 400 X magnification, no white or transparent calcium carbonate plaster particles were detected, either on top of or alongside the green pigment particles.

The fresco section was next submitted for analysis by scanning electron microscopic/energy-dispersive spectrometic (SEM/EDS) dot

'Table I in Heller, B., "The Conservation of Diego Rivera's Detroit Industry Fresco Cycle," Preprints of the American Institute for Conservation meeting, New Orleans, Louisiana (June 1-5, 1988), 94.

Fig. J.1a,b. Cross section of a small fragment of dark green from Diego Rivera's fresco Detroit Industry. Figure J.1a is shown in visible light and Figure J.1b in ultraviolet light. In both photomicrographs, the thin layer of green pigment appears to be on top of the white underlying plaster.

Fig. J.1a,b. Cross section of a small fragment of dark green from Diego Rivera's fresco Detroit Industry. Figure J.1a is shown in visible light and Figure J.1b in ultraviolet light. In both photomicrographs, the thin layer of green pigment appears to be on top of the white underlying plaster.

mapping technique.2 This technique allows the analyst to produce images showing the actual distribution of the elements of interest across the surface of the specimen being analyzed. Ideally, the dot maps of the elements contained in the Rivera cross section would enable us to conclusively show that the plaster particles (containing calcium from calcium carbonate) were located in the same top surface layer as the green pigment particles (containing calcium from viridian) thus proving that the green pigment had indeed become an integral part of the final plaster wall.

The SEM/EDS dot maps obtained are illustrated in Color Plate 28a-g. The portion of the cross section analyzed (a) is the top 270 ¡¿m (or 0.27

2The instrument used was the AMRAY 1830I scanning electron microscope fitted with an EDAX PV9800 energy-dispersive X-ray spectrometer.

mm) of the fresco. The individual dot maps for chromium (Cr) in red, calcium (Ca) in white, magnesium (Mg) in blue, and sulfur (S) in yellow are shown in b through e, respectively. Color Plate 28f and g are the "composite" dot maps, which exhibit the individual colored dot maps of Cr, Mg, and S, and Ca, Cr, Mg, and S, respectively, overlaid on top of one another. The location in this section of Cr from the green pigment— consisting of a layer having a maximum thickness of about 10 ¡¿m (or 0.010 mm)—is illustrated in the individual Cr dot map (b) and in the two composite dot maps (f and g). The important difference between these three dot maps is that g exhibits the location of Ca-containing species across the section, whereas the other two (b and f) do not. It can be clearly seen that the Cr-containing layer appears to be considerably thicker and more extensive in b and f than it is in g. This at first confusing observation is explained by the fact that when two or more different elements are located in the same small area represented by a single dot in a composite dot map, the SEM/EDS computer is programmed to produce an image displaying only the color of that element present in the largest amount. Thus, the amount of Cr in g appears to be less than it is in b and f because of the presence of a large concentration of Ca-containing species located in the same small area as many of the Cr-containing pigment particles. Conversely, those areas exhibiting red dots in g are displayed as such because of the preponderance of Cr over Ca species in that area. The difference in thickness and shape of the Cr-containing layer in the two composite dot maps can thus be accounted for by realizing that both Cr and Ca are located close together within the same small area—which is to say that we have concrete visual, if indirect, proof that the Cr-containing green pigment particles are intimately mixed with Ca-containing plaster particles. And this could result only if the green pigment particles are embedded in the present surface of the plaster wall.

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