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The curator-conservator collaboration: remembering Roy Sieber

African Arts, Summer, 2003 by Dana Moffett, Stephen Mellor

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For African art, microscopy is especially applicable to the examination of metals, painted surfaces, wood, textiles, and ceramics. Looking at the microstructure of metals can lead to the identification of techniques and postcasting working methods, and help evaluate alloys. In addition, through systematic studies of metallographic structure, the researcher can determine general manufacturing techniques to establish a baseline and thus identify variations. Corroded metal surfaces signal the effects of environment, postcollection coatings, and other treatments. For example, NMAfA's Benin collection includes many works whose surfaces were treated after the objects were removed from their original contexts. An unaltered surface can be seen on one plaque (Fig. 3), where indigenous red earth is detectable in the interstices of the design. On the other hand, the various metals expertly combined in a Benin pendant (Fig. 4) would originally have been distinguished by the differences in coloration, including that of the inlaid iron eyes and textured brass strip at the nose. However, the natural coloration is hidden under an coating of pigment (cadmium sulfide), applied to imitate clay investment. In another plaque (Fig.5), Prussian blue pigment, meant to replicate copper corrosion, obscures much of the surface.

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Under magnification, these altered surfaces do not exhibit the naturally occurring corrosion layers, such as red cuprous oxide and green, basic copper carbonate, that might be expected. Solvent tests as well as instrumental analysis (see below) can confirm their inappropriate nature. In addition, it can be seen that "protective" coatings applied in the early twentieth century to Benin materials have caused the surfaces to appear darker and produced corrosion while leaching out alloying components of the castings. High magnification supplemented by instrumental analysis can identify these Western modifications (Fig. 6).

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One can also use microscopy to examine the painted surfaces of African artworks. These studies might involve looking at the stratigraphy of paint layers to understand the history of indigenous refurbishment or Western embellishment of the surface (Figs. 7a, b). Sometimes this can be accomplished under low magnification, but often it is useful to make a cross-section slide from a minute sample of pigment to view the layers more closely. In addition, dealers and middlemen have been known to strip objects of their colorful paint to suit the Western aesthetic. Detected under high magnification, evidence of residual paint might indicate a stripped surface. Polarized light microscopy (PLM) allows one to identify pigments as well as fibers (e.g., wool, cotton, coconut) and particles of substances such as plant starch and kaolin (Fig. 8). Using PLM to interpret the morphology and refractive indices of these materials can provide another piece of information for the researcher wishing to evaluate an object's authenticity.

The curator-conservator collaboration: remembering Roy Sieber

African Arts, Summer, 2003 by Dana Moffett, Stephen Mellor

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