From Plant Press, Vol. 23, No. 4, October 2020.
By Julia Campbell-Such
The National Museum of African Art at the Smithsonian holds many beautiful works of art made from wood, but only rarely do our records indicate what species of wood the artist used. This is not unusual for a museum, but it is a shame since so much can be learned about a piece of art from understanding the materials that were used to make it. Artists choose their materials carefully for their physical characteristics but also, especially in the case of art originating on the African continent, for their symbolic value. Certain tree species can impart meaning, or even metaphysical powers, to an object made from its wood. The identification of which tree species used in African art and belongings can help researchers recreate the history of artworks with uncertain provenance and elucidate the symbolic worlds of those who created them.
Increasingly, species identification is also important in the context of the enforcement of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), an international treaty to prevent species from becoming endangered or extinct because of international trade. Without proper documentation demonstrating that objects are not made with endangered species, museum artworks travelling to exhibitions abroad may be held, confiscated, or even destroyed by customs officials. Knowing which wood was used can help protect artworks when they travel.
As an Andrew W. Mellon Fellow in Object Conservation at the Smithsonian National Museum of African Art, I have been investigating ways to answer the question, “What wood is that?”, for artwork in the museum’s collection for the past two years. Using the Jane Smith collection of 64 hand samples representing over 50 species of West African woods, I have been comparing traditional wood anatomy to newly developed methods that use the tree’s chemistry, as well as its anatomy, to identify it. With the generous support of the Botany Morphology Lab at the National Museum of Natural History, under the supervision of Kenneth Wurdack and the kind, generous tutelage of Stanley Yankowski, I have been preparing and examining wood slides from this collection. I have then been comparing the results of that anatomical analysis to direct analysis in real time mass spectrometry (DART-MS) of the same samples, conducted at the Smithsonian Museum Conservation Institute under the supervision of G. Asher Newsome.
One of the reasons wood species are not often identified for works of art is the size of sample required to do traditional anatomy. The 1 cm cube generally necessary for an accurate identification is a huge chunk for art conservators who usually measure their sample sizes in millimeters or even microns. DART-MS, which requires only a splinter-sized sample, is therefore a promising alternative, or compliment, to anatomy for wood species identification. This technique uses an ambient ionizing source, the DART, to desorb and ionize small molecules from the surface of the wood, which are then transmitted to a mass spectrometer for analysis. Spectra representing the mass-to-charge ratios of ionized molecules in the sample can then be compared to a database of knowns using computer learning algorithms. Specific chemicals can also be identified in the spectra to further confirm species.
The technique of using DART-MS to identify woods was developed at the U.S. Fish and Wildlife forensics lab by Cady Lancaster and Edgard Espinoza, who have been using this system to enforce CITES-based regulations on the trade of endangered tree species. Using vouchered samples from xylaria all over the world, including the Smithsonian’s own collection, they have developed a database of DART-MS spectra of known woods called the Forensic Spectra of Trees Database ©, or ForeST. DART, in combination with a time of flight mass spectrometer, has been used to successfully differentiate between red and white oak (Quercus) (Cody et al., J Anal Appl Pyrolysis 95: 134-137; 2012), various species of rosewood (Dalbergia) (Lancaster & Espinoza, Rapid Commun. Mass Spectrom. 26: 1147-1156; 2012) and mahogany (Swietenia) (Deklerck, Wood Sci. Technol. 53: 953-965; 2019), and others. At the Smithsonian Museum Conservation Institute, we have been working with Lancaster and Espinoza to adapt their protocol to our slightly different instrumentation and to identify the species of West African woods most commonly used by African artists. We hope that this collaboration will not only help us to protect and care for the important art in our museums but will also help to further the protection and care of the forests and communities who created those works.
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