In Part 1 of this post, we discussed the possible origin and significance of an Inca sheet metal corn stalk in our collection, which is one of only three known to exist.
In order to learn more about the object, our initial step was to seek out a scientific approach that could detect and reveal the composition of the metal(s) used to manufacture the piece. Our questions included: What exactly is the corn stalk made of? Are the stalk, corn cobs, and husks made of the same material? How does the composition compare to the other two existing pieces (in museums in Germany and North Carolina)?
Conservation ethics specify the use of non-destructive analytical techniques to study an object whenever possible. The most common non-destructive techniques are ultraviolet (UV), infrared (IR), and x-ray imaging. For the corn stalk, we used x-ray fluorescence (XRF) spectroscopy. XRF is an established analytical technique for identifying chemical elements, primarily metals. In the last decade, advances in electronics have allowed the development and refinement of hand-held, portable XRF analyzers for the field. These instruments are encased in protective plastic shells and look something like ray guns.
Currently, the DAM conservation department does not have an XRF analyzer but has in the past worked with the Colorado School of Mines (CSM) in Golden to undertake analysis on the Asian department’s Indian Bastar bronze collection. Fred Fraiker and Bruce Geller and their CSM colleagues are always enthusiastic to help, and we turned to them again to analyze the corn stalk.
So, how was XRF used to get information about the Inca corn stalk? The XRF analyzer was secured into a vertical position with nozzle facing up and the corn stalk placed on a metal plate above it.
The metal plate has a small rectangle cut out of it to allow for an x-ray beam to be directed onto the surface of an object. Once the corn stalk was situated on the plate, a protective metal cover was put down over the object in order to contain x-ray scatter. Various locations on the stalk, cobs, and husks were analyzed for localized identification and comparison purposes.
XRF systems include three components: an x-ray source, a detector, and a signal-processing unit. The x-ray source creates x-rays which are directed to the surface of the object being analyzed. When the corn stalk was irradiated with x-rays, the x-rays interacted with individual atoms on its surface. These atoms responded by “fluorescing” or producing their own x-rays at energy levels characteristic of the elements present. The XRF detector captured the fluorescing x-rays and sent information about their energy levels and numbers to the signal-processing unit. Finally, from the information sent by the detector, the signal-processing unit produced a spectrum or a graph with a series of peaks. The spectrum was interpreted to identify the elements present as well as information about their relative abundance. Higher peaks corresponding to a particular element meant that more of that element is present in the area analyzed.
The highest peaks in all spectra taken from the Inca corn stalk identified copper and silver as the main metal components of the object. Although more sensitive analysis can be undertaken in the future to better quantify the proportions of these elements in the metal of the corn stalk, we can now say that the sheet metal is copper based and likely has a silver surface produced by a technique known as depletion silvering. (Rather than plating or gilding with metal leaf, to create a precious metal surface over a base metal, pre-Columbian peoples usually depleted the base metal from just the surface of an alloy (using an acidic bath), leaving a surface enriched with the precious metal component of the alloy.) Curatorial and historical research confirms that silver and silvered metal, especially copper, is known and well documented in the Inca tradition and culture.
Next steps in the research process are to gather information about the two similar objects in Berlin and North Carolina for comparison. The XRF data combined with careful examination and other methods of analysis will hopefully further illuminate how these objects are related to each other and to other Inca artifacts whose history and origin we better understand.
Image credit: Corn stalk, 15th century, Inca culture. Denver Art Museum: Museum Exchange, 1960.64.