GEOCHEMICAL CHARACTERIZATION OF FRENCH GLASS TRADE BEADS FROM FORT MACKINAC, MI

Glass beads manufactured in Europe were brought to North America and used in trade with native American peoples. The style and composition of glass beads were unique to their manufacturing site in Europe. The synthetic glass trade beads are analogous to geologic glasses (i.e., obsidian) in that their unique internal chemical homogeneity allows for the identification of their place of origin, as well as for reliable tracking of the trade relations among native American groups and their European trade partners. Here, we characterize a series of turquoise-colored glass beads sampled from Fort Mackinac, MI, a vital 18^th century French trade post in the upper Midwest. Major and trace element chemistry are measured using energy dispersive x-ray spectroscopy on the scanning electron microscope (SEM). Water content and hydroxyl speciation of glasses are determined using Fourier Transform infrared spectroscopy. The bulk water content reflects the humidity level inside the kiln from which it was manufactured, and the speciation can be used to determine how quickly the glass cooled when it formed. The infrared spectra of the Ft. Mackinac beads display the same six absorption bands observed in a suite of trade beads collected from around North America, and include variable absorption at 3550, 3400, 3250, 3100, 2950, 2800, and 2650 cm^-1. Importantly, we show that carbon-coating of the historic artifacts is not required in order to measure accurate major element compositions (SEM analysis conditions under variable pressure mode return identical major oxide abundances as those measured on carbon-coated samples under high vacuum). We compare our analyses to those conducted on similarly blue-colored 18^th century glass artifacts provided by the Musee du Verrier in Saint-Nicolas-des-Biefs, Auvergne-Rhone-Alpes, France.