Corrosion concerns and metal soap formation in shea butter-containing Forawa brass vessels from Ghana
Author(s): Vanessa Muros; Sebastian Warmlander; Kym Faull; Ellen Pearlstein; David Scott
Year: 2015
Summary
Twenty-three forowa metal vessels from Ghana, housed in the Fowler Museum at UCLA, were investigated with regard to manufacture and deterioration. Technical examination revealed that all vessels were manufactured from skillfully hammered brass sheets, and purpose-built for storing shea butter, a multi-purpose substance derived from shea nuts. Most vessels contain remnants of shea butter, which has become discolored: while shea butter extracted using native methods is off-white to yellow, the current vessel remains display different shades of green. Typically, such discoloration is caused by diffusion of metal ions into the organic substance. Spectroscopic analysis of the discolored material using XRD, FTIR, and GC/MS techniques indicated formation of metal soaps, i.e. metallo-organic complexes formed between shea butter fatty acids and copper and zinc from the brass vessels. In order to investigate possible corrosion concerns, the interactions between shea butter and test coupons of copper, zinc, brass, and tin were monitored. Furthermore, the GC/MS data quantified the chemical compositions of shea butter from various sources, allowing us to evaluate the usefulness of such data for geographic shea butter provenancing. Together with the technical metal examination, these results increase our understanding of the uses, manufacturing, and other properties of forowa vessels.
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Cite this Record
Corrosion concerns and metal soap formation in shea butter-containing Forawa brass vessels from Ghana. Sebastian Warmlander, Vanessa Muros, Ellen Pearlstein, Kym Faull, David Scott. Presented at The 80th Annual Meeting of the Society for American Archaeology, San Francisco, California. 2015 ( tDAR id: 397424)
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Keywords
General
archaeological science
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Archaeometallurgy
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Archaeometry
Geographic Keywords
AFRICA
Spatial Coverage
min long: -18.809; min lat: -38.823 ; max long: 53.262; max lat: 38.823 ;