Angew. Chem. Int. Ed. http://doi.org/f2sp7w (2014)
The phoenician ivory carvings found at Arslan Tash in northern Syria date back to the end of the ninth century bc, and are renowned for their artistic quality. For archaeological conservationists, they also represented something of a conundrum. The carvings were originally covered with a thin layer of gold foil, and over the centuries this has reacted with the ivory to give rise to distinctive purple stains. Detailed chemical analyses carried out at the Louvre museum in Paris revealed that these stains consist of gold nanoparticles (AuNPs) with a relatively uniform size distribution. Ivory is, in essence, a collagen-based mineralized tissue with a high specific surface area arising from the presence of hydroxyapatite nanocrystals, and it is known that AuNPs can be made by photoreducing collagen–HAuCl4 complexes in solution. But their formation in archaeological finds had not been examined. Now, Ina Reiche and colleagues demonstrate that the collagen can indeed act as a stabilizer for AuNP formation in a simple reaction that occurs at ambient burial conditions. With the origin of the stains established, the authors propose they can be used as a marker of authenticity for other gold-plated ivory artefacts found in the future.
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Taroni, A. Stains of authenticity. Nature Mater 13, 769 (2014). https://doi.org/10.1038/nmat4054