American investigators' Brasso unexplained

It's now about a month since I enquired of the US team led by Roger Bagnall of New York University who were excavating at the town of Amheida in Egypt why they were reported as scouring Roman coins with "brasso". No explanation however was ever forthcoming. Perhaps the local Inspector has some explanation. Was the mistake the US journalist's or of the New York excavator? Just another of those mysteries of the archaeology of ancient Egypt, I guess.
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The picture used in Dr Bagnall's recent presentation is believed to be the only surviving representation of a Roman "Brasso" party. See how they shine.