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A portrait of two Roman men with the inscription PIE ZESES. This sort of glass pieces (known as gold glass) were made with a leaf of gold between two pieces of glass. Originally these pieces were parts of drinking cups and such like ('pie zeses' actually mean 'drink - may you live' in Greek and was used as a toast) but later cut out and set into the walls of catacombs. I don't actually know that this is what happened here (the label wasn't that informative), but it isn't unlikely given how this piece looks now. Dated to the first quarter of the 4th century A.D.
From a temporary exhibition (back in the summer of 2012) of Ancient glass, held at the Curia at Forum Romanum, Rome. (The photo is a bit dark - but the original back light was pure evil!)