Patina A beautiful surface deposit or fine rust, with which, in time, buried coins and bronzes become
covered. It is at once preservative and ornamental, and may be seen to advantage in the ancient bronzes
of Pompeii. (Greek, patane, a paten.)

Notice Brewer calls it "rust". Reminds me of joke about magician who brought statue of
Civil War general to life, and asked what was the first thing he was going to do.
"Shoot a million pigeons," said the general.