Rose-brown patinas on Roman imperial monuments are shown to be linked, either directly or indirectly, to biochemical activity of encrusting lichens, called scialbatura , the patina is found on monuments as well as historic buildings, artifacts and natural outcrops, on both marble and limestone, exposed to the open atmosphere. Analysis shows that the main components are whewellite and and weddelite, two calcium oxalates. It is proposed that they are formed by oxalic acid released by lichens. Color photographs, charts, references.