Researchers examined two terracotta pessoi, probably fragments from broken amphorae, found in the filling underneath Roman latrines close to excrement deposits. The fragments were recut to have smoother edges. Examination under a microscope found solidified and partially mineralized feces.
There is also artistic evidence of the use of pottery fragments for wiping. The Museum of Fine Arts in Boston has a kylix (a wine cup) that has a rather cut and dry depiction on the tondo, the flat round inside the cup.
Until now, the pessoi had been cataloged as pieces in an intellectual game. I guess they'll have to "wipe" the slate clean and start reclassifying these artifacts as a being used for a less cerebral purpose.