This article has the answer; "In Rome, people cleaned themselves after using a public latrine with a sea sponge lashed to a stick, stored in a bucket of salt w

This article has the answer; "In Rome, people cleaned themselves after using a public latrine with a sea sponge lashed to a stick, stored in a bucket of salt water or vinegar. It was considered polite to give the sponge a cursory rinse and a squeeze before putting it back in the bucket to get it ready for the next person."