Papyrus scrolls were commonly tied with string, and a lump of clay was placed over the knot. The sender then pressed a seal—bearing his name, his title and sometimes his father’s name—into the clay (called a bulla once it was stamped), making the document official. See the following articles in Biblical Archaeology Review: André Lemaire, “Royal Signature: Name of Israel’s Last King Surfaces in a Private Collection,” BAR 21:06; Hershel Shanks, “Jeremiah’s Scribe and Confidant Speaks from a Hoard of Clay Bullae,” BAR 13:05 and “In Private Hands,” BAR 22:02.