Jamieson, Fausset & Brown

a potsherd--not a piece of a broken earthen vessel, but an instrument made for scratching (the root of the Hebrew word is "scratch"); the sore was too disgusting to touch. "To sit in the ashes" marks the deepest mourning (Jon 3:6); also humility, as if the mourner were nothing but dust and ashes; so Abraham (Gen 18:27).