וַיִּֽקַּֽח־ל֣וֹ חֶ֔רֶשׂ לְהִתְגָּרֵ֖ד בּ֑וֹ וְה֖וּא יֹשֵׁ֥ב בְּתוֹךְ־הָאֵֽפֶר׃
And he took him a potsherd to scrape himself therewith; and he sat among the ashes.
Read kabbalistic (mystical) interpretation of Job 2:8: and classical mystical sources.