Jonah 3 is a good OT example of the idea of ashes. The idea is conversion back to God from sin.
1 Jonah began his journey through the city, and had gone but a single day’s walk announcing, “
Forty days more and Nineveh shall be destroyed,” 5 ** when the people of Nineveh believed God; they proclaimed a fast and all of them, great and small, put on sackcloth.** 6 When the news reached the king of Nineveh, he rose from his throne, laid aside his robe,
covered himself with sackcloth, and sat in the ashes. 7 Then he had this proclaimed throughout Nineveh, by decree of the king and his nobles: “Neither man nor beast, neither cattle nor sheep, shall taste anything;** they shall not eat, nor shall they drink water.** 8
2 Man and beast shall be covered with sackcloth and call loudly to God;
every man shall turn from his evil way and from the violence he has in hand.** 9** Who knows, God may relent and forgive, and withhold his blazing wrath, so that we shall not perish.” 10 When God saw by their actions how they turned from their evil way, he repented of the evil that he had threatened to do to them; he did not carry it out.I know that there is a passage in Job about sackcloth and ashes as well.