Ashes in Fausset's Bible Dictionary

Sitting down in, or covering one's self with, is the symbol of mourning (Job 2:8; Job 42:6; Esther 4:1; Isaiah 61:3; Matthew 11:21). To eat asides expresses figuratively mourning is one's food, i.e. one's perpetual portion (Psalm 102:9). "He feedeth on ashes," i.e., tries to feed his soul with what is at once humiliating and unsatisfying, on an idol which ought to have been reduced to ashes, like the rest of the tree of which it is made (Isaiah 44:20). The ashes of a red heifer burnt entire (Numbers 19), when sprinkled upon, purified ceremonially the unclean (Hebrews 9:13) but defiled the clean person.