Easton's Bible Dictionary: M. G. Easton, M.A., D.D.
The ashes of a red heifer burned entire (Num. 19:5) when sprinkled on the unclean made them ceremonially clean (Heb. 9:13).
To cover the head with ashes was a token of self-abhorrence and humiliation (2 Sam. 13:19; Esther 4:3; Jer. 6:26, etc.).
To feed on ashes (Isa. 44:20), means to seek that which will prove to be vain and unsatisfactory, and hence it denotes the unsatisfactory nature of idol-worship. (Comp. Hos. 12:1).