The ashes of a red heifer burned entire (Numbers 19:5) when sprinkled on the unclean made them ceremonially clean (Hebrews 9:13).
To cover the head with ashes was a token of self-abhorrence and humiliation (2 Samuel 13:19; Esther 4:3; Jeremiah 6:26, etc.).
To feed on ashes (Isaiah 44:20), means to seek that which will prove to be vain and unsatisfactory, and hence it denotes the unsatisfactory nature of idol-worship. (Compare Hosea 12:1).
These dictionary topics are from M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition, published by Thomas Nelson, 1897. Public Domain, copy freely.
Easton, Matthew George. "Entry for 'Ashes'". "Easton's Bible Dictionary".