Easton's Bible Dictionary (1897)
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).
Smith's Bible Dictionary (1863)
The ashes on the altar of burnt offering were gathered into a cavity in its surface. The ashes of a red heifer burnt entire, according to regulations prescribed in Numb. 19, had the ceremonial efficacy of purifying the unclean, (Hebrews 9:13) but of polluting the clean. [Sacrifice] Ashes about the person, especially on the head, were used as a sign of sorrow. [Mourning]
Hastings' Dictionary of the Bible (1898) & Schaff's Bible Dictionary
ASH'ES. Gen 18:27. To cover the head with ashes, or to sit in ashes, betokens self-abhorrence, humiliation, extreme grief, or penitence. 2 Sam 13:19; Esth 4:3; Job 2:8; Jer 6:26; Lam 3:16; Jon 3:6; Matt 11:21. The ashes of the altar of burnt-offering on the days of the great festivals were suffered to accumulate, and then taken away the next day by a priest chosen by lot to this work. There was a sort of lye made of the ashes of the heifer
sacrificed on the great day of expiation, which was used for ceremonial purification. Num 19:17,1 Sam 30:18. See Heifer.