It would seem that is the earliest known Biblical reference to ashes being in Genesis, when Abraham accepts that he is nothing but dust and ashes before the LORD, so it is a symbol of humility before God.
Then Abraham answered and said, “Indeed now, I who am but dust and
ashes have taken it upon myself to speak to the Lord..."
Nevertheless, I believe there is also another symbol of great significance. It is important to note that ashes were taken from the burnt offering in the earthly sanctuary:
And the priest shall put on his linen garment, and his linen trousers
he shall put on his body, and take up the ashes of the burnt offering
which the fire has consumed on the altar, and he shall put them beside
Ashes could symbolize something very important, purification:
Then a man who is clean shall gather up the ashes of the heifer, and
store them outside the camp in a clean place; and they shall be kept
for the congregation of the children of Israel for the water of
purification; it is for purifying from sin.
Many times sackcloth and ashes were put on for mourning a death, but other timer for mourning over sin and showing outwardly the repentance that should have been happening inwardly.
Here is the case of Ahab...
But there was no one like Ahab who sold himself to do wickedness in
the sight of the Lord, because Jezebel his wife stirred him up. And he
behaved very abominably in following idols, according to all that the
Amorites had done, whom the Lord had cast out before the children of
So it was, when Ahab heard those words, that he tore his clothes and
put sackcloth on his body, and fasted and lay in sackcloth, and went
And the word of the Lord came to Elijah the Tishbite, saying, “See how
Ahab has humbled himself before Me? Because he has humbled himself
before Me, I will not bring the calamity in his days. In the days of
his son I will bring the calamity on his house.”
1 Kings 21:25-29
And also the case of the city of Nineveh where even the animals were covered in sackcloth and ashes...
So the people of Nineveh believed God, proclaimed a fast, and put on
sackcloth, from the greatest to the least of them. Then word came to
the king of Nineveh; and he arose from his throne and laid aside his
robe, covered himself with sackcloth and sat in ashes. And he caused
it to be proclaimed and published throughout Nineveh by the decree of
the king and his nobles, saying,
Let neither man nor beast, herd nor flock, taste anything; do not let
them eat, or drink water. But let man and beast be covered with
sackcloth, and cry mightily to God; yes, let every one turn from his
evil way and from the violence that is in his hands. Who can tell if
God will turn and relent, and turn away from His fierce anger, so that
we may not perish?
And Job's repentance...
Therefore I abhor myself, And repent in dust and ashes.”
They symbolize the blood of Christ.
For if the blood of bulls and goats and the ashes of a heifer,
sprinkling the unclean, sanctifies for the purifying of the flesh, how
much more shall the blood of Christ, who through the eternal Spirit
offered Himself without spot to God, cleanse your conscience from dead
works to serve the living God? And for this reason He is the Mediator
of the new covenant, by means of death, for the redemption of the
transgressions under the first covenant, that those who are called may
receive the promise of the eternal inheritance.
We should take the example of how people mourned and repented and put ashes on their head, as repent in the same way. Instead of covering ourselves with ashes, claiming to be covered by the blood that Jesus spilled in the cross of Calvary for us. For that is how we are cleansed.
But if we walk in the light as He is in the light, we have fellowship
with one another, and the blood of Jesus Christ His Son cleanses us
from all sin.
If we say that we have no sin, we deceive ourselves, and the truth is
not in us. If we confess our sins, He is faithful and just to
forgive us our sins and to cleanse us from all unrighteousness.
1 John 1:7-9
TL;DR - ashes are representative of Christ blood which cleanses us from our sins.