THE TRADITION BEHIND ASH WEDNESDAY

The use of ashes and dust with religious, magical or medical meanings was common among the ancients, says the New Catholic Encyclopedia. Ashes were often symbolic of mortality, mourning or penance.
In the Hebrew Scriptures, after Jonah has announced the destruction of Nineveh and the news reaches the king, he rises from the throne, lays aside his robe, puts on sackcloth and sits in ashes. The king then proclaims a fast. Nineveh mourns its sins and does penance (Jonah 3:1-6).
In foreseeing the destruction to come upon Israel, Jeremiah says, "O daughter of my people,...roll in the ashes. Mourn as for an only child with bitter wailing, for sudden upon us comes the destroyer" (6:26). Jeremiah calls Israel to conversion.
In the Gospel of Matthew Jesus reproaches Chorazin and Bethsaida saying, "For if the mighty deeds done in your midst had been done in Tyre and Sidon, they would long ago have repented in sackcloth and ashes" (11:21).
Christians, then, seem to have taken the use of ashes as a sign of penance from Jewish tradition. According to the New Catholic Encyclopedia, ashes were originally signs of private penance. But early on they became part of the ritual for public penance.
Adolf Adam, in The Liturgical Year (Pueblo Publishing Company), points to Tertullian and Cyprian, in the third century, as evidence early Christians were familiar with ashes as a sign and part of the ritual for public penance.
As early as the 300's, local churches had a ritual for the beginning of public penance at the start of Lent. Those who had been guilty of public serious sins like murder, apostasy, heresy or adultery were clothed in a penitential garment and sprinkled with ashes.
The sinner was then expelled and led from the church as Adam had been cast out of Paradise. Later those ending their penance (which could go for years) were received back into the Church on Maundy Thursday. They were led back into the church in procession as part of a rite of reconciliation.
According to Herman Wegman in Christian Worship in East and West (Pueblo Press), texts and ceremonies were added to the rite for the reception of penitents in the ninth century, and carried over into the Pontificale Romanum of the Council of Trent.
Adolf Adam says public ecclesiastical penance disappeared around the end of the first millennium. Wegman suggests that the severity of the practice was at least partially the reason for its disappearance. But there was also a growing conviction that every person is a sinner and must do penance.
Pope Urban II (1088-1099) recommended the custom of all receiving ashes to all the churches. Ashes were put on the heads of men and the sign of the cross traced with ashes on the foreheads of women, presumably because their heads were covered.
In the 11th century there appeared a special prayer for the blessing of ashes. And the 12th century gave rise to the rule that the ashes used on Ash Wednesday are to be made from the palm branches of the previous year.

http://www.americancatholic.org/Messenger/Jul1996/Wiseman.asp