On Ashes

 Dust and ashes - אפר / עפר - are a Biblical symbol of mourning, repentance and prayer for mercy, e.g. Daniel 9.3, Jonah 3.6; there is warrant for the corporate use of ashes, e.g. Esther 4.3, Jeremiah 6.26, 25.34. Jesus assumes that corporate use of ashes in repentance is an acceptable response to him, e.g. Matthew 11.21. They symbolise that we come from the dust and to dust we shall return because of our sin (Gen 3.19). To make a sign of the cross on the forehead with ashes proclaims to us the judgement the cross passes on us: we sin and deserve death, so should repent and seek God's mercy.