The first mention of ashes are in relation to the ashes Moses tossed into the air, which then became boils on the Egyptians.
In another instance, Tamar put ashes on her head after she was raped by her brother.
Who cares about the "first mention?" That's a fallacious argument.
In another instance, Tamar put ashes on her head after she was raped by her brother.
As a sign of penitance and mourning. When Catholics put that ashen mark on their head, it is a sign of repentance from their sins.