Anonymous

1, 2 could be misconstrued as the coal turned around to face the ashes.

Anonymous

Coal and ashes.... Coal ash is either fly-ash, small airborne particles, or larger pieces left over after coal burns. Coal doesn't turn to or into ashes. The ash is the residue from burning coal. "As the coal burned, nothing but ash remained."

Anonymous

Both are OK given that the chemical context is understood, but the use of 'into' conveys less ambiguity.