In the Iron Keep, after lowering both ends of the bridge and while standing immediately in front of the fog door that leads to the Smelter Demon, turn around and jump over the gap (the chest here holds the Zweihander); look to the right and jump to that platform. The corpse hanging from the broken stairs holds the Dull Ember.
Two iron chests will appear in front of the player after they are transported from the Forest of Fallen Giants to the The Lost Bastille (after defeating the Pursuer). The chest on the right holds the Dull Ember.
When given to Steady Hand McDuff in the Lost Bastille, he will be able to use elemental stones to infuse weapons. Unlike the original game, only one non-standard titanite stone is needed to modify a weapon; the item keeps its current upgrade level and is upgraded further with regular titanite.