The simplest of these was the charcoal iron, whose hollow interior could be filled with smoldering coals. In addition to being rather smoky, it was difficult to get a sufficient draft to keep the coals burning. For this reason, they were equipped with high, spout-like openings, so that the coals could be fanned by inserting a bellows, or by swinging the iron back and forth vigorously. In the late 1800s, other types of slef-heating irons were developed that used gasoline and alcohol as fuel, which was stored in small metal tanks at the back of the iron. The major drawback to these was the smell, and the tendency for them to "pop-off" suddenly when escaping fumes ignited, which not only frightened, but also singed the user.