A steam iron uses superheated water to eliminate wrinkles in clothes and fabrics which may not be suitable for traditional dry ironing. Distilled water is usually poured into a holding tank and special heating elements convert it to steam. This hot mist comes out through a number of holes in the soleplate (bottom heating element) of the iron. As the steam loosens the individual fibers of the clothes, the steam iron's pressing action smooths out wrinkles.
Many traditional dry irons contain a steam option. Water is poured into a small reservoir and superheated until it becomes usable steam. A mechanical switch on the handle of the iron allows the user to select the steam setting. The steam itself should come out of small holes located near the tip of the pressing plate. For most conventional ironing needs, this combination of dry and steam settings should suffice. The steam generated by a traditional dry iron may not be overwhelming, but it will loosen wrinkled fibers.
Some ironing jobs such as curtains or quilts require a much more generous supply of steam. This is where a true steam iron can be useful. Distilled water is placed in a larger reservoir in the iron. Users can push a button to receive a burst of steam when needed. More holes in the soleplate means a more generous supply of steam while ironing heavier materials like suits or curtains.
Advanced steam iron systems may also feature handheld wands which can direct clouds of steam in and around vertically-hung garments or curtains. The steam itself should eliminate wrinkles without the need for a pressing element. Consumer rating boards suggest that the ability to generate 'vertical steam' is a major plus for a steam iron system. Some irons that combine dry and steam functions can only generate steam when the iron is held horizontally over an ironing board. Good vertical steaming means the iron can be used to iron curtains and quilts in place.
Another important feature to look for in a steam iron is a means to control the level of steam. Some models only allow for an on/off situation--either the user has steam or not. Better models have a dialing mechanism which can adjust the intensity of the steam produced. Consistency of steam is also a consideration. Once the water has become superheated, the steam should be steady and have some power behind it. Effective steaming means penetrating individual fibers, not merely wetting the surface.