How A Steam Iron Works
A steam iron contains a tank that holds water - usually somewhere around a quarter to half a litre. Also inside the iron is a heating element which quickly boils the water. As steam is produced, it is forced out of small holes in the irons soleplate. The holes are strategically placed to direct the steam evenly onto the fabric being ironed.
The amount of continuous steam produced is measured in grams per minute and is usually around 30g. Most irons can produce a short burst or shot of steam at a much higher rate - up to 200g in the case of top of the range Rowenta models. This shot is ideal for removing really stubborn wrinkles or producing really sharp creases.
In addition, the soleplate is heated too - this combination of heat, steam and the pressure the user places on the iron is enough to flatten the fabric creating a smooth, wrinkle free surface.
Both the temperature of the soleplate and the water (and therefore the amount of steam being produced) is carefully regulated by the iron, usually in conjunction with a dial that the user turns to the specific setting they want. Some irons have presets for different fabrics - all the user needs to know is the fabric type and the iron will ensure that the optimum heat and steam is produced. Generally, for light, delicate materials the iron will use low volume steam and a low setting for the soleplate temperature.
The material that the soleplate is made from can have a big impact on the iron's performance and therefore the time the user has to spend to get good results. A coated ceramic soleplate is usually the most efficient at conducting the heat to the fabric, and provides an ultra-smooth surface for the iron to glide easily over the fabric.
Because water can contain minute impurities that can solidify when boiled, such as lime-scale in hard water, many irons feature some kind of anti-scale or anti-calc devices. These may take the form of removable filters that need to be rinsed out periodically.
The iron may also contain another permanently installed cartridge which prevents clogging of the steam holes. This cartridge can be damaged by vinegar so it's not advisable to flush out these irons with vinegar as suggested by some people.
In addition to these anti-scale features some irons also have a self-clean programme that you should perform every 2-4 weeks in order to flush scale particles from inside the iron.
Some steam irons have other useful features such as auto-shut off which will stop the iron from heating up after a period of inactivity - useful for energy saving and fire prevention. Some top-end irons even feature digital displays that give the user more feedback and make it easier to adjust settings.