Water contains dissolved salts of calcium, magnesium and often iron in the form of bicarbonates, chlorides and sulphates present in the Earth’s crust. When such water is heated, the bicarbonates of calcium and magnesium decompose evolving carbon dioxide and leave behind sparingly soluble carbonates.
Bicarbonate of iron interacts with the carbon dioxide and water forming sparingly soluble ferric hydroxide (brown). These sparingly soluble salts form the layer or ‘scales’ seen in utensils and boilers.
In filters, there is no boiling, still similar chemical changes take place, though to a much less extent, when the remnants of water dry up. Chlorides and sulphates do not undergo these chemical changes, but form residues due to evaporation of the water.
The usual way to avoid this trouble is by using de-mineralized water, that is, ordinary water filtered through permutit (sodium aluminum silicate), manganese salts (to remove iron) and modern ion-exchange resins. The last one frees water from all mineral salts.
All these filters naturally get clogged when in continuous use but can be regenerated in most cases by simple chemical treatment.