The Calc-Clean function removes scale and impurities.
To do so, set the steam control to position '0', set the temperature dial to 'MAX', and fill water tank up to the 'MAX' level. Then heat up the iron until the pilot light on the handgrip goes out.
Remove the mains plug from the wall socket and hold the iron horizontally over the sink. Then press and hold down the 'CALC CLEAN' button.
Move the iron forwards and backwards while boiling water/steam squirt from the holes in the soleplate to wash away impurities/scales.
When the water tank is empty, heat up the iron to dry it.
Finally move the iron to and fro on a piece of cloth to dry the soleplate. Repeat the process if necessary.