Clay based minerals are the main components of unglazed porcelainized stoneware ("gres porcellanato") bodies and their amount generally ganges from 40 to 60 wt%. They confer plasticity and workability in the green state and fournish the main oxides involved, with fluxes and other sintering aids, in the strenghening mechanism of the body during firing. Kaolinitic clays can improve the formation of mullite and , in general, by widenong the sintering temperature range reduce swelling and deformation of tiles during firing. Low titanium and iron oxides content is foundamental in developing colours.