Porcelain stoneware tiles are characterized by a very low water absorption (<0.5% according to standard ISO 13006; <0.2% in current production) and excellent mechanical, tribological and functional properties for a building material.

Porcelain stoneware is a glass-bonded material manufactured in large size (up to 1 m2) with fast cycles and a wide range of decorative techniques in order to bestow outstanding aesthetical effects.

Therefore, pressing exigencies for the tile-making industry are: (a) enhancing the sintering kinetics; (b) actually controlling firing shrinkage to achieve a uniform densification; (c) keeping adequate mechanical strength in large size tiles; (d) obtaining colour of fired stoneware as lightest as possible. In fact, ceramic pigments used to decorate porcelain stoneware present a relevant cost; a light-coloured body, especially if even translucent, allows to lower significantly the amount of pigment necessary to get the desired coloration.

In order to fulfil these exigencies, some kinds of glass–ceramic materials have, in the latest years, come in a wide use in porcelain stoneware production. They are vitreous precursors, prepared like ceramic frits, that are expected to devetrify during the fast firing cycle of the tile-making industry (typically 50– 60 min cold-to-cold, with heating rates up to 80 8C/min and cooling rate up to 110 8C/min, maximum temperature 1200– 1240 8C for 5–10 min soaking). The glass–ceramic systems entered in use are silicate and alumino-silicate of Na, Mg, K, Ca,Zn, Ba and Zr, though some further components may be present in the most complex formulations.

The effects played by these glass–ceramic frits in the firing process have been essentially studied by the technological viewpoint. The role of glass–ceramic precursors on sintering kinetics, phase transformations and microstructure of porcelain stoneware is still to a large extent unknown.