Porcelain stoneware tile densification involves a liquid-phase
sintering process.8,9 During firing, at temperatures of about
900–1000 ◦C, a significant quantity of liquid phase is formed,producing a capillary pressure (PC) at the particle contact points
that brings the particles closer together, increasing shrinkage
and lowering porosity, while at the same time altering pore size
and shape.At an intermediate stage, pores start closing as interpore
connections are eliminated. The occluded pores contain
gas that exerts a pressure (Pg) on the pore walls, acting against
densification. At temperatures around 1200 ◦C, occluded pore
gas pressure is high and counteracts capillary pressure, causing
the product to expand. The sintering rate of this type of material
can be obtained from the equation shown below