Several studies have been carried out to establish a relationship
between the surface microstructure and stain resistance. The
roughness and the total amount, size, and morphology of surface
pores have been proved to be the most influent variables [7–9].
Porosity results from a number of complex phenomena simultaneously
occurring during sintering. A liquid phase forms at about
1100–1150 C and it diffuses through the interconnected pores filling
them [10]. Nevertheless, due to mullite crystallization and
quartz dissolution in the liquid phase, the viscosity of the latter
grows upon firing. As a consequence, the suppression of open
porosity is delayed [11]. Indeed, closed porosity starts to increase
before the open porosity has totally disappeared. Open porosity
could be strongly reduced by increasing firing temperature.
Though, above a certain temperature (i.e. the optimum firing
temperature), the gas pressure inside the pores rises opposing
the progressive densification of the material. This leads to the so
called ‘‘body-bloating’’ with drastic deterioration of the mechanical
properties [12]. Therefore, an optimum firing temperature must be
established for each porcelain stoneware mixture. The optimum
temperature is usually reached through slight over-firing
conditions [13].
Beyond open porosity, pore size and morphology have been
shown to play a crucial role. However, their correlation to stain