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.