Since all the fired clay contains high proportion of Silica it is obvious that its presence in the free Crystalline state must always be expected beside Mullite. The presence of Silica can be demonstrated by X-ray methods. The study on a dilatometer occasionally discloses a well developed Cristobalite effect at its conversion temperature of about 230oC and more rarely the Tridymite effect at 117oC.
The Mullite formation from the clay is not quantitative due to the possible solution of mullite in the glass formed from the fluxes. The glass in fireclay bricks consists of an alkalis alumina glass rich in SiO2. The size of the individual crystals depends on the presence of a molten phase and on firing conditions. Both favour crystal growth so that relatively large crystals are formed in fire clays containing large amounts of fluxes.
By taking advantage of this knowledge crystal growth can be artificially promoted. In the absence of fluxes the crystals are granular. Mullite is always formed from the clay, never from Felspar.
… to be contd.
