Using phase diagrams to deal with moisture expansion

Consensus has it that the post-fabrication moisture expansion of fired clay-based materials is governed by the presence of amorphous and/or vitreous phases. In this work, very simple calculations using the lever rule and the phase diagram of the Al2O3–SiO2–K2O (A–S–K) system, were used to show that the experimentally observed dependencies between moisture expansion and the A/S and K/A weight ratios in clay-based compositions can be correlated with the amount of liquid phase formed during the initial melting of the composition, which remains in the fired body as a vitreous phase. This correlation might, if judiciously used, help ceramists in the choice of additives and/or processing conditions, while avoiding changes in the firing temperatures. Also, the results obtained throw a sharper light in the dual and, in this respect, antagonistic role played by the liquid phase during firing.