Global insight into microwave stoneware firing: Crystallochemical transformations

Abstract

Microwave heating technology is presented as an alternative to conventional stoneware firing, with focus on the crystallochemical transformations. Scanning electron micrographs of microwave fast-fired samples show similar features as those of conventionally fired in a natural gas kiln, taken as reference samples. Electrically fast-fired samples present incipient mullite type-II crystals, while in the microwave and conventionally (gas) fired samples it is fully formed. X-ray diffraction pattern show that the crystallochemical transformations occur at lower temperatures when samples are microwave fired, particularly for temperatures lower than the vitrification temperature. Kaolinite dehydroxylation is fully accomplished at 500 °C when the samples are microwave fired, being still incomplete at 800 °C in electrically fired samples. Stoneware crystallochemical and microstructural transformations point to lower microwave firing temperatures (up to 100 °C lower) than that required by electric firing. These observations are in good agreement with what is reported in the literature, explaining at the microscopic level what happens at the macroscopic level.