Microwave versus conventional porcelain firing: Colour analysis

Abstract

Colour of microwave fired porcelain in a home-built multimode furnace is compared with electrically and gas (industrial) fired samples. Coffee cups were fired from room temperature up to slightly above 1400 °C using both heating technologies, presenting similar colour and functional properties. During firing, porcelain undergoes several transformations, with associated colour changes, which depend on the used heating technology. Results show that the colour of microwave fired samples is within the green-blue spectra, and that the colour of electrically fired samples is within the yellow-red spectra. Above the eutectic temperature of the K2O-Al2O3-SiO2 system, the colour of porcelain converges to the yellow-red spectra for any of the considered heating technologies. Microwave heating was found to promote faster crystallochemical transformations, which are responsible for the observed colour differences. This study aims a better understanding of microwave porcelain firing, focusing on the reasons behind the observed colour differences when comparing with conventional firing.