Color modification of ceramics with controlled firing

This study explores vacuum firing to achieve a dark color of ceramics produced with iron-bearing clays. Four local low-quality clays were used as the ceramic body. The clay samples were ground, compacted into pellets, and fired at 1150 °C in either a conventional muffle or vacuum furnace. The resulting ceramics were evaluated by visual observation, optical microscopy, XRD, TGA, and nanoindentation to assess their color, chemical, and micromechanical properties. The study revealed that vacuum firing produced dark grey ceramics, and longer vacuum retention during cooling led to a darker shade. Metastable phases were decomposed, and mullite and cristobalite were formed from the kaolinitic constituent. More magnetite existed under vacuum firing, contributing to the dark shade of the ceramics. However, the indentation elastic modulus was lower under vacuum firing due to more formation of the softer magnetite and less of the stiffer hematite. The findings have implications for the large-scale production of dark-color ceramics with low-quality clays.