Effect of oxide additives on densification of terracotta

In this study, the effect of the addition of a few oxides (TiO2, Cr2O3, Fe2O3 CaCO3, and MnO2) on the sintering behaviour and sintered properties of a raw clay are reported. Static sintering study revealed that, except CaCO3, all remaining compositions showed similar values of firing shrinkage, which is greater than the control batch. Both TiO2 and MnO2 addition leads to lower open porosity at 1000 °C. Dynamic sintering studies at a heating rate of 5 °C.min−1 reveal that the onset of vitrification is in the following order: MnO2 > CaCO3 > control, TiO2 > Cr2O3, Fe2O3 and few oxides (TiO2, Cr2O3 and Fe2O3) improve the firing range and sintering compared to control composition. The terracotta body's flexural strength was influenced by porosity and chemistry. An overall negative linear correlation between flexural strength and open porosity was evident. The beneficial role of Cr2O3 and TiO2 over other oxides can also be clearly discerned. The lightness value (L*) of the terracotta body did increase only with TiO2 addition. A clear trend of change in chromacity components (a* and b*) was affected by oxide addition and firing temperature. The incorporation of Fe2O3 improved the characteristic red colour of the terracotta, along with other positive effects on the sintering of the terracotta body.