Firing at low temperature and soaking time of the studied mixtures of kaolinitic-clay and feldspar-rich materials resulted in the formation of spinel, and the technical ceramic properties were below the required magnitudes for vitreous ceramic products. The use of high temperature and soaking time yielded mullite and a glassy phase, and consequently ceramic properties were improved.
The full factorial design was proven a useful tool for evaluating the effects of feldspar content, temperature and soaking time on vitreous ceramic properties. In this respect, it was concluded that the increase in temperature had a marked and positive effect on the measured properties. Positive effects also were due to the increase of the flux content and soaking time. The weights of interactions between the factors were less significant and varied for properties.