In this study, the effect of composition on the pyroplastic deformation of a sanitaryware porcelain bodies was investigated. Systematic compositional arrangements were made to combine different amounts of SiO2/Al2O3 and Na2O/K2O ratios. It was found that fleximeter analyses can be successfully utilised not only to measure the viscosity of a system but also to determine the pyroplastic deformation behaviour of the vitreous bodies. Al2O3 and SiO2 sources affect the mullite formation in the body formulation in that mullite formation increases as the clay kaolin fraction increases. The composition variation with different Na2O/K2O and SiO2/Al2O3 ratios affects the viscosity of the system as the amount of formed mullite increases. The formation of mullite crystals increases the viscosity and diminishes the pyroplastic deformation. In the current study, the formation of about 28 wt% mullite phases with higher viscosity (similar to 10(8.34) P) values through the combination SiO2/Al2O3:5 and Na2O/K2O:4 ratios was sufficient to reduce the pyroplastic deformation of sanitaryware porcelain bodies. (C) 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved.