Rheological properties of commercial porcelain stoneware slip and compositions modified by nepheline syenite were studied at different milling times. Firstly, the investigation was focused on the effects of the content of nepheline syenite and the particle size distribution on the rheological properties. For this purpose, flow curve data and yield stresses were obtained for the porcelain stoneware slips using a rotational rheometer. It was found that the viscosity and yield stress of porcelain stoneware compositions significantly depend on the above parameters and a minimum viscosity is achieved when 10 wt.%, nepheline syenite is added to the nux. The optimum milling time was determined for the modified composition which resulted in the best rheological properties. Finally, a new rheological model was developed for these types of slips using an hyperbolic function. The new model satisfactorily correlates the shear stress to the shear rate for the slips at different milling times. The results showed that the proposed model predicts the behavior of the flow curves, yield stress and infinite viscosity for the porcelain stoneware slips both with and without the yield stress.

Keywords: Porcelain stoneware slip; Rheology; Flow curve; Yield stress; Nepheline syenite