A Discrete Phase Model (DPM) is adopted to investigate the particle behavior during viscosity measurement of solid-bearing slags. The particle-particle interaction is interpreted by the Discrete Element Method (DEM). The results show that the particle migration is a function of shear time, leading to continuous changes in variable viscosity before a critical shear time. The critical shear time and therefore the viscosity measurement depend on the operational conditions through the particle migration behavior that is influenced by the shear time, rotational speed, gap width and particle content, shape and size. The study of particle size indicates a size segregation of the initially homogeneous system. Based on our results a number of guidelines are formulated to assist in developing more reproducible viscosity measurement methodologies. (C) 2019 Elsevier Ltd. All rights reserved.