ViscosityViscosity is a critical property of slagSlag, essential in pyrometallurgical operationsOperation and phase equilibriaPhase equilibria research. Accurate viscosityViscosity values are necessary for reliable modelingModelling and process optimizationProcess optimization. To achieve this, an integrated experimental and modelingModelling approach is applied. However, many viscosityViscosity datasets used in modelModel development are prone to errors due to experimental inaccuracies, affecting modelModel reliabilityReliability. Common sources of inaccuracy include poor control over slagSlag composition, crucible dissolutionDissolution, shifts in Fe3⁺/Fe2⁺ and Cu2⁺/Cu⁺ ratios due to oxygen partial pressure variations, challenges in measuring highly viscous slagsSlag, and the inability to quench samples for compositional analysisAnalysis. To address these issues, this study refines an existing methodology to generate consistent and accurate viscosityViscosity data. A coupled viscosityViscosity measurement and electron probe micro-analysisAnalysis (EPMAElectron Probe X-ray Microanalysis (EPMA)) method is introduced to improve compositional accuracy. Preliminary experimental and modelingModelling work focused on the Al₂O₃–CaO–SiO₂ ternary system to validate the methodology. This research demonstrates the ability to obtain reliable experimental data, which will be used to optimizeOptimize existing viscosityViscosity modelsModel. In-house experimental data, along with literature data, will be employed to optimizeOptimize viscosityViscosity modelsModel for fully liquid slagsSlag within the Cu₂O–PbO–ZnO–FeO–Fe₂O₃–CaO–Al₂O₃–MgO–SiO₂ multicomponent system. This chapter presents an overview of recent progress in this research.