Process models that simulate industrial processes are increasingly being used to reduce costs and improve productivity. 1–8) The success of such models is highly dependent on the thermophysical property data used in the simulation calculations. Unfortunately high temperature thermophysical data for liquid metals and slags are scarce. Also what data that are available often have a high uncertainty associated with them.1,2,9–11) Therefore regardless of how well a process model may be physically based it is likely, given the current state of high temperature thermophysical property data, in particular heat capacity, viscosity and thermal conductivity/ diffusivity, that the models predictions will be limited by the uncertainty associated with the data. It is unclear how much such models are being used by industry but extreme care should be exercised when attempting to realise or understand the predictions of process simulation models.