On-line Experimental investigations have been carried out to examine parameters which show marked changes in the performance of coke fired cupolas during melting such that a decrease in optimum conditions can be predicted. This decrease in performance usually results in increase in pollution levels, especially that of particulate emissions from the stack. Previous models by other workers have been examined and classified into three groups, those involving the use of heat transfer coefficients, thermodynamic and chemical methods, and models based on the statistical analysis of data. From the measurements of the top gases it is shown that the carbon monoxide levels identify problems within the melting zone and that a drop in the carbon monoxide level indicates that the iron temperature is about to fall and that foaming slag is developing.