The ironmaking blast furnace forms part of the dominant process route for steelmaking worldwide. In the lower zone of this shaft reactor, liquid iron and slag descend countercurrent to reducing gases through a packed bed of coke. The fundamental characteristics of these liquid flows are not fully understood, but influence the product quality, production rate, fuel use and asset life of the process, and hence the greenhouse gas emissions and competitiveness of the integrated steel industry. The present study aimed to establish the criteria for the passage of slag through the narrow pore necks that form between coke particles. To simulate the flow of slag through the pore necks, a new experimental technique was developed. This involved melting a slag pellet in a coke funnel with a channel of known diameter cut out of synthetic coke. Synthetic coke was used to minimise experimental uncertainty associated with the use of variable industrial coke and allow control of the coke mineralogy. Coke and slag were heated to 1500°C under argon and held at temperature for 30 minutes. After cooling, the penetration of slag into, or passage of slag through the channel was determined and the interactions of the slag with coke were characterized. Variables assessed included slag composition in the CaO-SiO2- MgO-Al2O3 system, coke mineralogy and channel diameter. For the slags and cokes studied, the minimum channel diameter that allowed slag to flow was between 4.4 and 5.0 mm. The results were in good agreement with a simple gravity and capillary force balance.