Experiments have been performed to investigate the morphology and chemistry of the interaction between mould slag, steel, and SEN refractory that are potentially happening during slag crawling. The experiments were designed to capture the initial stages of the reactions between original reactant partners. It was found that mould slag completely wets the refractory interface to steel, preventing steel-refractory contact. Mould Slag simultaneously reacts with the steel exchanging Si for Al; dissolves alumina from refractory, and undergoes silica reduction by carbon of the refractory. The net effect is that mould slag changes into a mixed silicate-aluminate composition which remains fully liquid and mobile, able to spread out across the submerged surface of the SEN, linking the more reactive parts to the mould slag by intense reaction driven flow.