Slag foaming plays an important role in new in-bath smelting processes for producing iron and in state-of-the-art electric furnace steelmaking; both processes will be important in future steelmaking in the United States. Most previous work on slag foaming simply measured a foaming index which was the foam height or foam life for arbitrary conditions. Whereas such qualitative information indicates what slags may foam easily it gives little information concerning the basic phenomena. The present experimental work will measure foaming as a function of slag composition, gas flow rate, slag viscosity slag surface tension and temperature. The measurements will not only include foam height and life but also the void fraction as a function of position, which is one of the critical parameters in any quantitative analysis of foaming. The slag compositions to be studied will include those relative to in-bath smelting and electric furnace steelmaking. The effects of small concentrations of surface active elements such as P2O5, and of suspended second phase particles will also be investigated. The foaming of the slags will be correlated to the properties of the slag and, the equations for the conservation of mass, momentum and energy will be used to generalize the behavior. A new model concerned with foam life will also be tested. The surface tension of the slags will be determined if no such information exists for that particular slag. In addition, the rate of nitrogen transfer through a foamed slag and its reaction with liquid iron will be measured.