The Bureau of Mines has conducted studies of the slags related to ferrous technology to evlauate experimentally acceptable substitutes for the auxiliary flux, mineral fluorspar, in steelmaking operations. Comparative heats were made in a 1/4-ton, pilot-scale, basic oxygen furnace (bof) to evaluate a waste material known as "used potlining." Used potlining is recovered from alumina reduction cells after its useful life and contains significant levels of fluorine and sodium in various compounds. This material in both lump and pelletized form provided adequately fluid slags, based on observations made during bof operation. No adverse effects were found on furnace performance, metal and slag chemistry, the strength of the steel produced, or the environment. Fluorine recovery in the slag was about the same as that obtained when using fluorspar, as were the losses to the scrubber water and solids. No difference was observed in furnace-refractory attack, as determined by the level of magnesia in the slag. The sodium recovered in the slag apparently also contributed to fluidity, since the fluorine input was at about two-thirds that of the fluorspar. Used potlining was determined experimentally to be a satisfactory substitute for fluorspar in basic oxygen steelmaking.