BASIC SLAG, a by-product of steel manufacture, is formed when phosphatic ores are used in converters lined with lime or magnesia, as in the Thomas-Gilchrist or "basic" process. The phosphorus containing material of the ore combines with the lin ing material of the converter to form basic phosphates. This ma terial, if finely ground, serves as a valuable fertilizer. Its value is due in large part to its phosphatic content, although the lime con tent also is valuable. (See STEEL MANUFACTURE; FERTILIZERS.)