Previous studies in steel cleanness have been hampered by limited control of inclusions’ phase and composition. This limited control is often a result of studying inclusion formation and development using industrial steel melts and the inclusions therein as starting material. This is an extremely complex situation and not always readily amenable to analysis. A new experimental laboratory technique has been developed that allows addition of inclusions of known, size, phase and composition to be added to liquid steel. These inclusions can then be tracked and changes in morphology with time can be assessed for different melt conditions. The technique involves the formation of a steel-oxide compact, where high-purity inclusions are sandwiched in steel foil at high pressure. This compact is added to a steel melt of controlled and known composition. The use of this sandwiching technique overcomes problems of passing small particles (inclusions <50 μm) through the gas-melt interface and inclusion agglomeration. Details of the technique are presented and discussed and results applying this new technique in a reactivity study using cerium oxide as a synthetic inclusion in steel are given.