[en] At the Institut fuer Nukleare Entsorgungstechnik (INE) experiments have been performed in recent years on sintering ceramic masses by means of microwaves. The goal has been to fix radioactive wastes in the waste forms to be sintered. While searching for a suitable shape of the furnace, we have faced questions of a fundamental nature regarding ceramic mass sintering in the microwave field. It had become apparent that the single pellet sintering of waste specimens in a radio frequency resonator yields reproducible results when metal oxides with high dielectric losses are added. The prerequisite has been that the material composition remains unchanged. Even minimum variations in the composition gave defective waste forms. The results obtained in the experiments make evident that sintering of ceramic masses still needs to be studied further in order to enable validated statements to be made about this technique. (orig.)