Suction molds used in the suction removal process must first be prepared ore they can receive vitrified radioactive waste materials. A method is disclosed for preparing the suction mold which includes a suction pipe welded into the base of the mold with a desired-break location whereat the suction pipe is broken off during the suction removal process. An evacuation stub is mounted in the mouth of the suction pipe and communicates with the interior of the mold. The method of the invention includes evacuating the mold through the evacuation stub and then heating the mold to remove moisture and organic impurities. The mold is then filled with a dry inert gas and the evacuation stub is cleaned. The mold is then evacuated to a predetermined residual pressure. Thereafter, the evacuation stub is cold-press welded so as to close off the interior of the mold from the ambient.