Abstract: A container, which may be a burial casket or a toxic waste container, is protectively sealed by enclosing it in an initially open-topped receptacle, which may be a burial vault or similar outer container or a grave, spacing the container from the bottom of the receptacle, partially filling the receptacle with a foamable plastic still in its liquid form, and, following completion of the foaming of such initial charge, supplying sufficient additional foamable plastic in one or more additional steps to completely cover and surround the container with the resulting plastic foam. In the preferred method, a film-like plastic sheet is placed over the open-topped receptacle after the final charge of liquid foamable plastic is applied, and held in place around the edges of the opening by a collar which permits the sheet to yield upwardly as the foam expands, thereby partially confining the expanding foam to assure full filling of all voids in the receptacle.