Abstract: A dished foil lid is secured to a container having a flange at its top by placing the lid over the open top of the container with the periphery of the foil on the flange and the dished portion of the lid extending downwardly into the container, and by applying a heating tool to the exterior of the foil adjacent the inner edge of the top of the container. The heating tool preferably has a tapered tip portion so that when it is urged into the depression formed by the dished portion of the lid, the side surfaces of the tip bear against the part of the foil adjacent to the inner edge of the top of the container. The time, temperature and pressure of application of the tip are such as to cause the adjacent inner edge of the container to soften and, upon subsequent removal of the tool, a narrow seal is formed between lid and container at the inner edge of the top of the container. The resultant seal is not only uniform and hermetic, but is easily parted when one later peels off the lid.