A method is provided in which a lower box comprising a base, walls that surround the base and an open side, and an upper box comprising a cover, walls that surround the cover and an open side are provided. One or more objects are arranged on the base of the lower box. The object(s) are covered with the upper box such that the open side of the upper is oriented towards the base of the box, the walls of the upper box are arranged on the base of the lower box and a gap is formed between the walls of the upper box and the walls of the lower box. A powder material is introduced into the gap in order to form an assembly having an interior. The powder material provides a mechanical obstacle to gas exchange between the interior and the environment. This assembly is then heat treated.