The muffle consists of a compact double walled trough-like hollow body which is gastight. The thermal insulation (17) is located within the body's cavity (4), and is additionally enriched by a heavy gas, such as xenon or fluorine-coal-hydrogen. On its upper side the body has a pressure compensating hole (18) which opens out in a compensating envelope located in a housing (19). The housing is a flat construction and extends parallel to the upper side of the muffle. The back wall (5) of the body is curved outwards or as a spherical segment.