A furnace is described which comprises an insulated muffle comprising a ceramic fibrous body into which work pieces may be directly introduced through an opening. A closure member is arranged to close the opening, and means are provided for energizing a coiled heating element disposed in an open channel within the body and for controlling the rate at which the muffle is heated. The opening is arranged at the top of the muffle so as to ensure its rapid cooling subsequent to removal of the closure member therefrom. To enable the muffle to withstand rapid cooling, and in particular to retain the heating element in position, a plurality of strips of ceramic fibrous material are cemented diagonally across the top of the open channel. The closure member may be latched in the closed position by atmospheric pressure as the result of evacuating the interior of the furnace.