The furnace (2) has a combustion chamber (6) including a pressing stamp (4) for applying a pressing force to a heated blank (14) e.g. ceramic blank, in a muffle (8), where the stamp is guided into a guide opening (11). An optical fiber (36) is partially guided by the stamp, and guides temperature in form of temperature radiation, of the blank in the combustion chamber from the combustion chamber. An optical temperature sensor (34) is connected to the optical fiber outside of the combustion chamber, and detects the temperature of the blank. The fiber is made of quartz glass or sapphire.
