Abstract: A vacuum chamber (1), notably for steel decarburisation, having: an upper part (2), or the top of the tank, which is in the form of a vacuum bell; and a lower part (3), or the bottom of the tank, having two jambs (4, 5) which are fitted with two plungers (7, 8), namely a rising plunger (7) and a falling plunger (8), which are intended to be submerged in the steel in the cavity containing the steel to be treated. The plungers (7, 8) are connected to the jambs (4, 5) of the tank by flanges (10) and a joint (11) supporting a sealing device. The flanges (10) are used to mount and disassemble the plungers. The joint (11) has at least two parallel planes (12, 13), and forms at least one collar (14), thereby providing a mortar and tenon-type joint.