Apparatus for closing the discharge aperture of a metallurgical vessel is presented. This apparatus includes a closure element attached to a pivoted lever in order to prevent undesirable discharge of slag at the beginning and end of the casting process and during refining. The lever pivotally opens and closes the aperture by means of a piston rod and pressure cylinder. The closure element, preferably made from wood, is temporarily attached to the lever by a friction fit and is designed to be used only once.