The gas-tight closure comprises a thin walled tubular piece projecting from a feeding duct into a filling space, a thin walled conical jacket having its larger bottom base provided with a rigid seal reinforcing member cooperating with the lower rim of the intake tubular piece. The outer periphery of the reinforcing member is slanted about a small angle to snugly fit into the tubular piece and adjust the form thereof when upward pull is exerted upon the conical jacket by means of an actuation rod passing outwardly through the wall of the feeding duct. The actuation rod has preferably the form of a hollow shaft which accommodates the manipulation rod of a scraper projecting radially below the reinforcing member into the filling space. The outside portion of the actuation rod is provided with a control device in the form of clamping plates cooperating with abutment plates resting on the guiding sleeve for the actuation rod. The clamping plates are manipulated by means of separate levers.