Abstract: A freely oscillating moving element (12-14-16), according to a radial axis with respect to the furnace wall (1) and having an inner appendix (12) deviated from the axis and with an outer counter-weight (16), can oscillate between two angular positions defined by pawls, towards the first of such positions said moving element is cyclically moved during the moving element lowering trajectory with the rotation of the furnace while towards the second angular position said moving element is cyclically moved during the moving element raising trajectory with the furnace rotation; said moving element can reach said second position only when the inner appendix (12) emerges from the material contained in the furnace.