The slag  stopping ball density is between the molten steel and slag density.for that reason it floats between the molten steel and the slag.the size is determined on the converter type and the taphole diameter,if it's too small it will float even on the slag.if it's too large it will plug in the taphole prematurely.when1/3 of the molten steel in the converter is left,the slag stopping ball is entered to rhe converter,and by the delivery equipments calibration the guiding rod is positioned inside the taphole mouth and then released.it is weighted lighter on the rod side and heavier on the ball side.