the invention relates to a sealing jaw for sealing thermoplastic materials or paper or cardboard coated with thermoplastic material , preferably packaging material . the sealing jaw is constructed from two ceramic materials , of which one is electrically insulating and the other electrically conductive : the electrically conductive ceramic material forms a continuous strand embedded in the insulating material and is , at or in the proximity of its end points , metallized so as to make good connection with an electric current source . the electrically conductive ceramic materials are treated with high pressure and heat so that they together form a pore - free , hard and abrasion - resistant body which can be applied to a steel rail .