plasma induction or the cvd chemical vapour deposition plasma deposition process the core is preferably provided with grooves which may be produced by machining into a planar core or may be provided for when the core is moulded especially in the case where it is found more economical to manufacture the latter by moulding the core may advantageously be a disc or a segment of a commercial refining disc or a cone a conical frustum or a segment of a cone or of a commercial refining conical frustum the coating of wear resistant material is then preferably applied so that all the shapes of the commercial disc disc segment cone conical frustum or segment of a cone or of a conical frustum are preserved it is obvious that the coating may completely cover the core that is to say its working face as well as its securing face or its side faces a filling according to the invention is thus available which produces better refining than the commercial disc disc segment cone conical frustum or segment of a cone or of a conical frustum the lining according to the invention thus obtained also has a better wear resistance than the commercial disc disc segment cone conical frustum or.