The refractory metals are insoluble in silver and copper. They do not form alloys and cannot be made by the usual melting and casting methods. Instead they are made by powder metallurgy techniques. Elkonite® is made using the press-sinter-infiltration procedure whereby refractory powders are pressed and sintered to form strong bodies with a pre-determined amount of porosity. The pores are then filled with molten silver or copper to form essentially fully dense bodies. There is usually an excess of silver or copper on one of the surfaces which in most cases is removed by machining. With the press-sinter-infiltrate process, the refractory content of the materials can range from about 35% to 90% by weight. The materials made by the press-sinter-infiltration process normally have the greatest resistance to arc erosion. They are used for almost all of the larger switchgear contacts as well as for some of the lighter duty applications.