Abstract: An improvement in an apparatus for the production or refinement of metals comprising a furnace and a hood disposed thereover, which improvement comprises an upper chamber surrounding the sides and the top of said furnace, said upper chamber having at least one chamber side wall, said wall having a first movable door therein, a runway disposed within said chamber and partially out of said chamber, a charging crane disposed on said runway having disposed on one side thereof a second movable door movable therewith, means connected to said charging crane for moving said first movable door to permit entry of said charging crane, the size of said first and second movable doors being such as to substantially occupy the entire entrance way to said upper chamber.