The invention provides an improved furnace structure and methods of melting material in furnaces, particularly aimed at reducing the start up time for such furnaces. The furnace (2) defines a container for the molten metal having a maximum depth of molten metal in that container during use. A flow generator (30) for the molten metal within the furnace is fed through a first conduit (32) and supplies material back to the furnace through a second conduit (36). The first conduit entrance is provided in the upper 60% of the maximum depth of the molten metal in the container and/or the second conduit exit is provided in the lower 25% of the maximum depth of the molten metal in the container. The flow generator may receive material preferentially from around the periphery of the container and/or direct material towards the centre of the container. The inlet and outlet to the flow generator may be angled downwards towards the container. All of these features improve heat transfer and material circulation within the furnace.