Abstract: A cover is provided for a launder, or an equivalent open-topped vessel for containing molten metal, the cover including a heat-insulative layer spanning the open top of the vessel, with a spacer arrangement to maintain a gap of uniform width between the heat-insulative layer and the open-top. A housing defines an evacuation plenum, and connections are made to allow an exhaust device to remove gaseous materials from the plenum. Adjacent the gap is an air-guide arrangement which directs outside air into the evacuation chamber along a path which is intercepted by any gaseous materials exiting through the gap, whereby such gaseous materials are entrained into the evacuation chamber.