Alternatively, a gas, for exam: ple an inert gas such as argon or nitrogen or a reactive gas such as an oxidisingor reducing gas, may be passed up through the or each passageway 23 into the space defined between the upper end of the sleeve 8 and the undersurface of the flange 16 and maintained at a pressure sufficient to inhibit the ingress of molten metal between the opposed surfaces of the sleev