Furnace elements 10 surround gas impermeable mold vessel 12 which can be constructed of a material such as steel, quartz, alumina or other metals or ceramics which are gas impermeable and wherein molten infiltrant charge 14 is separated from staple preform 15 by filter 16 which can be made of alumina staple fiber, fiberglass, mullite fiber, carbon fiber, Fiberfrax???, silicon carbide foam, carbon