0 6903.90 Retorts, crucibles, mufflers, nozzles, plugs, supports, cupels, tubes, pipes, sheaths, rods and other refractory ceramic goods (excl. those of siliceous fossil meals or similar siliceous earths, those of heading 6902, containing > 50 % carbon or containingg > 50 % by weight of alumina [Al203] or a mixture or compound of alumina and silica [SiO2]) 10 4 3 2 1