Customs exemption for silicon carbide imports used in crucible manufacture limits duty above a prescribed ad valorem threshold. The Central Government, invoking section 25(1) of the Customs Act, exempts silicon carbide falling under Chapter 28 when imported for manufacture of crucibles from so much of the customs duty in the First Schedule to the Customs Tariff Act, 1975 as is in excess of the amount computed at an ad valorem rate of thirty five percent.