Product classification: slagwool and rockwool classed under mineral wool heading despite substantial blast slag content, clarified by tax circular. The Board clarifies that goods known or marketed as slagwool, rockwool and similar mineral wool are appropriately classifiable under heading 68.03 even if they contain more than 25% by weight of blast furnace slag, based on the Conference finding that these products are produced from blast furnace slag and sold as bulk or sheets, and the Conference recommendation that the specific mineral wool tariff entry is the appropriate classification.