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This heading covers float glass in sheets. Its raw materials are melted in a furnace. The molten glass leaves the furnace and is fed on to a float bath of molten metal. On the float bath, the glass acquires the flatness of a liquid pool and later retains the smooth finish of liquid surfaces. Before it reaches the end of the bath, it is cooled to a temperature at which it is hard enough to be passed over rollers without being marked or distorted. From the float bath the glass moves through an annealing lehr, at the end of which it is cooled and can be cut. This glass is not surface ground or polished : it is perfectly flat as a result of the manufacturing process.
The heading also covers the types of glass of headings 70.03 and 70.04, which have been surface ground or polished (usually the two processes are combined).
In the surface grinding process the glass is subjected to the action of rotating iron-shod discs which, in conjunction with a flow of water containing abrasives, wears the glass surface down to smoothness. Transparency is achieved by polishing in a machine with felt-covered discs impregnated with rouge (iron oxide). Surface grinding can be continuous and twin-grinding machines are capable of working both surfaces of the glass simultaneously. A final polishing is sometimes done.
The glass of this heading may be coloured or opacified in the mass, or flashed with glass of another colour during manufacture or may be coated with an absorbent, reflecting or non-reflecting layer.
Glass of this heading is frequently used in windows and doors, motor cars, ships, aircraft, etc., for the manufacture of mirrors, table and desk tops, shelves, display cases, etc., and in the manufacture of safety glass of heading 70.07.
Glass in sheets which has undergone working not provided for in the heading text or in Note 2 (b) to this Chapter, including bent or curved glass, is excluded (headings 70.06, 70.07, 70.09, etc.).
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