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This heading covers a range of preparations used in the ceramic industry (china, earthenware, etc.), in the glass industry or for colouring or finishing metal articles.
(1) Prepared pigments, prepared opacifiers and prepared colours are dry mixtures formed by the heat treatment of oxides (of antimony, silver, arsenic, copper, chromium, cobalt, etc.) or salts (fluorides, phosphates, etc.) with or without fluxes or other substances, and are fired at high temperatures, generally above 300 °C, after application. The goods are used to produce a coloured or opaque surface in the course of ceramic firing. They may be incorporated in the glaze or enamel, or be applied as a coating before glazing. (2) Vitrifiable enamels and glazes are mixtures of silica with other products (feldspar, kaolin, alkalies, sodium carbonate, alkaline-earth metal compounds, lead oxide, boric acid, etc.) giving a smooth surface, either matt or glossy, by vitrification under heat. In most cases some of the constituents have been fused together in a preliminary process and are present in the mixture in the form of powdered frit (see below).
They may be transparent (whether or not coloured) or rendered opaque by the addition of opacifiers or pigments; sometimes substances (e.g., titanium or zinc oxides) are added which produce decorative crystalline effects on cooling after the firing. These vitrifiable enamels and glazes are generally in the form of powders or granules. (3) Engobes (slips) are semi-fluid pastes with a basis of clay, whether or not coloured, used to coat ceramic ware, either completely or in the form of a pattern. They are applied either before firing or after a preliminary first firing. (4) Liquid lustres are solutions or suspensions of metal compounds in spirits of turpentine or other organic solvents, used for decorating ceramics or glassware. The most widely used are gold, silver, aluminium or chromium lustres. (5) Glass frit and all other varieties of glass (including vitrite and glass obtained from fused quartz or other fused silica) in the form of powder, granules or flakes, whether or not coloured or silvered.
These products are used in the preparation of coatings for ceramic, glass or metal articles as well as for other purposes. For example, frit is used in the preparation of the vitrifiable products referred to in paragraph (2) above. Glass powder and granules are sometimes sintered to form discs, plates, tubes, etc. for laboratory use.
Vitrite is generally used for insulating electrical parts (e.g., contact terminals for electric lamp caps).
Other varieties of powdered glass are used as abrasives, for decorating postcards, Christmas tree decorations, for obtaining coloured glass articles, etc.
When the products referred to in paragraph (5) above are in forms other than powder, granules or flakes, they are excluded, and generally fall in Chapter 70. This applies in particular to "vitrite" and "enamel" glass in the mass (heading 70.01), to "enamel" glass put up in the form of bars, rods or tubes (heading 70.02) and to small regular spherical grains (microspheres) used for coating cinematograph screens, road signs, etc. (heading 70.18).
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