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Slag wool and rock wool (e.g., of granite, basalt, limestone or dolomite) are obtained by melting one or more of these constituents and converting a stream of the resulting liquid into fibres, usually by centrifugal action and stream or air blast.
This heading also includes a class of "alumino-silicates" known as "ceramic fibres". They are formed by fusing a blend of alumina and silica, in varying proportions, sometimes with the addition of small amounts of other oxides such as zirconia, chromia or boric oxide, and by blowing or extruding the melt into a mass of fibres.
The mineral wools of this heading, like the glass wool of heading 70.19, have a flocculent or fibrous appearance. They differ from the latter by their chemical composition (see Note 4 to Chapter 70), while their fibres are generally shorter and not as white in colour.
Expandedor exfoliated vermiculite is obtained from vermiculite (heading 25.30) by heat treatment which causes a very large expansion of the material, sometimes up to 35 times its original volume.
The heading also includes the expanded forms of perlite, chlorites, obsidian, etc., similarly obtained by heat treatment. These generally consist of very lightweight spheroidal grains. Perlite activated by heat treatment is in the form of shiny white microlamellar powder and is classified in heading 38.02.
Expanded clays are made either by calcining specially selected clays, or by calcining a mixture of clays with other materials (e.g., sulphite lye). Foamed slag is made by adding small amounts of water to molten slag, and should not be confused with granulated slag which has a much higher density; the latter is made by pouring molten slag into water and is classified in heading 26.18.
All the above materials are incombustible and excellent heat-insulating, sound-insulating, or sound-absorbing products. The heading includes them even when in bulk.Subject to the tolerances concerning the asbestos content (referred to below), this heading also covers heat-insulating, sound-insulating or sound-absorbing mixtures of mineral materials in bulk, e.g., mixtures composed essentially of kieselguhr, siliceous fossil meals, magnesium carbonate, etc., often with added plaster, slag, powdered cork, sawdust or wood shavings, textile fibres, etc. The mineral wools described above may also form part of such mixtures which, in the mass, are used as packing materials, in the insulation of ceilings, roofs, walls, etc.
The heading includes articles, usually of low density, made from the above products or mixtures (e.g., blocks, sheets, bricks, tiles, tubes, cylinder shells, cords, pads). These articles may be artificially coloured in the mass, impregnated with fireproof substances, faced with paper, or reinforced with metal.
The mixtures and articles classified here may contain a small quantity of asbestos fibres, in particular to facilitate use. The proportion of asbestos added is generally not more than 5 % by weight. The heading excludes articles of asbestos-cement (heading 68.11) and mixtures with a basis of asbestos or of asbestos and magnesium carbonate (and articles thereof) (heading 68.12).
The heading also covers diatomite or other siliceous earths sawn into blocks or other shapes.
Articles of lightweight concrete (including concrete made with an aggregate of exfoliated vermiculite, expanded clay or the like) are excluded (heading 68.10).
Articles obtained by firing fall in Chapter 69.
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