| Mixed with poor coal are certain unburnable materials that melt and stick together as it burns and form what are known as clinkers. |
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| It is so unburnable that NASA uses it for the protective shell of its space shuttles. |
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| The rest of the material forms char, which is nearly pure carbon, and ash, which is all of the unburnable minerals in the wood. |
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| I ask people to remove all unburnable rubbish in bin liners. |
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| Their fireproof yarns and fabrics could create superlight firefighting clothes, lifesaving shelters for fire jumpers, and unburnable housing materials. |
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| Elm, apart from being nearly unsplittable, is almost unburnable. |
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