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The non-combustible residue of a combusted substance; it is composed primarily of alkali and metal oxides. In the case of coal, ash consists of silica, iron, alumina, and other noncombustible matter. Ash increases the weight of coal, adds to the cost of handling, and can affect its burning characteristics Ash content is measured as a percent by weight of coal on an "as received" or a "dry" (moisture-free, usually part of a laboratory analysis) basis.

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