Bureau of Mines has made a general survey of the "fusing" or "softening" temperatures of the ash from coals in the United States.
Data on the fusibility of coal ash should he of considerable value to the coal consumer, mainly in connection with the troublesome formation of clinker resulting from the melting of the ash constituents in burning coal. When used together with the large number of coal analyses published by the bureau, it will assist the consumer in comparing different coals and in selecting the coal best adapted to his purpose.
The ash is the incombustible residue after complete combustion, and is derived from the inorganic mineral constituents. 'I'hese are (I) inherent impurities in intimate mixture with the coal substance, (2) impurities such as clay, shale, pyrite, and calcite that are present as partings, layers, or nodules in the coal bed, and (3) fragments of roof or floor mixed with the coal in mining.
Coal ash varies so widely in chemical composition that no typical composition can be given, but generally falls within the following limits : Silica, 40 to 60 per cent. ; alumina, 20 to 35 per cent. ; iron oxides, 5 to 25 per cent. ; lime, I to 15 per cent. ; magnesia, 0.5 to 4 per cent., and alkalies, I to 4 per cent. The fusibility depends on several factors, such as ratio of bases to silica present, the particular bases, and the percentage of alumina. For example, ash extremely high in silica or in bases is not readily fusible. Ash low in iron is usually so high in silica that it is not readily fusible. Ash of coals high in pyrite contains much iron, and is likely to form clinker, the iron converting to ferrous okide which unites with silica to form easily fusible silicates. Some coal ash is so infusible that little clinker is formed, and ash slightly fusible often forms a porous, spongy clinker that does not obstruct flow of air through the fuel bed and is easily