The fine ash from combustion processes that becomes dispersed in the air. To the casual observer it might appear as smoke, and indeed it is often found mixed with smoke. Fly ash arises because fuels contain a small fraction of incombustible matter. In a fuel like coal, ash has a rock-like siliceous composition, but the high temperatures at which it is formed often means that metals such as iron are incorporated into the ash particles, which take on the appearance of small, colored, glassy spheres. Petroleum produces less ash, but it is often associated with a range of oxides such as vanadium (in the case of fuel oils) and, more noticeably, hollow spheres of carbon. In traditional furnaces, much ash remained on the grate, but modern furnaces produce such fine ash that it is carried away in the hot exhaust gas.

Early power stations dispersed so...