Coal the heat and pressure of deep burial causes the loss of water, methane and carbon dioxide and an increase in the proportion of carbon. Thus first lignite (also called "brown coal"), then sub-bituminous coal, bituminous coal, and lastly anthracite (also called "hard coal" or "black coal") may be formed. The wide, shallow seas of the Carboniferous Period provided ideal conditions for coal formation, although coal is known from most geological periods. The exception is the coal gap in the Permian–Triassic extinction event, where coal is rare. Coal is known from Precambrian strata, which predate land plants—this coal is presumed to
Coal breaker more marketable product. Here, a series of interlocking, toothed crushers or rollers would break lump coal into progressively smaller sizes. On the fourth level down, the coal was further cleaned of impurities. This was originally done primarily by hand, but hand picking was gradually supplanted after 1910 by improved screens and jigs. Although breaker boys worked at all levels of the coal breaker, most of the removal of impurities by hand occurred on this level. (Some picking did occur on the ground level of the coal breaker, where boys would locate good pieces of coal in the culm and return