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 19th century and early 20th century led to the use of mechanical devices for separating impurities from coal in coal breakers. Screens and sorters were used for dry coal. Some examples from the beginning of the 20th century are: To handle wet coal, coal jigs were used. Coal jigs separated coal from impurities by using gravity. Since the relative density of uniformly sized pieces of coal, slate, ash, dirt, and sulphur vary, pieces of each element will descend through water at different speeds—allowing them to be separated. Some examples of coal jigs from the beginning of the 20th century include: