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
Bituminous coal sulfur, and other impurities as possible. The constituents of the coal should be as follows: Cannel coal is a coal which ignites easily producing a bright flame. The name derives from the Scottish pronunciation of "candle coal". It contains a high volatile content, is non-coking and was a source for coal oil in West Virginia during the mid-1800s. While the use of Cannel has greatly diminished over the past century, it is still valued by artists for its ability to be carved and polished into sculptures and jewelry. When used for many industrial processes, bituminous coal must first be "coked"