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 screen (e.g., was of acceptable steam size or smaller). Raw coal often contains impurities such as slate, sulphur, ash (or "bone"), clay, or soil, which requires that it be cleaned before shipment to market. Mine workers sampled the coal as it came out of the mine to determine whether the level of impurity recommended washing (if washing was available). Slate, sulphur, and ash have a higher relative density than coal, and will sink in agitated water. Passing the coal through the tipple was an essential pre-treatment process for coal washing, however, because the impure coal must be of similar sized