Coal breaker remaining. The sorting by size is particularly important for anthracite coal. In order to burn efficiently, air must flow evenly around anthracite. Subsequently, most anthracite coal is sold in uniform sizes. In the 1910s, there were six commercial sizes of coal (with the smallest size having three subsets): Coal pieces smaller than in size were considered "culm," and unable to be separated from the impurities (and thus useless). The grade of coal ranged from a low of 5 percent impurities for steam or broken coal to a high of 15 percent for pea-size coal and its subsets. Coal breakers were
Coal assay in the crucible. The cross sectional profile of this coke button compared to a set of standardised profiles determines the Free Swelling Index. Several international standards classify coals by their rank, where increasing rank corresponds to coal with a higher carbon content. The rank of coal is correlated with its geologic history, as described in Hilt's law. In the ASTM system, any coal with more than 69% fixed carbon is classified by its carbon and volatiles content. Coal with less than 69% fixed carbon is classified by its heating value. Volatiles and carbon are on a dry mineral free base;