Anthracite latter in two or three sizes, all three being above in (38 mm) size on round-hole screens. High grade (HG) and ultra high grade (UHG) anthracite are the highest grades of anthracite coal. They are the purest forms of coal, having the highest degree of coalification, the highest carbon count and energy content and the fewest impurities (moisture, ash and volatiles). High grade and ultra high grade anthracite are harder than standard grade anthracite, and have a higher relative density. An example of a chemical formula for high-grade anthracite would be CHONS, representing 94% carbon. UHG anthracite typically has a
Anthracite difficulty of igniting it. This was a particular concern in smelting iron using a blast furnace. With the invention of hot blast in 1828, which used waste heat to preheat combustion air, anthracite became a preferred fuel, accounting for 45% of US pig iron production within 15 years. Anthracite for iron smelting was later displaced by coke. From the late 19th century until the 1950s, anthracite was the most popular fuel for heating homes and other buildings in the northern US, until it was supplanted by oil-burning systems and more recently natural gas systems. Many large public buildings, such as