Although its potential for water treatment has been recognized since ancient times, anthracite coal was not used for this purpose until the beginning of the 20th century. Crushed Anthracite makes an excellent medium density filtration media. Clack Anthracite is mined from the finest Pennsylvania coal. It is specifically selected for water treatment, and during its production goes through several sizing inspections. Representative samples are randomly chosen for a complete laboratory quality control analysis for effective size, uniformity coefficient, specific gravity, acid solubility and hardness. Because of its angular shape, some of the sediment penetrates deeper into the bed. When compared to equivalent filter sands, this means longer filter runs and less head loss. Backwash rates are also reduced. Because of its unique density, Clack Anthracite can be used in multi-media filters. At 50 lbs/ft³, it will hydraulically classify and remain above heavier media such as Filter Sand or Manganese Greensand, providing a prefiltration layer. When combined with filtering sands, anthracite is an effective filter media for water clarity in potable or industrial applications. It is among the most widely used filters. It works well in combination with sand or green manganese sand in mixed filters. It allows suspended pollutants to be kept in the depth of the filtering bed due to the unique form of its particles. This filtering media leads to greater flow, a decrease in pressure loss, and a faster and more efficient backwash than a sand filter. Anthracite filtration media are hand-picked from underground mines that have the maximum carbon content.