Portland cement is essentially an artificial product made by burning various minerals in a kiln at intensive heat to produce clinker. The upright dome kiln was the first type used in America. An improvement was the Schoefer vertical kiln, with chambers for heating, burning, and cooling the material. The kiln was in continuous production except when closed for repairs, and bituminous coal served as fuel. Coplay Cement Company built the kilns preserved in Saylor Park in 1892-1893 and operated them until 1904. They were replaced by the rotary kiln, one of the most important advances in cement manufacture.