Coke is a gray, hard, porous fuel with high carbon content and few impurities, used both as a fuel and as a reducing agent in smelting iron ore in a blast furnace, where carbon monoxide from its combustion reduces iron oxide to iron. It is made by baking bituminous coal at high temperatures, 1800 to 3600 degrees Fahrenheit, until impurities burn off without consuming the coal itself, leaving carbon; although coke can form naturally, the man-made form is more common. Because it has a higher proportion of combustible carbon than coal, coke produces more heating power per ton and is strong enough to bear the weight of iron ore in an iron furnace. Coke was first used in an iron furnace in the states at Issac Meason's Plumsock pudding furnace and rolling mill in Fayette County, Pennsylvania, around 1817. Early production stacked lumps of coal in mounds for air circulation, ignited them, and covered them with large pieces of turf so the coal would not burn too quickly; this took up to eight days and converted only about half the coal to coke. A major advance came with the beehive oven, a large masonry dome built in long rows for easier loading and unloading, in which workers dumped coal through an opening in the top, ignited it, and sealed the oven so the coal could smolder. This process took two to three days and converted 70 percent of the coal to coke, though the rest burned off into the air as noxious smoke. As the steel industry grew and demanded more coke, the number of beehive ovens rose from about 200 to 31,000 between 1870 and 1905, then peaked at almost 48,000 in 1910. Beehive ovens were first used in Fayette County in the 1830s, and the Dilworth Mine in Rices Landing used 190 coke ovens along the Monongahela River to load coal and coke directly into barges. In the early 1900s, by-product coke ovens of various designs ended the dominance of beehive ovens by capturing and recycling contaminants from the coking process, producing more coke per ton, using lower-quality coal, and reducing the release of noxious smoke.